close

Вход

Забыли?

вход по аккаунту

?

Methods and Applications of Nuclear Magnetic Double Resonance.

код для вставкиСкачать
[7] R . E . Watson: Solid State and Molecular Theory Group. Tech.
Rep. No. 12, MIT 1959; Phys. Rev. 118,1063 (1960).
[S] K I . Goldanskii, At. Energy Rev. I , 3 (1963).
[9] J . Danon: Applications of the Mossbauer Effect in Chemistry and
Solid State Physics. Tech. Rep. Ser. Int. At. Energy Agency 50,89 (1966).
[lo] V 1. Goldanskii, E. F . Makarov, and R . A . Stukan, Teor. Eksp.
Khim. Akad. Nauk. Ukr. SSR 2, 504 (1966).
[ll] E. Simanek and 2. Scroubek, Phys. Rev. 163,275 (1967).
1121 E . Siriidnek and A . Y. C. Wong, Phys. Rev. 166, 348 (1968).
[13] C. K . Jbrgensen, Progr. Inorg. Chem. 4,73 (1962).
[14] A . Viste and H . B. Gray, Inorg. Chem. 3,1113 (1964).
[l5] G. Kaindl, E! Porzel, F. Wagner, U . Zithn, and R. L. Mossbauer,
Z. Physik 226, 103 (1969).
[I61 H . B. Gray and N . A. Beach, J. Amer. Chem. SOC.85,2922 (1963).
[17] R. C. Shulman and S . Sugano, J. Chem. Phys. 42,39 (1965).
[IS] H . G. Drickamer, R. W Vaughan, and A. R. Champion, Accounts
Chem. Res. 2.40 (1969); S. C. Fung and H.G. Drickamer, J. Chem. Phys.
51,4353, 4360 (1969); H . G. Drickamer, G . K . Lewis, jr., and S . C. Fung,
Science 163, 885 (1969).
1191 U! A. Graham, Inorg. Chem. 7,315 (2968).
[20] H . D. Bartunik, W Potzel, R. L. Mossbauer, and G . Kaindl, Z .
Physik 240, 1 (1970).
[21] G. M . Bancroft, M . J . Mays, and 8. E. Prater, J. Chem. SOC.A 1970,
956.
[22] C. K. Jmgensen: Absorption Spectra and Chemical Bonding in
Complexes. Pergamon Press, Oxford 1962.
[23] J. Danon in K I . Goldanskii and R. H . Herber: Chemical Applications of Mossbauer Spectroscopy. Academic Press, New York 1968,
Chapter 3.
[24] B. D. Josephson, Phys. Rev. Lett. 4, 341 (1960).
[25] G. A . Sawatzky and d . Hupkes, Phys. Rev. Lett. 25, 100 (1970).
Methods and Applications of Nuclear Magnetic Double Resonance
By Wolfgang von PhiIipsbornr''
In double resonance spectra, transitions between energy levels of a nuclear spin system are
measured in the presence of two (or more) oscillating magneticfields. Experiments of this nature
form the basis of what is nowadays one of the most important techniques of N M R spectroscopy.
Depending on the method selected, they can be used to unravel complex spectra, to measure
hidden or weak resonances, or to determine the relative signs of coupling constants, as well as in
stereochemical or kinetic studies. This wide and steadily growing range of applications of double
resonance is described with the aid of specific examples.
1. Introduction
NMR spectroscopy is nowadays an essential part of molecular spectroscopy and an indispensable aid in the investigation of the structures of organic and inorganic compounds.
Several progress reports on various aspects of the field
have appeared in this journal" -61. Particularly great
progress has been made in recent years in the development
and application of double and multiple resonance methods.
An effort is therefore made below to describe the methods
that are of most interest to chemists, and to demonstrate
their use in spectral analysis and for the solution of structural problems. As the literature is extremely abundant,
a comprehensive review is impossible here, and the examples were therefore chosen for their illustrative character.
turbation of a second type of nucleus. The method is based
on a proposal made by Bloch in 1954, and was successfully
realized in the very same year[71.This first reported experiment was concerned with the determination of the Larmor
frequency (o=yH,) of I3C in 13CH31;the procedure used
2. Phenomenology and Instrumental Principles
I
In double resonance experiments, transitions between
energy levels Of a
'pin system in a polarizing static
magnetic field HO are measured in the Presence of two
oscillatingmagnetic fields H, and H,. H, is used to observe
the resonance of one type of nucleus, and H, for the per[*] Prof. Dr. W. v. Philipsborn
Organisch-chemisches Institut der Universitat
CH-8001 Zurich, Ramistrasse 76 (Switzerland)
472
EmJ
w,
1
WC
Fig,1, Schematic illustration ofthe proton spectrum of13CH31(bottom)
and Royden's 1H-{13C] double resonance experiment [7]. Observing
field H I , perturbing field H,.
was to look for the frequency w, of the perturbing field
H, that simplifies the proton doublet observed with H I
to an optimal singlet (Fig. 1). The experiment is thus also
Angew. Chem. internat. Edit. 1 Vol. 10 (1971)
/ No. 7
the simplest case of spin decoupling, since the strong perturbation of the 13Cnuclei leads to a breakdown of protoncarbon spin coupling. However, double resonance and spin
decoupling are not synonymous, since decoupling effects
occur only under certain conditions.
In view of the large number ofdouble resonance experiments
known, some classification is necessary. First of all, one
distinguishes between homonuclear and heteronuclear
experiments, according to whether the nuclear spins in
question belong to nuclei of the same type or of different
types. This affects the experimental conditions, inasmuch
as a second radio frequency can be used for H, in the
heteronuclear case, whereas in the homonuclear case the
second frequency is produced mostly by audio modulation
of the spectrometer frequency. The technique of heteronuclear double resonance has been described in two
reviews18.91. A comprehensive description of the theory
and practice of double resonance with special reference to
homonuclear experiments has been published by Hoffman
and Fors&n‘lol.Since the present article is concerned
mainly with the phenomenology and application of double
resonance, the reader is referred to the literature for a more
profound theoretical study 18, lo].
The experimental methods can best be further classified
according to the energy of the perturbing field. The energy
yH,[Hz] (y=y/2n; y=gyromagnetic ratio of the nuclei
irradiated by H, ; H, = amplitude of one of the rotating
components of the perturbing rf-field) may be compared
here with the nuclear spin interaction JNN,[Hz], the line
width Av,,, [Hz], or the reciprocal relaxation times I/T,
and 1/T, [Hz]. As the amplitude of the perturbing field
decreases, it is possible to distinguish the following cases,
where A is the nucleus under observation and X is the
perturbed nuccleus in any spin system :
Amplitude of the
perturbing field
[Hzl
Effect on the observed
A resonance
Terminology
-
Reduction of the
yH, 9 n. J A X
( n depends on the
number of resonance
nature and number lines
of equivalent X
nuclei
Reduction of the
YH,% J A x
number of resonance
lines by selective
irradiation of the X
resonance
Additional splitting
by submultiplet
formation
Changes in the relative
intensities of the
multiplet lines
Spin decoupling
Selective spin
decoupling
Spin tickling
Generalized
Overhauser effect [a]
[a] Cf. [lo] and Section 3.4
The various double resonance experiments yield different
kinds of information, and are illustrated in Section 3 with
the aid of some typical examples. The following abbreviation is commonly used for the formal description of such
experiments: A-{X} denotes an experiment in which
the X resonance in an AX system is totally or partly
irradiated by H,, whereas the A resonance is observed by
means of H,. This can be achieved under various conditions,
Angew. Chern. internat. Edit. 1 Vol. I D (1971) / No. 7
which determine the appearance of a double resonance
spectrum. Since there are five experimental parameters,
i.e. H,, w, ;H,, w,, and H,, and since w, and o,are related
to H, by the Larmor conditions for A and X, at least one of
the quantities H,, ol,and w, must be varied to enable a
double resonance spectrum to be recorded.
The following measuring techniques are distinguished :
1. Variation of field
“field sweep”:
o1-02=const., H,+AH variable, 1.c.
theX spectrum is scanned wilh wz(H2),
while A is scanned with w I f H I ) .
2. Variation of frequency
“frequency sweep a,”:
w,/H,=const., o1*A@ variable; the
additional irradiation is fixed for X.
whereas w,(H,) sweeps over the entire
A spectrum.
o , / H , =const., 0 2 + A w variable; the
same line in the A spectrum is always
observed, whereas the perturbing field
H, scans the resonance lines of X.
3. Variation of frequency
“frequency sweep 02”:
(INDOR [a])
[a] INDOR = abbreviation for “internuclear double resonance”.
In contrast with normal “single resonance” spectra, fieldsweep and frequency-sweep double resonance spectra
differ from one another. The field-sweep method can be
readily used in the homonuclear case when o, and o2
are produced by audio modulation of the transmitter frequency 0,. It is particularly suitable for spin decoupling,
and is historically the older technique. However, the number of experiments required for the decoupling of the A
nuclei from several nuclei with different chemicals shifts
(e.g. (X) (Y) ...) is obviously equal to the number of different nuclei. If oA-oxp 2 n JAx(and this is so in first-order
spectra), the difference in the modulation frequencies
o,- o2corresponds to the relative chemical shift of A and
X, o A - w x . It follows therefore that spin decoupling can
also be used to determine the (relative)chemical shift of the
X nucleus if this resonance is hidden under the resonances
of other nuclei. In a frequency-sweepexperiment, one of the
two fields, H, or H,, must be accurately fixed at a certain
point in the spectrum (at w1or 02);
this is facilitated by field
frequency stabilization of the spectrometer. If the perturbing
field H, is fixed at the resonance of a nucleus which is
coupled to several partners the coupling with all the partners can be eliminated in one experiment. The techniques
most commonly used at present are 2 and 3.
Another important factor is whether the magnetic fields
H, and H, are stationary or transient, i.e. whether their
application is of short duration. An additional parameter
in double resonance experiments is therefore the rate
of change of frequency for the observing and perturbing
fields (sweep rate). This is particularly important in
experiments in which intensity changes are observed as
a result of changes in the populations of the energy levels
(Overhauser effects). Time-dependent intensity changes of
this nature in double resonance experiments are discussed
in Section 3.5.
3. The Methods and their Uses
The double resonance methods indicated in Section 2 can
be applied to a wide variety of problems, as is clear from
the following survey :
473
Method
Use
Noise decoupling
(broad-band decoupling)
heteronuclear
Simplification and sensitivity enhancement in spectra of nuclei that
give low intensities, e. g. I3C-{'H) and
also "F-{ IH} experiments.
Spin decoupling
Structure elucidation by definite idenheteronuclear and homonuclear tification of spin-coupled nuclei and
simplification of complicated multiplets; detection of hidden resonances; simplification of spectra of multispin systems to obtain preliminary
sets of parameters for computer calculations.
Assignment problems within complex
Selective spin decoupling
and overlapping mulliplets; determination of the relative signs of scalar
nuclear spin couplings.
Accurate measurement of the frequenSpin tickling
cies of hidden resonance lines; assignments in strongly coupled spin systems; identification of subspectra; determination of the relative signs of
coupling constants; construction of
energy level diagrams.
Overhauser effects
(a) Generalized OE
(spin pumping)
Accurate measurement of the frequencies of hidden resonance lines by
INDOR spectroscopy, determination
of proton sequences, elucidation of
structures; determination of relative
signs of coupling constants; construction of energy level diagrams ;indirect
determination of the chemical shifts of
nuclei giving low intensities, e.g.
I4N, 31P, by heteronuclear INDOR
spectroscopy.
Investigation of relaxation processes
in liquids and gases.
(b) Classical infermolecular
internuclear OE
(c) Intramolecular internuclear Semiquantitative determination of internuclear distances, solution of sterOE (NOE)
eochemical problems ; measurement
of relaxation and correlation times;
intensity enhancement of weak nuclear resonances, e. g. I T .
Transient double
resonance experiments
quantization of X in the direction of H, (x), i. e. perpendicular to Ha, the zdirection, in which the A nucleus observed
with H,(H,< H2) and coupled with X is quantized. Since
the scalar nuclear spin coupling is given by the product
J,,. i,. i,, orthogonal quantization of A and X results in
a collapse of the spin coupling.
The analytical importance of spin decoupling for the unraveling of complex NMR spectra was recognized very
early, and experiments with this aim were carried out first
on heter~nuclear[~.
'*
and later on homonuclear
systems1131.Heteronuclear experiments of the type H-{X}
and X-{H} can greatly simplify the spectra of protons in
compounds containing 2H, "B, I4N, I9F, or 31Pand facilitate the interpretation of the spectra of the corresponding
hetero n~clei"~.It is essential in experiments of this type
that the transmitting coil of the spectrometer be able to
accommodate both Larmor frequencies and that it be tuned
accordingly or that a second transmitting coil be incorporatedIL4';the second radio frequency may be advantageously
supplied by a frequency generator.
''3
An important and frequent application is the decoupling
of the protons from deuterium in specifically deuterated
organic compounds. The proton resonances in such cases
are greatly broadened as a result of 'H, 'H spin coupling
and deuterium quadrupole relaxation effects (Fig. 2),
However, for kinetic studies by NMR line shape analysis the
line width should be as small as possible. An example
is provided by the determination of the free enthalpy of
activation (AG" = 8.1 kcal/moIe) for the ring inversion of
cyclooctane on the basis of the 'H resonance of C,D,,H
with 'H-(*H} de~oupling['~'.
Similarly, AG" = 10.2 kcaI/
mole was found for the ring inversion of cyclohexane from
the temperature-dependent 'H spectra of C,Dl 1H['6a-c1.
Kinetic studies on spin systems with
chemical exchange, determination of
exchange rates and spin-lattice relaxation times.
Their value will now be demonstrated with the aid of
specific problems. The space devoted to the various
examples does not necessarily reflect their relative importance, which should in fact be greatest for the less well
known, more recent methods.
3.1. Spin Decoupling
This is the oldest method. The disappearance of spinspin coupling in an A-{X} system can be best understood
if one considers the double resonance experiment in a
rotating system of coordinates ("rotating frame"). For a
system of Cartesian coordinates rotating about the direction of the static magnetic field H, (z axis) with a frequency
0 2 ,the time-dependent perturbation caused by H2 becomes a stationary perturbation in the x direction. Ha
then appears to be reduced to Hb=H,-w2/y, and the X
nucleus perturbed by H, is exposed to an effective field
Hb R2. If o2 corresponds to the resonance frequency of
the X nucleus, this leads to Hb=O and to an effective
+
474
Fig. 2. Proton spectrum of [D,,]-cyclohexane at -100°C (bottom)
and with decoupling from the deuterium nuclei (top); v,-v,=28.7 HZ
(60 MHz) [I~c].
The advantage of this double resonane technique is that
the kinetics in complex spin systems such as in C,H,, and in
C6H1, can be reduced to a simple two-center exchange
in CsD,,H and C6D,,H respectively (Fig. 2), with the
result that more accurate thermodynamic activation parameters can be obtained. The vicinal proton coupling conhave also
stants in 2,2,3,3,4,4,5,5-octadeuteriocyclohexane
been determined from the 'H-{'H} double resonance
Heteronuclear 'H-{ 14N} and 'H- { "B} decoupIing have proved excellently suited for the analysis
Angew. Chem. internat. Edit. / Vol. 10 (1971) No. 7
of nitrogen heterocycles, such as pyrrole['81and pyridine"'!
and of boranes["! "B-('HI decoupling has been used
for the analysis of the "B spectra of carboranes'"!
For
further examples and instrumental details the reader is
referred to 18*91.
80%
+
decoupling of the 'H resonance, the carbon spectra can
be greatly simplified and the signal/noise ratio for this
low-intensity nucleus can be improved (cf. also Section
3.4.3). Figure 4 shows the application of this double resonance process to the I3C spectrum of pyridine that contains
20%
i
100
120
110
140
130
6 lppmfmCFCI,I
--3
Fig. 3. "F spectra of a mixture of two substituted cyclobutanes, which are themselves mixtures of cis-trans isomers; top: normal spectrum, bottom: proton noise-decoupled spectrum [22].
Difficulties are encountered in the use of the decoupling
technique when the resonances of the types of nuclei to
be irradiated extend over a large frequency range. This is
so in 'H-{"F) and "F-{'H) experiments because of
the magnitude of the proton-fluorine coupling. A sjmitar
problem arises when one wishes to measure the I3C
resonances of a molecule with complete decoupling of all
protons. The required frequency range (a few hundred to
one thousand Hz) of the perturbing field H, is achieved
by a method known as noise decoupfing"". In this method,
the perturbing field H, can be applied pseudo-randomly
over a wide frequency range (e.g. lo00 Hz) with the aid
the carbon isotope in natural abundance (1.1%). The
importance of suchL3C-{'H} double resonance spectra for
the elucidation of structures becomes obvious when one
considers the noise-decoupled carbon spectrum of 4-allyl4-rnethyl-2,5-cyclohexadien-l-one
(Fig. 5)1231.
-L'a
1
,
-25
I
I
-5
15
35
S lppm 1 -+
55
75
Fig. 5. Carbon I3C spectrum (25.2 MHz) of 4-allyl-4-methyl-2,5-cyclohexadien-1-one in CHCI,, { 'H} noise decoupling; chemical shifts
relative to CS, [S(CHCI,)= 115.0 ppm].
L
36.5
L@Q
44.5
52.4
60.4
68.3
76.3
81ppm1-
Fig. 4. Carbon I3C spectra (25.2 MHz) of neat pyridine: (a) 16 computer-accumulated spectral scans; (b)1 scan, measured with {'HI noise
decoupling; chemical shifts relative to CS, (Varian Assoc.).
of a shift pulse generator, so that all the nuclei are covered
in a very short time. The first examples were described by
Ernst for the 'H,19F double resonance (Fig. 3); however,
the most important use nowadays is in the measurement
of the 13C resonance of organic compounds. By complete
Angew. Chem. internal. Edit.
I Yol. 10 (1971) J No. 7
The conditions for homonuclear (e. g. 'H, 'H) experiments
are nowadays satisfied by the equipment of standard
spectrometers, and have been described in detailFs- 'O1.
We shall therefore confine ourselves here to a brief description of the difference between homonuclear field-sweep
and frequency-sweep double resonance spectra. Figure 6
shows the 100 MHz spectrum of the alkaloid fruticosine,
the structure of which was verified by double resonance
experiments[241.The region between 2.4 and 4.8 ppm
contains the signals of twelve protons, six of which are
assigned to two separate AMX systems, H-C(4)-C(3)HOH
and H,C( lO)-C(ll)H. Six separate irradiations are required
here, since only one spin coupling is eliminated in each
475
experiment. This disadvantage disappears in the frequencysweep method.
The irradiation of the H5 resonance in mannosan triacetate
(Fig. 7) simplifies the signals of all the protons coupled
with this nucleus, i.e. H2, H3, H4, H6I and H6212s1.This
fixing
0,.In
the field-sweep technique, the variable is
w1-a2, so that eq. (1)and (2) enter into the correction
formula :
(3)
The frequencies of the lines observed in double resonance
spectra can thus be regarded only as approximations to
the values for the unperturbed spin system. This is important when spin decoupling is used to simplify complicated
systems in accurate analyses. Nevertheless, an important
application of this double resonance method is the determination of preliminary sets of parameters for iterative
computer calcuiations (cf. Fig. 16).
-30°C
A
8 00 7’55
i
H-I7
\
-
7 65
483
3.2. Selective Spin Decoupling
331 359316286255241 16
Gippmi
Fig. 6. ‘H spectrum of the alkaloid fruticosine (100 MHz) and fieldsweep double resonance spectra; irradiation position A, observation
position 1[24].
The spin decoupling method cannot be applied to strongly
coupled spin systems or to systems with overlapping
multiplets, since perturbation of the lines of neighboring
nuclei is unavoidable. However, double resonance spectra
that are easy to interpret and that can be used for assignment purposes are obtainable by reduction of the amplitude
of the perturbing field H, so that only part (an accurately
defined part) of a multiplet is perturbed. This “selective”
decoupling method can be best illustrated for a three-spin
AMX system (I=+) (Fig. 8).
M
X
1234“
1. Irx>O
5.50
ias2571
5 OC
-
12
L.00
3 50
Slppmi
Fig. 7. ‘H spectrum of mannosan triacetate (100 MHz) and frequencysweep double resonance spectrum; irradiation at 4.62 ppm from TMS
~ 5 1 .
example can also serve for the demonstration of the frequency shifts that occur in double resonance spectra. In
the double resonance spectrum of Figure 7, the signals of
all the decoupled protons are displaced to different frequencies in relation to their original position, the shift being
greatest in the neighborhood of o,(H,). The irradiation
of the X resonance with H, causes a shift in the Larmor
frequency of the nucleus A under observation as a result
of the Bloch-Siegert effect[z61:
In experiments in which oA- ox4271J,, is not satisfied,
moreover, the value of o2for optimum decoupling differs
from the chemical shift of the X nucleus ox:
34
M: ap ap X. ab a!
I,,~>O A - aap p A aa
4 50
A
pp
2. IAx<0 M: ag a@ X: ap ap
1,,>0 A: p p aa A- acl p p
1 2 3
”
X a
L
pa p
M: a ap p
1 ap a
M - a a @ fi
X:
:i
Spin-
orien -
tation
1182581
Fig. 8. Schematic three-spin AMX spectrum (I =+) and spin orientations of the nuclei for the various lines with like and unlike signs of
J , and J M X .
Irradiation on the right-hand M doublet produced by J,,
simplifies only one of the corresponding X doublets to a
singlet. This is understandable if one considers that the
two M doublets (MI, M,) and (M,, M4) are produced by
different molecules, which differ in the spin state (a or J3)
of the A nucleus. The same is true of the two X doublets.
Irradiation of the M resonance in one type of molecule
can decouple only theX nuclei in the same type of molecule.
The information obtained from selective decoupling of
this nature is the same as from a total decoupling experiment, and it can be used e.g. in questions of assignment in
the elucidation of structures.
The question which of the two X doublets is decoupled by
irradiation of M, and M, depends only on the relative
signs of J,, and JMx(Fig. 8). This double resonance (DR)
method is thus suitable for the determination of signs of
coupling constants, and was first applied to the diethylthallium ion (C2Hs),TI 1281, furan-a-carboxylic acidtzg1,
and fluorinated ethane~[~”.
Figure 9 shows the M and X
part of the 100MHz spectrum of the vinyl protons of
+
Both correction terms are easily explained[271.
In frequencysweep experiments, eq. (2) must be taken into account in
476
Angew. Chem. internat. Edit.J Vol. 10 (1971) j No. 7
furan-a-carboxylic acid and a selective decoupling experiment carried out by frequency sweep"]. It shows that
J, =J4, and J,, = J,, are of like sign (cf. Fig. 8).
,
Similar DR experiments can be carried out in heteronuclear
systems. Figures 10a and 10b show the 100 MHz proton
spectrum and the 94.1 MHz fluorine spectrum of 2-fluoro4-allyl-4-methyl-2,5-cyclohexadien-6-one.
Irradiation of the
two lowest-field lines of H-5, i. e. elimination of J,,,, simplifies the left-hand branch of the fluorine spectrum from a
septet to a sextet (Fig. 10c).It follows that the signs of J,,
J,,,, J,,,, and J6,Fare the same1311.
,,
X
M
J,,
'nx
Fig. 9. 'H spectrum of the M and X protons of furan-a-carboxylic acid
(bottom) and selective decoupling experiment by irradiation of the
two highest-field lines of M.
[*] The original experiment by Freeman and Whr%fen[29]was carried
out under field-sweep conditions.
It is even possible in certain cases to determine the magnitudes and relative signs of unresolved spin couplings. It
can be seen from Figure 11 that elimination of J,, in the
X part of an AMX spectrum by selective irradiation of
suitable A transitions produces an unsymmetricalX triplet,
from which ]JMxl
= /(A1- A,)]/2 and the relative signs of J,,
and JAMcan be found even when JMx is very small and
not resolved in the original spectrum. Irradiation of the
four lowest-field lines of H-5 (Fig. 10d) changes the two
doublets of H-3 ( J 3 , 6 not resolved) into two triplets that
are unsymmetrical toward the right, from which one
obtains J 3 , 6 z 0 . 1 5Hz, with opposite signs for J,,, and
J3,6.Since J,,,, as a vicinal H,Hcoupling, has a positive
sign[3z',J 3 , must have a negative sign.
The relative signs of the nuclear spin coupling constants can
be determined by various DR experiments (cf. the survey
F
JV
3,F
0
H
HQF
H
CHp
+ 131:8
ppm
-CHz-CH
N C " ,
*
20 Hz
700
H-5
690
.
650
H-3
640
-630
620
Hz
H-6
Fig. 10. 'H spectrum (a) and I9F spectrum (b) of 2-fluoro-4-allyl-4-methyl-2,5-cyclohexadien-1-one;
(c) selective 19F-{'H) decoupling experiment; (d) 'H-{'HI decoupling to eliminate J 3 , 5 [31], cf. Fig. 11.
Angew. Chem. internat. Edit. / Vol. 10 (1971)
/ No. 7
411
on p. 474). Together with the magnitude of J and the chemical shift, it is a further source of information on which
structural and theoretical conclusions can be based. Since
the relative signs of most of the typical H,H couplings are
now known, their determination can be of decisive value
in the investigation of unknown structures, particularly in
a’
G&*
I I
I I
x, x,
x3
X‘
bl
3.3. Spin Tickling
If the amplitude of H, is further reduced and yH, reaches
the order of magnitude of the line width A v , , ~ or of the
reciprocal transverse relaxation time T;*, the perturbation is confined to one transition and the associated energy
levels. Under these double resonance conditions, spectral
lines that have an energy level in common with the perturbed transition will be split into ~ubmultiplets[~~?
The
magnitude of the doublet splitting observed in nondegenerate systems[*’is proportional to the amplitude of H,
and the square root of the intensity of the irradiated line.
Lines that are regressively[401connected to’the perturbed
transition normally give well resolved doublets, whereas
progressively connected lines give poorly resolved (broad)
doublets. The tickling experiment can be best illustrated
by the schematic spectrum and energy-level diagram of an
X‘
Cl
Fig. 11. Schematic representation of the X part of an AMX spectrum:
(a) normal spectrum, (b) and (c)double resonance spectra with selective
irradiation of the A part and elimination of JAX.
stereochemical problems, in cases where IJI alone does
not allow any definite conclusions. For example, the
preferred conformation ( I a ) of 7-substituted cycloheptatrienes was deduced from the (negative) sign of the allylic
coupling J , , 7[331.
There is no doubt that too little attention has been given
to the significanceof the relative signs of coupling constants
for structural problems.
On the other hand, these experimentally determined signs
have proved to be important test quantities for quantumchemical calculations of the scalar nuclear spin coupling.
H
Fig. 12. Schematic representation of an AB spectrum and energy-level
diagram (top); tickling experiment for the B, transition (bottom) and
energy-level diagram with transitions in the rotating frame.
This spin coupling takes place via the bonding electrons,
is an intrinsic phenomenon of the molecule, and is therefore
independent of nuclear magnetic resonance. It is thus
of direct interest to the theory of molecular structure.
Calculations have been carried out for (r and particularly for 7c electron systemst341.A comparison of the
results with experimentaI coupling constants and their
relative signs enables interesting conclusions to be drawn
regarding the validity of the underlying theories, the mechanisms of scalar spin coupling, and the nature of the chemI[ systems[36],and organoical bonding in 0
metallic compounds[3’I.
478
AB system (Fig. 12). Freeman and Anderson[38a1have
developed a theory that has been extended by Lippmaa
et aZ.[38b1,
and which can explain the phenomena observed
in tickling experiments. It is again advantageous to choose
a system of Cartesian coordinates that rotates around the
direction of the magnetic field H, ( z axis) with a frequency
a, (“rotating frame”).Ifthe transition B, (q-r) in Figure 12
is perturbed by H,, the line A, that is progressively connect-
p] Splitting into several lines is observed in degenerate systems [27];
for spin tickling with nuclei having I = l , cf. [39].
Angew. Chem. internal. Edit. 1 Vol. 10 (1971) / No. 7
ed to B, splits into a doublet, since the eigenstate q appears
in the two new states q' and r' as a result of mixing of the
states q and r. From the standpoint of the rotating system of
coordinates, the two A, lines can then be assigned to the
transitions p-q' and p-r'. The same is true of the transition A, that is regressively connected to B,. If o,is exactly
the same as o,,(B,), the two doublet lines A; and A;' have
the same intensity and appear at distances of +&H, from
A,. On the other hand, if o2differs somewhat from a,,,an
unsymmetrical doublet is observed. Since the additional
irradiation with H, is fixed on a certain line (kO.1Hz) in
tickling experiments, a field frequency stabilized spectrometer is required for this method. Corresponding experiments have been carried out under field-sweep conditions,
and give different
is to be expected for the vicinal cis and trans proton-phosphorus couplings. However, JZ; can be found only from
the hidden H, multiplet, the lines of which can be determined very accurately by tickling experiments (Fig. 13).
The coupling constant J;;=40.3 Hz obtained from the
four lines is greater than J&?=30.2
Hz in trivinylphosphane, whereas J$ in this compound is only 13.5 Hz. This
result proves the cis configuration of the protons in the
phosphane oxide (2)[431.
The spin tickling technique can also be used for the determination of the chemical shifts of low-intensity, heavy
nuclei. The chemical shifts of "%e in xenon
and of Io3Rhin triphenylphosphane-rhodiumc o m p l e x e ~ [ ~ ~ J
have been measured in this way.
However, this method of establishing the positions of
hidden or weak resonances is very laborious and timeconsuming if a very large frequency range must be scanned
in very small intervals ( ~ 0 . Hz)
2 with 0,. In such cases the
INDOR technique (a2sweep) is superior to spin tickling.
This technique is discussed in Section 3.4.2.
This double resonance method is excellently suited for the
determination of the exact frequencies of hidden or overlapping resonance lines, since the doublet splitting depends
very sensitively on the perturbing frequency 02[411.
A
9
Another important use of spin tickling is in the determination of the relative signs of coupling constants. Figure
14 shows the energy-level diagrams for a three-spin system
with the four possible combinations of signs for JAB,J,,,
and JBc.Reversal of the sign of a coupling constant leads
to interchange of the transitions of the two nuclei in question in the corresponding rectangles. This changes the
connection of the energy levels and hence the tickling pat-
typical application in the elucidation of structures is shown
in Figure 13. The problem here is to determine the configuration of the olefinic double bond in styryldiphenylphosphane oxide (2) from the 'H-NMR spectrum.
oc.
.
7176
7642
7315
1231
;
:I4 0 Hz
='9.3 HZ
l~H,franll=40.3HZ
Cal
J?," l g C r l
I
80
7.0
7.5
c-
I
6.5
6.0
Glpprn)
Fig. 13. 'H spectrum of styryltriphenylphosphaneoxide (2) (100MHz) and tickling experiments to determine the frequencies of the four hidden lines
of H, 1431.
[ a ] Calculated from spin tickling data.
Whereas the p vinyl proton is visible, the u vinyl proton
is completely masked by the phenyl resonances. Thus only
y HHi c - 14.0 Hz can be found from the 'H spectrum, while
G; cannot. The magnitude of J;
does not allow a definite
distinction between the cis and trans arrangements of the
vinyl protons[421 On the other hand, a greater difference
Angew. &hem. internat. Edit. / Vol. 10 (1971) 1 No. 7
tern. It has been shown in this way[461that the coupling constants of the geminal methylene protons in the u,P-epimino
CSHS-CH,,CHz
N
H
(3)
C&-CH,-/CH2
S
(4)
479
derivative (3) and in the u,p-epithio derivative (4) of
styrene have opposite signs.
Figure 15 illustrates the spectra of (3) and ( 4 ) and a comparison of the double resonance spectra on irradiation of
the A, transition. Whereas the lines B,, B, and C,, C , have
+ ++
strong irradiation at the methyl resonance, and one R line
was simultaneously perturbed by a weak field in a triple
resonance experiment (Fig. 16). The alternance of the signs
of the long-range H,H spin coupling constants in acridine
(6) have been demonstrated similarly by a triple resonance
analysis of I-methylacridine and 2-methyla~ridine[~'~.
This result confirms theoretical predictions concerning the
relative signs of proton-proton coupling in aromatic
++-
Since tickling experiments not only give the relative signs
of the coupling constants but also the nature of the connection (progressive or regressive) of the observed transition with the irradiated transition, double resonance experiments of this type are the best method for the construction
+--
---
Fig. 14. Energy-level diagrams and transitions for a three-spin system
( v A ~ v B % vwith
c ) the four combinations of signs of the coupling constants, I J A & > IJAcI > I JBA .
energy levels in common with A, in the imine (3), the
transitions that share energy levels with A, in the case of
the sulfide (4) are B,, B, and C,, C,. This is in agreement
with the energy-level diagrams (a) for the imine and (d) for
the sulfide, i. e. the coupling constants JBc=J,,, have
opposite signs in the two styrene derivatives. Since the
signs of all three couplings are the same for (3) and vicinal
H,H coupling constants are absolutely positive, the geminal
coupling in ( 4 ) must have a negative sign, as is observed
for cyclopropanes. The change in the sign of J,,, in threemembered rings is a particularly interesting case of the
structure dependence of the signs of nuclear spin coupling
constants.
Tickling experiments are particularly suitable for multiple
resonance studies. Thus the same sign was found for the
three coupling constants of the vinyl protons in 4-methyl-
o-benzoquinone (5) in one experiment[471.The six-spin
system (AKRX,) formed by the three ring protons and the
methyl group was first simplified to an AKR system by
7a:
J I , and
~ J3,0:negative[*]
'\
J z , and
~ J4,9:positive
4
(6)
[*] Based on a positive sign for Jorih0
480
Fig. 15. 'H spectra and tickling experiments on the side-chain protons
of (a) styrenimine ( 3 ) and (b) styrene sulfide ( 4 ) [46].
of complete energy level
Other important
uses occur in the identification of the components of subspectra in complicated multispin systems and in the analysis
of the spectra of oriented moleculest491.
The splitting into submultiplets of the lines connected to
the perturbed transition is always accompanied by a change
in the intensities of these lines. The population changes in
the levels of the perturbed transition increase the intensity
of the progressive transitions and reduce that of the
regressive transitionscz7].Intensity changes of this nature
always also occur in the double resonance experiments
discussed so far, since a weaker field H, is sufficient for this.
The condition for these pure population changes is
-yZH$TlT2
21, and the effect on the intensities is known as
the generalized Overhauser effect (GOE)l'ol.A wide range
of applications can be found for such effects.
Angew. Chem. internat. Edit. / VOI. I 0 (1971) / N o . 7
of certain energy levels as compared with the thermal
equilibrium populations. A quantitative treatment152a1of
the processes requires a precise knowledge of the (thermal)
relaxation mechanisms and times, which would have to
be determined for each spin system. The reader is referred
to a detailed
for a discussion of the physical
processes involved.A qualitative description of the situation
that is adequate for practical purposes has been presented
by Kaiserrs3],who also described the first practical applications of this effect. Under certain conditions regarding the
relaxation times, the following general rules can be established (cf. Fig. 17):
j i
H-5
I
1
Fig. 17. Connection of transitions in the energy level diagram: q-r
progressive and p u s regressive with respect to p-q.
is
1. Irradiation of a line (transition) influences only lines
that have an energy level in common with it.
7.0
[Ae25.161
6.9
6.5
6.2
Sipprn)
Fig. 16. ’H spectra of 4-methyl-o-benzoquinone ( 5 ) [47]; (a) normal
spectrum (CDCI,, 100 MHz); (b) double resonance spectrum with
irradiation of the methyl protons; (c) triple resonance spectrum with
irradiation of the methyl protons and tickling of the lowest-field line
(700.5 Hz) of H-5.
3.4. Overhauser Effects
In this section we shall discuss double resonance experiments that are based essentially on population changes in
the energy levels and which cause corresponding intensity
changes in the spectra. The Overhauser effect is really the
polarization of nuclei as a result of the saturation of the
electron spins in metals1s0*
’IJ. However, it has become
common practice to apply the same name to corresponding
intermolecular and intramolecular internuclear polarization effects based on the relaxation mechanism (dipolar
coupling). On the other hand, intensity changes observed in
double resonance for connected transitions in spin systems
with exclusively scalar coupling are collectively known as
the “generalized Overhauser effect”“ ‘I. Since this effect
follows directly on the last section, it is convenient to
discuss it here.
3.4.1. Generalized Overhauser Effect (GOE)
The first effect of this type was observed by Anderson and
in tickling experiments. These authors perturbed one line in theX, part o f t h e m , spectrum ofacetaldehyde and found a change in the relative intensities in the
quartet of the A part. These intensity changes occur because
the irradiation with H, causes a change in the populations
Angew. Chem. internat. Edit. / Vol. 10 (1971) 1 No. 7
2. Irradiation of a nondegenerate line reduces the intensity
of a regressive line and increases the intensity of a progressive line.
This Overhauser effect may also be regarded as a consequence of “spin pumping”. The information obtained
from such experiments is fundamentally the same as that
obtained from spin tickling, i. e. the relative signs of coupling constants, the assignment of transitions to certain
energy levels, and the location of hidden or weak resonances. This will be illustrated here by the normal spectrum and the double resonance spectrum of m-dinitrobenzene. This compound gives an AB,C spectrum (Fig. 18),
which can be described approximately as a first order
spectrum153! Very weak irradiation of the transition
23 increases the intensities of lines 8 and 10 and decreases
those of lines 7 and 13. This can be understood from the
energy level diagram (Fig. 19), and the signs of all four
coupling constants of the aromatic protons are found to
be the same. Other examples are the analysis of the AX,
system of acetaldehyde and the ABM,X system of trarisc r o t ~ n a l d e h y d e [and
~ ~ ~the
, determination of signs for 2,6-
dinitr~benzaldehyde~~~].
The intensity changes, which are sometimes very small,
must be detected by comparison with unperturbed lines.
However, measurement by the INDOR sweep technique
is recommended, since signal intensities are also influenced
by instrumental effects (e.g. momentary changes in the field
then be used above all to find the positions of hidden or
h~mogeneity)‘~~].
The generalized Overhauser effect can
weak resonance lines.
3.4.2. INDOR Spectroscopy
In the INDOR sweep technique, as was mentioned in
Section 2, the frequency w, of the perturbing field Hz is
48 1
varied over the entire spectral range, while the observation
field H I is used for the measurement of a monitor line (a,).
This method was first described and used by Bakerrss1.
The characteristic feature of such an experiment is that a
A
B
intensity change is plotted as a function of wz, and the
corresponding INDOR signals are positive for progressive
transitions and negative for regressive transitions. The
relative intensity changes due to the Overhauser effect are
C
- _ - - - _ _ _ _ - Z e r oline _ _ - _ _ _ _ _ _ _ _ _ _ _ _ _ - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
r z m q
Fig. 18. (a) 'H spectrum (AB,C) of rn-dinitrobenzene (56.4 MHz). The differences between the full and the dotted lines in the
splitting diagram would disappear in the limiting AM,X case. (b) Double resonance spectrum with weak irradiation of line 23;
lines 7 and 13 1 (regressive), lines 8 and 10 T (progressive). (c) Double resonance INDOR spectrum (a2sweep) with line 23 as
the monitor line: positive INDOR signals at the frequencies of lines 8 and 10, negative INDOR signals at the frequencies of
lines 7 and 13. Weak ZNDOR signals also appear at the frequencies of lines 12 and ! 5 because of partial overlapping of lines
23 and 24. Cf. [53, 101and energy level diagram in Fig. 19.
M
-2
11
-
0
d)
[ m i
I,=l
Fig. 19. Energy level diagram of the symmetrical states (IB= 1) for the
four-spin system of m-dinitrobenzene with an illustration of spin
pumping on irradiation of line 23 (Fig. 18) [lo, 531.
change in the intensity of the monitor line occurs whenever
% coincides with the frequency Of a line that has an energy
level in common with the monitor transition at a,.This
482
thus recorded as more easily recognizable, absolutely
positive or absolutely negative signals[*l.
INDOR experiments have been carried out both on heteronuclear and on homonuclear systems. However, applications so far have not been very numerous. According to a
~ ~ ~are
~ , concerned either with
review by K o ~ a l e w s k ithey
the determination of the chemical shifts of nuclei other
than protons or with the determination of signs of coupling constants. Heteronuclear 'H- { 31P}INDOR experiments have been successfully used for the analysis of mixtures of organophosphorus compounds[57].In the homonuclear version, the method is also excellently suited for
the accurate determination of the frequencies of hidden
resonances of a proton that exhibits scalar coupling with
the monitor proton. This double resonance technique has
great potential here for the elucidation of the structures of
mo1ecu1es'581.
The INDOR principle is illustrated in Figure 20 for a threespin AMX system. If a nondegenerate monitor line is
chosen (Al), two INDOR signals appear for each proton
p] The use of higher amplitudes of H,
leads to tickling effects that
result in an increase of the negative INDOR signals and a decrease
of the positive signals
Angew. Chem. internat. Edit. / Vol. 10 (1971) 1 No.7
coupled with A. However, if J,,=O and the monitor line
is degenerate (Al, A2), four signals are obtained for M,
but none for X. To obtain the entire INDOR signal of the
M nucleus, two experiments with different A lines are
signals are hidden in a practical case, therefore, not only
can the line frequencies of M and X be determined with
great accuracy (kO.1 Hz), but the coupling constants of
these nuclei can also be found.
/M\
necessary in the A-X
is sufficient in the A-M-X
The intensive application of the INDOR technique allows
the complete determination of the structure of a thermal
dimerization product (8) of 11,13-dioxo-12-methyl-l2aza[4.4.3]propellane (7)[58'.
case (Fig. 20b), whereas one
case (Fig. 20d). If the M andX
x
A
M
X
Fig. 20. Schematic spectra of an A M X system and energy-level diagram: (a) normal spectrum,
J,,>J,,>J,,>O;
(b) INDOR spectrum for case (a), monitor transition A,; (c) normal spectrum,
J,,>J,,>O,
J,,=O; (d) INDOR spectrum for case (c).monitor transitions A,, A,.
The 100 MHz and 220 MHz spectra of the dimer (Fig. 21)
are extremely complex, and conventional double resonance
methods cannot be applied. Only four of the sixteen ring
protons are resolved in the spectrum. Of these, the two
220 MHz
8
6
7
z
5
i
100 MHz
7
jA825.211
I
I
6
5
I
I
3
61ppml
2
1
Fig. 21. Proton spectra of the propellane dimer (8) at 100 MHz and
220 MHz (CDCI,) [58].
Angew. Chem. internal. Edit. f Vol. I0 (1971) / N o . 7
vinyl protons H - 4 and H-8' in particular may be used as
monitor protons. Figure 22 shows the determination of
the four lines of H-S', while Figure 23a illustrates the
detection of ten lines of the protons H-9', H-7', and H-10,
all of which are coupled with H-8', but are hidden in the
spectrum. INDOR lines can also be used as monitor lines
for consecutive INDOR experiments (Fig. 23 b), and this
allows the elucidation of proton sequences. The technique
has been applied for the same molecule, and Figure 24
illustrates that starting with only three observed protons,
eleven of the twelve protons of the skeleton are detected
in this way. The use of the INDOR method is not restricted
to first order spectra. Thus the chemical shift and coupling
parameters of the ABCX system of the four protons of the
dimer (8) have been established by determination of 32
INDOR frequencies and subsequent evaluation with the
aid of an iterative computer program. Other uses of 'H,'H
INDOR in structural studies have recently been publ i ~ h e d [ ~ ' - ~This technique has also proved useful in the
''.
483
'550
632.8
640.3
H-5'
pfiEKEj
599.0
'
Fig. 22. 100 MHz spectrum of the vinyl region of the dimer (8j and INDOR experiments for the determination of the four hidden lines of H-5'; frequencies in Hz [58].
al
H-8'
655 1
H-9'
H-1'
H-10'
Vinylic
Aliphatic
bl
I
/
H -9'
588.8
I
I
IaEzJ
H-10'
H-7'
Fig. 23. (a) INDOR spectra of H-9', H-7', and H - 1 0 of the dimer ( 8 ) , measured with a monitor line (655.1 Hz)
of H-8'. (b) Consecutive INDOR spectra of H-7' and H-10, measured with an INDOR line (588.8 Hz) of H-9'
as the monitor frequency [58].
analysis of the l9F spectra of threolerythro mixturesI6*1.
It may be expected that INDOR spectroscopy will find
wide use in the elucidation of structures.
The first applications, however, were in heteronuclear
double resonance, and made use of the high sensitivity of
the 'H or 19Fnuclei for the measurement of the resonances
484
of nuclei whose direct measurement is difficult because of
low natural abundance and small gyromagnetic constants
(i. e. poor sensitivity) and because of long relaxation
times[551.This group includes the nuclei 13C, 14N, 29Si,
and "P. The measurement of the carbon resonance in
organic compounds is particularly facilitated in this
way[41. 63.6%. 641. H owever, methods of this type have now
Angew. Chem. internat. Edit. J Vol. 10 (1971) / N o . 7
been displaced by Fourier spectroscopy, combined with
noise decoupling (see Sections 3.1 and 3.4.3).
Whereas the experiments described so far proceed under
quasi-stationary conditions, and the new populations for
the energy levels in question are established under the
influence of relaxation processes, INDOR experiments can
also be carried out under conditions that correspond to a
1 1
H-8 Z H-9
For the homonudear case (e.g. protons), Mp=MS“ and
ys=yl, and we obtain
P_
H -10
CH -2’
Fig. 24. Proton sequence for the dimeric propellane ( 8 j , deduced from
INDOR experiments. The arrows point from the monitor proton to
the INDOR proton. Protons that can also be observed directly in the
spectrum are marked with an asterisk.
fast adiabatic passage across the {XI resonance. The
“transient nutations”r651(Torrey oscillations1661)that then
occur in the monitor resonance, which is also saturated,
provide information similar to that obtainable from normal
INDOR signals (see Section 3.5.3).
3.4.3. Internuclear Overhauser Effects (NOE)
Apart from the chemical shifts and coupling constants, the
relaxation times T, and T, are the most important parameters in NMR spectroscopy, since they determine the
excess of nuclei that remain in the lower energy level when
the spin system absorbs energy from the rf field H I . For
protons,
where nr is the population excess in the lower level in the
presence of HI, and no is that in the absence of HI. Since
the signal intensity is proportional to the population excess
ns, the intensity of an NMR signal can be increased by
reducing or T, by means of external factors.
One of the mechanisms that allow proton relaxation in
liquids is the magnetic dipole-dipole interaction between
nuclei in different molecules, e. g. as a substrate-substrate
or substrate-medium interaction. The movement of the
molecules produces fluctuating dipole fields, with the
result that transitions are induced between Zeeman levels
--
Fig. 25. Energy-level diagram and transition probabilities for a twospin system with dipole-dipole interaction.
Angew. Chem. internat. Edit. 1 Vol. I0 (1971)
2
=
+(
W,+2W,+W2
MP = magnetization of I in the absence of H, ;
MI = magnetization of I on saturation of S by H,.
Calculations based on a pure dipole-dipole interaction
between I and S have shown that (wz- w,)/(wo+2wl + w2)
assumes the value i,i. e. that the maximum increase in the
intensity of the I resonance to be expected for protons
under these circumstances is 50%. The internuclear
Overhauser effect (NOE) was first observed by
for the signal of the chloroform proton in a mixture of
chloroform and cyclohexanewhen the cyclohexaneprotons
were saturated. The relaxation time TI of the CHCl,
protons can also be determined here as the time constant
of the build-up and breakdown of the additional magnetization, which agrees well with the value obtained from
the direct saturation of the CHCl, resonance. However,
the observed increase in intensity was only 34%. This
means that the chloroform protons are not relaxed exclusively via the cyclohexane protons, but that there are
also other relaxation paths (e.g. v i a the paramagnetic
oxygen in the solution) that contribute to the total transition probability (w2 +2w, + wo) and hence to the effective
relaxation time T I .
(7)
If there is only scalar coupling between I and S, the saturation of S causes more or less saturation also of the I-spins,
i. e. a decrease in the signal inten~ity~’~!
The two effects may
therefore be expected to be superimposed if a chemical
bond exists between the nuclear species.
However, the significance of the NOE to structural chemistry, which was recognized by Anet and Bournt711,
is that
it can be observed between protons of the same molecule
if these are close together in space. The contribution to the
longitudinal relaxation time T, of a nucleus I due to intramolecular dipole-dipole relaxation by a second nucleus S
is given by
+ 3
rn
of the nuclei‘671.We now consider a pair of nuclear spins
(1,s) that exhibits no scalar coupling and for which the
only relaxation path available is the internuclear dipoledipole mechanism.The transition probabilities corresponding to the change in the quantum number m (Am=O, *I,
+2) are denoted by wo, wlr and w2 (Fig. 25). If one of the
nuclear resonances ( S ) 15 murated by a strong perturbing
field H, (i.e. ni=O), the change in the longitudinal magnetization of I is given1681by
1 No. 7
(8)
485
where s is a correlation time for the molecular rotation
and d is the internuclear distance between I and S. The
intramolecular NOE should therefore depend very critically on the internuclear distance. It was soon found
that this effect is suitable for the study of molecular
structures, particularly for the solution of stereochemical
problems. This can be illustrated by the differentiation
between the two isomeric ethylideneazabicyclo[2.2.2]octanes (a) and ( b ) (Fig. 26). The compounds were in a
mixture and gave different chemical shifts for the methyl
and bridgehead protons. Saturation of one methyl resonance (principal component) increased the intensity of one
bridgehead proton signal by 31 % and had no effect on the
signal of the second bridgehead proton. Irradiation of the
methyl resonance of the minor component gave no Overhauser effect, so that the main component must have
structure (a) and the minor component structure (b) i721.
It has not been possible to achieve this differentiation with
certainty by any other spectroscopic method.
it
It wouid be possible in principle to use the NOE for the
measurement of relaxation times, correlation times, and
interatomic distances. Bell and Saunders[811have described
a correlation of a large number of NOE values with the
internuclear distances d, and have experimentally confirmed the d6 dependence required by the theory (Fig. 27).
Quantitative relaxation studies using the NOE have been
carried out on amides by Brownstein and Bystrot,@"'.
50!
,
26
2.7
28
29
log d [HI
30
31 32 3 3 3 6
36
-------z
Fig. 27. NOE observed in methyl compounds as a function of the internuclear distance CH,-H
(double logarithmic plot). The straight
line has a slope of - 6 [81].
Ho
-
Fig. 26. 'H spectrum of a mixture of the isomeric azabicyclo[2.2.2]octanes ( a ) and (6); A and B correspond to the methyl groups on C-9,
and X and Y to the C-4 methine protons. Saturation of the A resonance
produces a 31 % increase in the intensity of the X resonance and has
no effect on Y. Saturation of the B resonance has no effect on the X
and Y resonances, i. e. A and X may be assigned to the structure (a) [72].
Similar investigations have been carried out on innumerable
different
e. g. for alkaloidsi74,751, terpene~"~],
and penicillin derivativesi781,and the method
is extremely elegant for the solution of difficult stereochemical problems. The NOE has been used for the investigation of the preferred conformation of nucleotides
with respect to the N-ribose linkage (the syn-anti problem)1791.It must be noted, however, that great care is
necessary in the evaluation of NOE data in systems with
mobile conformations, since transfer of spin saturation
becomes possible if the time constant of the conformational
change is smaller than the relaxation time of the nucleusi8'?
In such cases it is necessary to use the temperature dependence of the NOE or transient methods. Negative as well as
positive NOE values are sometimes observed. It has been
shownc75]that alternating signs are to be expected when
dipole-dipole interaction is the dominant relaxation
mechanism in a series of neighboring protons.
A ............. B . . _ . _ . _ . _ .C. .__. _ . _ . . _ _D
_._.
saturated
NOE
NOE
NOE
positive
negarive
positive
486
Another important application of the Overhauser effect
lies in the signal enhancement of low-intensity nuclei.
The direct measurement of the I3C carbon resonances
is nowadays usually carried out under the conditions
of complete decoupling of the carbon nuclei from the
protons of the same molecule (noise decoupling; see
Section 3.1). Not only is the entire intensity thus concentrated in one line, but this is additionally enhanced by a
heteronuclear intramolecular NOE. The factor yJy, must
be taken into account in eq. (5) for heteronuclear systems,
the value of this factor for a {lH)-13Csystem being 3.976.
13C-(1H} noise decoupling thus leads to much greater
NOE values than in the homonuclear case. An enhancement of 200% has been observed in the I3C resonance
of H-13COOHi83a1. This effect can also be seen in
Figure 4, which shows the improvement of the signal/noise
ratio in the 13C spectrum of pyridine as a result of the
combination of decoupling and the Overhauser effect.
Carbon atoms that are not directly bonded to hydrogen
atoms give practically no NOE, so that the relative intensities of the I3C resonances often allow the differentiation
of tetrasubstituted from monosubstituted, disubstituted,
and trisubstituted C atoms. However, the factors that
determine the relative intensities of I3C signals are extremely diverse and are the subject ofintensive
The 'H, I3C spin coupling information is normally lost
in noise-decoupled spectra. However, if the perturbing field
for the protons is applied for only a short time, one can
obtain non-decoupled 13Cspectra that are still intensified
by the NOE[841.If the NOE is undesired for quantitative
evaluations of 13C spectra it may be eliminated by an
addition of paramagnetic
Angew. Chem. internat. Edit. / VoI. 10 (1971) 1 No. 7
3.5. Transient Double Resonance Experiments
eq. (12)and eq. (9) then gives TIAand T~ The time constants
T , ~ ,T ~ .and TIBare obtained in a similar manner from a
B-{A) experiment.
3.5.1. Double Resonance and Chemical Exchange
The NMR spectra of nuclei that are taking part in a chemical exchange, e. g. between the two centers A and B,
are influenced by this exchange if the average residence
times in A and B, and T ~ are
, so short that, they become
comparable with the transverse relaxation times TZAand
and TIB. This is the basis of line shape analysis, which
allows the study of kinetic processes by means of highresolution NMR spectroscopyr6]. Since
> T, in most
cases, the range of application of NMR spectroscopy can
be extended toward slower reactions by linking T~ and T~
with TIAand TIB.Hoffman and F o r ~ t n [have
~ ~ ]developed
a double resonance method for this purpose ;the method has
been applied to proton exchange reactions. When a proton
is transferred between the centers A and B, the effective
lifetime of a spin state in a given location (e.9. H bonded
to A) is determined by the new time constant T ~ ~ :
1
1
TIA
TI,
_ = _
1
f -
?A
Figure 28 shows the exponential decrease and increase in
the OH signal intensity of ( 9 ) when the strong perturbing
field for the O H resonance of (10) is switched on and off.
The evaluation of the decisive time interval (a few seconds)
and the measurement of the intensities of HA without the
perturbing field H, and after the prolonged action of H,
(stationary case) gave ~,=11.53, rB=2.17, TlA=5.43, and
TlB=3.70s. The time constants T~~ and T , are
~ 3.75 and
1.33 s respectively.
a)
c
1
(9)
The change in the z magnetization of the proton in A is
then given1861by
Strong additional irradiation of the proton in B causes
saturation of its resonance (M?=O), and solution of eq. (10)
then gives
In the steady state (t==
m), the ratio of the magnetization
of H in A with and without double resonance is
which is the ratio of the signal intensities of HA. T ~ TIA,
~
<
i. e. comparable with TIA,then leads to a decrease in the
intensity of HA, which means a transfer of saturation from
HB to HA by chemical exchange. The time constant ?,,can
beobtained by asemilogarithmicplot ofM$-M$(.r,JT,,)
6” 0
6
against t after T ~ J has
T been
~ ~ found from the stationary
double resonance experiment (eq. (12)). Combination with
Angew. Chem. internal. Edit. / Vol. 10 11971) 1 No. 7
This method can be used for the quantitative investigation
of the intermolecular exchange of the O H protons in a
5.7: 1 mixture of salicylaldehyde ( 9 ) and 2-hydroxyacetophenone (10)[851.
b,
1
t
jA825.281
Fig. 28. (a) Decrease and increase in the OH signal intensity of salicylaldehyde ( 9 ) when the saturation field H, for 2-hydroxyacetophenone
( l o ) is switched on (1) and off (T) in a mixture of the two compounds.
(b) Analogous experiment for the OH resonance of i10). The scale
interval is one second [92].
Other applications of the relaxation method have now
been reported. It was possible in this way to extend the
NMR kinetics of the ring inversion in undecadeuteriocyclohexane to the temperature range -97 to -137°C and T~
values of 3.8 to 236 s[16c1. The energy barrier for internal
rotation in the cyclopropyldimethylcarbonium ion ( 1 1J,
has also been determined1871.
The method can also be extended to exchange processes
between three non-equivalent centers. Thus the average
residence times of the protons at the oxygen, methylene,
and vinyl centers in the keto-en01 equilibrium of acetylacetone have been determined in this
487
The residence times in this case are again so long (T* = 14.0,
T ~ 14.2,
=
~ , = 3 . 8 s) that the chemical exchange has no
effect on the normal NMR spectrum at room temperature.
The saturation transfer technique can also be used qualitatively to detect the presence of exchange processes.
The conformational mobility of [18]-annulene at room
temperature was detected in this way, since saturation of
the “inner” vinyl protons results in the disappearance of
the resonance of the “outer” vinyl protons[*’! The same
principle forms the basis of an experiment in which the
OH resonance in phenols is detected by irradiation of the
water signal and transfer of saturation to the OH protons[*’!
The phenomenon can be described in general form in the
density-matrix formalism[89a1.In spin systems with scalar
coupling, the saturation transferred by chemical exchange
is transferred further to adjacent coupled nuclei, i. e. a
negative Overhauser effect (NOE) is observed[**. 911.
This is due to the fact that for scalar coupling, wo> w, in
eq. (6), and hence MJMP <Or701.
The sudden spin pumping initiates positive or negative
“Torrey oscillations” in the monitor resonance, which
allow the differentiation of regressive and progressive
transitions (Fig. 29)[651.This technique also allows the
detection of weak lines, e.g. combination lines, and has
been used for the indirect measurement of the 13C resoAs a transient method, it is less influenced by
relaxation effects than normal stationary Overhauser
experiments, and gives results similar to those obtained
by the TSI technique.
a1
”3
bl
3.5.2. Transitory Selective Irradiation (TSI)
In the use of the generalized Overhauser effect to deduce
the energy-level diagram of a multispin system, the changes
in population should preferably be confined to the levels
of the irradiated line. Only then do Overhauser effects
occur exclusively in “directly connected” lines. In the
stationary experiment, the perturbing field H, must act
for a time Ti corresponding to the reciprocal line width
of the irradiated transition. The optimum observation
time (t)for the intensity changes after H , is switched on is
T:e
t Q
Ti
where TI may be regarded as the shortest reciprocal thermal
transition probability Tp4 (spin-lattice relaxation time).
Maximum intensity changes are observed when the
perturbing field H, traverses the line in an adiabatic fast
passage. Such TSI experimentsry2]give particularly clear
Overhauser effects, since relaxation losses are minimized
and population changes occur only in the levels of the
perturbed line.
3.5.3. Transient Nutations (Torrey Oscillations)
This double resonance method[6s1again allows the construction of the energy-level diagram, i. e. the determination
of the relative signs of coupling constants. The phenomenological picture is simiIar to that in INDOR spectra,
but the monitor line is also saturated by an intense field
H,. Whenever H, meets a line that has an energy level in
common with the monitor line, a sudden transfer of population occurs to the common energy level, since the populations of the levels are reversed in the perturbed line‘661.
488
475
tn825.291
410
405
400
395
390
Fig. 29. (b) ’H spectrum of2-chlorothiophene (60 MHz) [ 6 5 ] . “Transient
nutations” obtained (a) in the monitor line 3 and (c) in the monitor line
13. Signals for which the initial deflection is positive indicate progressive
transitions, and negative signals indicate regressive transitions. (d) and
(e) illustrate Torrey oscillations that correspond to very weak transitions, and which are obtained by increasing the amplitude of H,.
3.5.4. Double Resonance and Fourier Transform
Spectroscopy
When a nuclear spin system is stimulated by an intense rf
pulse of short duration, all the spectral information is
contained in a time-dependent signal that describes the
decrease in the transverse magnetization. This signal can
be converted into the conventional spectrum by Fourier
tran~formation~’~~.
The technique requires only very short
“exposures” of the order of 10-100 ps, and so allows the
study of population changes produced by double resonance
(Overhauser effects) and their relaxation processes. Since
the H i pulse need only be applied after the perturbing field
H, has been switched off, the Overhauser effects can be
observed in this way without additional decoupling or
tickling effects[y41.
The importance of the use of the Fourier
transform technique in double resonance experiments will
undoubtedly increase in the future. It can be expected that
Fourier-transformed 13C-{’HI pulse spectra in particular
Angew. Chem. internat. Edit. 1 Vol. 10 (1971) No. 7
Hz
will find a wide field of application^^^^]. Besides the 13C
chemical shifts, spin-lattice relaxation times ('T,). of the
individual carbon atoms also may be determined with
this technique[96! In this way a new parameter for structural studies becomes accessible.
4. Closing Remarks
Nuclear magnetic double resonance has developed during
the past few years into a versatile research field. Whereas
some of the techniques described here are already in
routine use in commercial instruments, others require
special instrumental arrangements. With the development
of more versatile NMR spectrometers that are not designed
only for the measurement of protons, more use will be
made of heteronuclear double resonance experiments. This
may be expected to lead in particular to rapid advances in
carbon resonance, which holds a new dimension for organic
chemistry. The broad spectrum of applications of double
resonance to structural and spectroscopic problems has
been illustrated in this article by a number of examples to
provide the reader with a picture of the present situation.
New methods and new applications may be foreseen for
the future.
I am grateful to Dr. W Regel, Dr. 0.Sciacovelli, Dip1.-Chem.
R. Hollenstein, R . Wagner, and K . Hochreutener for their
assistance and experimental skill and to Dr. ?: Winkler for
the critical examination of the manuscript.
Received: December 10,1970 [A 825 IE]
German version: Angew. Chem. 83,470 (1971)
Translated by Express Translation Service, London
[I]K . H . Hausser, Angew. Chem. 68, 729 (1956).
[2] J . D. Roberts, Angew. Chem. 75,20 (1963); Angew. Chem. internat.
Edit. 2, 53 (1963).
[3] 8. Dischler, Angew. Chem. 78. 653 (1966); Angew. Chem. internal.
Edit. 5, 623 (1966).
[4] A . Saupe, Angew. Chem. 80. 99 (1968); Angew. Chem. internat.
Edit. 7, 97 (1968).
[5] H . J . Keller and K . E. Schwarzhans, Angew. Chem. 82, 227 (1970);
Angew. Chem. internat. Edit. 9,196 (1970); K . E. Schwarzhans, Angew.
Chem. 82,975 (1970); Angew. Chem. internal. Edit. 9, 946 (1970).
[6] H . Kessler, Angew. Chem. 82, 237 (1970); Angew. Chem. internal.
Edit. 9, 219 (1970).
[7] !T Royden, Phys. Rev. 96, 543 (1954).
[8] J . D. Baldeschwieler and E. W Randall, Chem. Rev. 63, 81 (1963).
[9] W McFarlane, Annu. Rev. NMR Spectrosc. I , 135 (1968).
[lo] R . A . Hofftnan and S . Forsin, Progr. Nucl Magn. Resonance
Spectrosc. I , 15 (1966).
1111 A . L. Bloom and J . N . Shoolery, Phys. Rev. 97, 1261 (1955); J . N
Shoolery, Discuss. Faraday SOC.19,215 (1955); R A . Ogg and J . D. Ray,
ibid. 19, 239 (1955).
[I21 L. H . Piette, J . D. R a y , and R . A . Ogg, J. Mol. Spectrosc. 2, 66
(1958); J . A . Happe, J. Phys. Chem. 65, 72 (1960); J . D. Baldeschwieler,
J . Chem. Phys. 36, 152 (1962).
[I31 W A . Anderson, Phys. Rev. 102, 151 (1956); J . l t o h and S. Sato,
J. Phys. SOC.Japan 14, 851 (1959); R . Kaiser, Rev. Sci. Instrum. 31,
963 (1960); R . Freeman, Mol. Phys. 3, 435 (1960); J . N. Shoolery,
Discuss. Faraday SOC. 34, 104 (1962); G. Albers-SchGnberg, W v.
Philipsborn, L. M . Jackman, and H . Schmid, Helv. Chim. Acta 45, 1406
(1962); D. W Turner, J . Chem. Soc. 1962,847.
[I41 A . Charles and W McFarlane, Mol. Phys. 14, 299 (1968); G. J .
Long and A . G . Moritz, ibid. 15, 439 (1968); R . Burton and L. D. Hall,
Can. J. Chem. 48, 59 (1970).
[ I S ] F. A . L. Anet and J . S . Hartman, J . Arner. Chem. SOC.85, 1204
(1963).
Angew. Chem. internat. E d i f . / Vol. 10 (1971) / N o . 7
[I61 a) F. A . L. Anet, M . Ahmad, and L. D Hall, Proc. Chem. Soc.
(London) 1964.145; b) F . A . Bocey, F . P . Hood 111, E . W Aniferson,and
R . L. Kornegay,ibid. 1964.146: J . Chem. Phys.41.2041 (1964),c) F. A . L.
Aner and A . J . R . Bourn. J . Amer. Chem. SOC.89. 760 (1967).
[I71 E. W Garbisch, j r . and M . G. Gryjith, J. Amer. Chem. SOC.90.
6543 (1968).
[ I S ] G. F. Katekar and A . G. Moritr, Austral. J . Chem. 22,1199 (1969);
E . Rahkamaa, Z. Naturforsch. 24a, 2004 (1969); H . Fukui. S. Shimokawa,
and J . Sohma, Mol. Phys. 18, 217 (1970).
[I91 J . D.Baldeschwieler and E. W Randall, Proc. Chem. SOC.(London)
1961, 303; J . B. Merry and J . H .Goldstein, J. Amer. Chem. SOC.88,
5560 (1966); S. Castellano, C. Sun, and R . Kostelnik, J. Chem. Phys. 46,
327 (1967).
[20] J . N . Shoolery, Discuss. Faraday SOC.19,215 (1955); R . Schaeffer,
J . N . Shoolery, and R . Jones, I. Amer. Chem. SOC.79, 4606 (1957);
H . H . Lindner and 7: Onak, ibid. 88, 1890 (1966).
[21] G. D. Vickers, H . Agahigian, E. A. Pier, and H . Schroeder, Inorg.
Chem. 4,693 (1966); J . A . Potenza, W N . Lipscomb, G . D. Vickers, and
H . Schroeder, J. Amer. Chem. SOC.88,628 (1966).
1221 R . R . Ernst and H.Primas, Helv. Phys. Acta 36, 583 (1963); R . R.
Ernst, J. Chem. Phys. 45, 3845 (1966); Mol. Phys. 16, 241 (1969);
R . Burton and L. H . Hall, Can. J. Chem. 48, 59, 2438 (1970).
[23] W u. Philipsborn and 0.Sciacovelli, unpublished.
[24] A . Guggisberg, M . Hesse, W u. Philipsborn, K . Nagarajan, and
H . Schmid, Helv. Chim. Acta 49, 2321 (1966).
[25] L. F . Johnson, Varian Assoc. Techn. Inform. Bull., Summer 1965.
p. 5.
[26] F. Bloch and A . Siegert, Phys. Rev. 57,522 (1940).
[27] M! A . Anderson and R . Freeman, J. Chem. Phys. 37, 85 (1962).
[28] J . P. Maher and D. F. Evans, Proc. Chem. SOC.(London) 1961,208.
[29] R . Freeman and D. H. Whlffen, Mol. Phys. 4, 321 (1961).
[30] D. F. Evans, Mol. Phys. 5, 183 (1962); Discuss. Faraday SOC. 34,
139 (1962); S. L. Manatt and D. D. Elleman, J . Amer. Chem. SOC.84,
1305 (1962).
[31]
W Regef and W
li.
Philipsborn, Helv. Chim. Acta 52, 1354 (1969).
[32] P . C. Lauterbur and R . J . Kurland, J . Amer. Chem. SOC.84, 3405
(1962).
[33] H . Giinther, Z. Naturforsch. 24b, 680 (1969).
[34] M . Barfield and D. M . Grant, Advan. Magn. Resonance I , 149
(1965); M . Barfield and B. Chakrabarti, Chem. Rev. 69, 757 (1969).
[35] J . A . Pople and D. P. Santry, Mol. Phys. 8, 1 (1964); J . A . Pople,
J . W Mcluer, and N . S . Ostlund, J . Chem. Phys. 49, 2965 (1968); G. E.
Maciel, J . W Mclver, N . S. Ostlund, and J . A . Pople, J. Amer. Chem. Soc.
92, 1, 11, 4151 (1970); M . Barfield, J. Chem. Phys. 41, 3825 (1964); 44,
1836 (1966); 46, 811 (1967).
[36] H . M . McConnell, J . Chem. Phys. 24, 460 (1956); 30, 126 (1959);
J. Moi. Spectrosc. 1, I 1 (1957); M . Karplus, J . Chem. Phys. 33, 1842
(1960); J . V Acriuos, Mol. Phys. 5 , l (1962); M . Barfield, J . Chem. Phys.
41, 3825 (1964); 48, 4458, 4463 (1968); G . E. Maciel, J . W Mcluer.
N.S. Ostlund, and J . A . Pople, J. Amer. Chem. SOC.92,4497,4506 (1970).
[37] H . Dreeskamp, Z. Naturforsch. 19a, 139 (1964); !I Breuninger,
H . Dreeskamp, and G . Pfisterer, Ber. Bunsenges. Phys. Chem. 70, 613
(1966); H . Dreeskamp, H . Elser, and C. Schumann, ibid. 70, 751 (1966):
H . Dreeskamp and C . Sregmeier, Z. Naturforsch. 22a, 1458 (1967);
H . Elser and H . Dreeskamp, Ber. Bunsenges. Phys. Chem. 73,619 (1969);
G. Pfisterer and H . Dreeskamp, ibid. 73, 654 (1969): C. Schumann and
H.Dreeskamp, J. Magn. Res. 3, 204 (1970).
[38] a) R . Freeman and W A . Anderson, J . Chem. Phys. 37,2053 (1962);
b) E. Lippmaa and S . Rodmar, ibid. 50, 2764 (1969); V Sinicee, Mol.
Phys. 17, 41 (1969).
[39] W McFarlane and D. W W h w e n , Mol. Phys. 17, 603 (1969).
[40] W A . Anderson, R. Freeman, and C. A . Reilly, J . Chem. Phys. 39,
1518 (1963).
[41] R . Freeman and W A . Anderson, J. Chem. Phys. 39, 806 (1963).
[42] L. M . Jackman and S . Sternhell: Applications of Nuclear Magnetic
Resonance Spectroscopy in Organic Chemistry, 2nd Edit. Pergamon
Press, Oxford 1969, p. 301.
[43] L. F. Johnson, Varian Assoc. Techn. Inform. Bull., Summer 1965,
p. 6.
[44] 7: H . Brown, E. B. Whipple, and P . H . Verdier, Science 140, 178
(1963); J. Chem. Phys. 38, 3092 (1963).
[45] 7: H . Brown and P. J . Green, J . Amer. Chem. SOC.91, 3378 (1969).
[46] S. L. Manatt, D. D. Elleman, and S. J . Brois, J . Amer. Chem. SOC.
87, 2220 (1965).
[47] R . Hollenstein and M! u. Philipsborn, unpublished.
[48] 0. Sciacovelli and W v. Philipsborn, Org. Magn. Res., in press.
489
[49] B. M . Fung and M . J . Gerace, J. Chem. Phys. 53,1171 (1970).
[SO] A. Ocerhauser, Phys. Rev. 89,689 (1953); 92,411 (1953).
[51] A. Abragam: The Principles of Nuclear Magnetism. Oxford University Press, Oxford 1961, Ch. 9.
1521 a) A. Kumar and S. L. Gordon, J. Chem. Phys. 54, 3207 (1971);
h) R. H. Webb, Amer. J. Phys. 29,428 (1961).
[53] R. Kaiser, J. Chem. Phys. 39, 2435 (1963).
[54] C . L. Bell, S . S. Danyluk, and 7: Schaefer, Can. J. Chem. 47, 3529
(1969).
[55] E. B. Baker, 3. Chem. Phys. 37,911 (1962); E. B. Baker and L . W
Burd, Rev. Sci. Instrum. 34, 238 (1963); E. B. Baker, L. W Burd, and
C . !I Root, ibid. 34, 243 (1963).
[56] V: J . Kowalewski, Progr. Nucl. Magn. Resonance Spectrosc. 5,
l ( 1 9 6 9 ) ; J. Mol. Spectrosc. 30, 531 (1969).
[57] R. Kosfeld, G. Hiigefe, and W Kuchen, Angew. Chem. 80, 794
(1968); Angew. Chem. internat. Edit. 7, 814 (1968).
[5S] 0. Sciacovelli, W D. Philipsborn, C. Amith, and D. Ginsburg, Tetrahedron 26,4589 (1970).
1591 F. W can Deursen and A. C. Udding, Recl. Trav. Chim. Pays-Bas
87, 1234 (1968); F. W van Deursen, Org. Magn. Res. 3, 221 (1971).
[60] D. H . R. Barton, P. N . Jenkins, R . Letcher, and D. A. Widdowson,
Chem. Commun. 1970, 391.
[61] R. Burton, L . D. Hall, and P. R. Steiner, Can. J. Chem. 48, 2679
(1970); G. W M . Grant and L. D. Half, ibid. 48, 3537 (1970).
[62] E. E . Baker, J. Chem. Phys. 45,609 (1966).
[63] F. W Wehrli and W Simon, Helv. Chim. Acta 52, 1749 (1969).
[63a] R. Burton, L. D. Hall, and P. R. Steiner, Can. J. Chem. 49, 588
(1971).
[64] D. Ziessow and E. Lippert, J. Mol. Struct. 1968,248; Ber. Bunsenges. Phys. Chem. 74, 335, 568 (1970).
[65] J . A. Feretti and R. Freeman, J. Chem. Phys. 44, 2054 (1966).
[66] H. C. Torrey, Phys. Rev. 76, 1059 (1949).
[67] H . C. Torrey, Phys. Rev. 92,962 (1953).
[68] I. So(omon, Phys. Rev. 99, 559 (1955).
[69] R. Kaiser, J. Chem. Phys. 42, 1838 (1965).
[70] 1. Solomon and N . Bloembergen, J. Chem. Phys. 25,261 (1956).
[71] F. A. L . Anet and A . J . R . Bourn, J. Amer. Chem. SOC.87, 5250
(1965).
[72] J . C . Nouls, G. Dan Binst, and R. H. Martin, Tetrahedron Lett.
1967,4065.
[73] G. Moreau, Bull. SOC.Chim. Fr. 1969, 1770
[74] J . C . Nouls, P. Wollast,J . C . Braekman, G. van Binst, J . Pecher, and
R H. Martin, Tetrahedron Lett. 1968, 2731 ; G. R. Newkome and N . S .
Bliacra, Chem. Commun. 1969,385.
[75] R . A . Bell and J . K . Saunders, Can. J. Chem. 46, 3421 (1968).
[76] A. R. Battersby, E. S. Hall, and R . Southgate, J. Chem. SOC.C 1969,
721 ; Y. Ishizaki, Y. Tanahashi, T Takahashi, and K . Tori, Chem. Commun.
1969, 551,; H . Hikino, C . Konno, T Takemoto, K . Tori, M . Ohtsuru. and
1. Horibe, ibid. 1969, 662; S. Ito, M . Kodama, M . Sunagawa, H . Hohma,
Y. Hayashi, S . Takahashi, H. Ona, ?: Sakan, and 7: Takahashi, Tetrahedron Lett. 1969,2951.
[77] J . Lugtenburg and E . Hazinga, Tetrahedron Lett. 1969,2391.
[78] R. D. G. Cooper, P. V: De Marco, J . C. Cheng, and N . D. Jones,
J. Amer. Chem. SOC.91, 1408 (1969).
[79] P. A. Hart and J . P. Davis, J. Amer. Chem. SOC.91, 512 (1969);
93, 753 (1971); Biochem. Biophys. Res. Commun. 34, 733 (1969);
R . E. Schirmer, J . H . Noggle, J . P. Davis, and P. A. Hart, J. Amer. Chem.
SOC.92, 3266 (1970).
[SO] J . K . Saunders and R . A. Bell, Can. J. Chem. 48,512 (1970);S . Combrisson, B. R o p e s , P. Rignj, and J . J . Basselier, ibid. 49, 904 (1971).
[81] R . A . Bell and J . K . Saunders, Can. J. Chem. 48, 1114 (1970).
[82] S. Brownsrein and V: Bystrov, Can. J. Chem. 48, 243 (1970).
[83] a) K . F. Kuhlmann and D. M . Grant, J. Amer. Chem. SOC.90,7355
(1968); b) K . F. Kuhlmann, D. M . Grant, and R. K . Harris, J. Chem.
Phys. 52, 3439 (1970).
[84] J . Feeney, D. Shaw, and P. J . S . Pauwels, Chem. Commun. 1970,
554.
[84a] G. N . La Mar, J. Amer. Chem. SOC.93, 1040 (1971).
[85] S . Forshn and R. A . Hoffman, Acta Chem. Scand. 17,1787 (1963);
J. Chem. Phys. 39, 2892 (1963); 40, 1189 (1964).
[86] H . M . McConnell, J. Chem. Phys. 28, 430 (1958).
[87] D. S. Kabakoff and E. Namanwirth, J. Amer. Chem. SOC., 92,
3234 (1970).
[88] 1. C. Calder, P. J . Garrat, and F. Sondheimer, Chem. Commun.
1967, 41.
[89] J . Feeney and A . Heinrich, Chem. Commun. 1966, 295.
[89a] P. P. Yang and S . L. Gordon, J. Chem. Phys. 54,1779 (1971).
[90] B. M . Fung, 3. Chem. Phys. 47, 1405' (1967); 49, 2973 (1968);
8.M . Fung and R. D.Stolow, Chem. Commun. 1967,257; B. M . Fung,
J. Amer. Chsm. SOC.90, 219 (1968); 91, 2811 (1969); B. M . Fung and
P. L. Olymytu, Mol. Phys. 19, 685 (1970).
[91] I . C. Calder, P. J . Garatt, H. C Longuet-Higgins, F. Sondheimer,
and R. Wolowsky, J. Chem. SOC.C 1967,1041.
[92] R. A. Hoffman, B. Gestblom, and S . ForsPn, J. Chem. Phys. 39,
486 (1963);40, 3734 (1964);J. Mol. Spectrosc. 13, 221 (1964).
[93] R. R. Ernst and W A . Anderson, Rev. Sci. Instrum. 37, 93 (1966).
[94] R. Freeman, J. Chem. Phys. 53,457 (1970).
[95] W Bremser, H. D. PV Hill, and R . Freeman, Messtechnik 79, 14
(1971).
[96] R. Freeman and H . D. W Hill, J. Chem. Phys. 51, 3140 (1969);
53, 4103 (1970);54, 3367 (1971).
C 0 M MU N I C AT1 0N S
Crystal and Molecular Structure of Diethyl
2,4-Bis(diethylamino)cyclobutadiene-1,3dicarboxylate[**'
Crystal data: yellow needles, m.p. 50-52°C. M = 338.2,
monoclinic ;lattice c o n s t p s :a= 17.62,b = 13.81,~
=9.76&
p=125.3", U=1954.2 A3, Z=4, D,,,,,=1.1495 g/cm3.
By Hans Jorg Lindner and Brigitte Don Gross[*]
We have carried out an X-ray structure analysis to clarify
the molecular structure of diethyl 2,4-bis(diethylamino)cyclobutadiene-1,3-dicarboxylate
which should
provide further insight into the n-electron system of this
compound. For these measurements single crystals were
sealed under nitrogen in capillaries[*!
[*I Dr. H. J. Lindner and B. von Gross
Institut fur Organische Chemie der Techntschen Hochschuie
61 Darmstadt, Schlossgartenstrasse 2 (Germany)
[ * * I Thls work was supported by the Deutsche Forschungsgemexnschaft.
490
Space group P2,ln. The intensities of 2715 symmetryindependent reflections from layers 0.. . I 4kl were measured
with an automatic Weissenberg diffractorneter with nickelfiltered Cu-K, radiation; of these, 986 reflections were
so weak that they could not be used for determination
of structure.
Angew. Chem. internat. Edit. 1 Vol. 10 (1971)
1 No. 7
Документ
Категория
Без категории
Просмотров
0
Размер файла
1 656 Кб
Теги
nuclear, application, magnetic, method, double, resonance
1/--страниц
Пожаловаться на содержимое документа