вход по аккаунту


Effects of 35Cl37Cl 13C Residual Dipolar Coupling on the Variable-Temperature 13C CPMAS NMR Spectra of Solid Chlorinated Sodium Acetates.

код для вставкиСкачать
Effects of 35C1/37CI,I3C Residual Dipolar
Coupling on the Variable-Temperature
I3C CP/MAS NMR Spectra of
Solid, Chlorinated Sodium Acetates **
Sergio H. Alarcon, Alejandro C. Olivieri,"
Stephen A . Carss, and Robin K. Harris
The effects of combined residual dipolar and scalar coupling
between spin 1,'2 nuclei and a large variety of quadrupolar nuclei have been recently reviewed."] Interest from organic
chemists in this phenomenon has been usually restricted to the
( 14N. I3C) case.[21whose manifestation has almost disappeared
from the literature due to the use of high-field N M R spectrometers.['] Very recently, however, 13C CPiMAS N M R spectra
(CP = cross polarization, MAS = magic angle spinning) of
solid chlorine-containing organic compounds have been shown
to display splittings arising from residual coupling (mostly dipolar in nature) between the observed spin 1;'2 13C nucleus and
the 35.37Clisotopes (S = 3;2 in both cases) at bonding distan~es.1~1
In all cases reported so far. carbon atoms bonded to :I
single chlorine atom were studied.131We report here on the effects of residual dipolar (35Cl;37C1.13C) coupling at both bonding and nonbonding distances in compounds containing CH,CI,
CHCI,, and CCI, groups.
Detailed theory has been produced on this subject, including
a fully computational solution of the (spin 112,spin 3i2) dipolar
coupling under MAS conditions,14. first-order perturbation
theory (valid when lx/vsl < 1 ; x is the quadrupole coupling constant and vs is the resonance frequency for the quadrupolar S
nuclei).r2*'. 61 and a polynomial fit to the exact solution (valid
for 1 < Ixivsl < 3).['] According to first-order perturbation theory,[61the '3C CP/MAS N M R signal of a carbon atom affected
by a single chlorine nucleus at a distance I' should give a symmetric doublet with a splitting .s = 6D,(3cos2,' -1)/2Ov, [ D =
BD is the angle between the vector r and the
zEbGaxis of the quadrupole tensor at the 3s.37C1]. Since 1 is
substantial for the 3s.37CIisotope bonded to the carbon nucleus. deviations are expected from the first-order regime even at
high fields. in which case Equation (a) can be used (in the
range 1 < l ~ / v s l < 3).15] where .Y is still a symmetric splitting. It
should be noted that the 3'C1 and 37C1 isotopes have similar
nuclear properties. and their separate effects are thus difficult to
detect. This means that the calculated effect on the observed
[*] Prof A. C . Olivieri. S. H. Alarcon
Departamento de Quimica Analitica
Facultad de Ciencias Bioquimicas y Farniaceuticas
Universidad Nacional de Roaario
Suipacha 531. Rosario (2000) (Argentina)
Telerax. Int. code +(41) 395980
carbon nucleus should be the weighted average of those corresponding to the individual isotopes.
Values of ; c ( ~ ~ Cfor
I ) chlorinated organic compounds usually
lie in the vicinity of -70 MHz."] so that j ~ / i ' ~isI about 2.4 at
7.05 T [and thus Eq. (a) can be used] and 3.6 at 4.7 T. Therefore.
in this latter case one has to resort to exact calculations, which
predict a broad triplet with intensities in the ratio 1 : 1 : 2 (in the
direction of increasing frequencies; the intrinsic asymmetry of
these triplets depends on the sign of x ) . ['I These results have
been experimentally confirmed in organic compounds for carbon atoms directly bonded to a single chlorine.131The l 3 C N M R
signal resulting from coupling to two and three equivalent chlorine atoms is expected to be a 1 :2: 1 triplet and a 1 :3: 3 : 1 quartet, respectively, at high fields (Ix/vsl < 3). whereas more complex line shapes are predicted at lower fields. Figure 1 shows the
This urork was supported by the University of Rosario. Conaejo Nacional de
Investigaciones Cientiticas y Tecnicas. Argentina, (CONICET) and Fundacion
Antorchas (A.C 0..S H . A ) We thank the U.K. Science and Engineering Research Council (SERC) for access to the Varian VXR 300 spectrometer under
the National Solid-State NMR Service arrangements. and for provision of the
Chemagnetics CMX 200 system (rcuearch grant GR;H96096). We are grateful
to R. Challoner. B. J Say, and D. C. Apperley for assiatance with spectrometer
operation. S.A.C. thanks the University of Durham for a Research Studentship
- _---
- 6
Fig. 1. "C CP:MAS NMR spectra for the compounds 1-3 in the solid state. a)
Spectrum of 1 at ambient probe temperature, B, =7.05 T. '("C) =75.4 MHr. h)
Spectrum of 1 at ambient probe temperature, B, = 4.7 T. v("CC) = 50.3 MHz. c)
Spectrum of 2 at 158 K (nominal temperature), E , =7.05 T. d) Spectrum of 3 at
ambient probe temperature. B, =7.05 T. The low-temperature spectra (down to
163 K ) of 3 were essentially identical to that shown in this figure. The inserts in
spectra a) and b) show expansions of the carboxyl region Asteritks denote spinning
aide bands.
experimental results obtained for sodium monochloro-(l),
dichloro-(2), and trichloroacetate (3); the relevant chemical
shift information is presented in Table 1. At ambient probe temperature, compound 1 gives. for both the carbon of the CHzCl
group and for that of the carboxyl group, the expected 1 : l
doublets at 7.05 T and 1:1:2 triplets at 4.7 T. Using
Table I . Chemical shifts observed for 1-3 in the "C MAS NMR spectra.
6 values
7.05 T
S. A . Carss. Prof. R. K . Harria
Department of Chemistry
Universitk of Durham
South Road. Durham DH13LE ( U K )
4.1 T
CI [b]
C2 [b]
175.7 [a]
42.2 [a]
174.0 [c]
177 7
61.8 [d]
4 0)
= 0.4)
[a] 1 - 1 Doublets. [b] 1 :1 : 2 Triplets. [c] Shoulder of the signal at 6 = 175.7.
[d] 1 . 2 . 1 Triplet.
l ) - 53.4 MHz; Q("Cl)/
x(~'CI)= - 67.8 MHz['l [ ~ ( ~ ' c =
Q(35C1)= 0.788]['l, V ~ ( ~ ~=C29.4
I ) M H z (24.5 M H z for 37Cl),
and a typical chloroacetate geometry ( r = 1.77 A, b" = 0 for
C 2; r = 2.74 A, 0"= 30" for C 1. see Fig. 2) ,[''I first-order per-
Fig. 2. Relative orientation of the internuclear vector Y and the 3sCl;3'Cl zr,(, axis
for the carbon atoms C 1 and C2 in 1. E F G = electric field gradient
turbation theory in a magnetic field of 7.05 T gives symmetric
splittings s of 9.2 ppm for the doublet of C 2 and 1.6 ppm for the
doublet of C 1 (averaged in both cases according to the natural
abundances of the CI isotopes. "CI. 75.5%; 37Cl, 24.5%). On
the other hand. if Equation(a) (jx/vsl= 2.31 for 3sCl; 2.18 for
37Cl at 7.05 T) is used to calculate the splittings, one obtains
values of s of 8.4 pprn and 1.4 ppm for the C 2 and the C 1
doublet, respectively. which are in better agreement with the
experimental data (Table 1, s = 7.8 ppm for C 2 and 1.O pprn for
C 1 at 7.05 T). At 4.7 T neither first-order perturbation theory
nor Equation (a) are appropriate. In this case a broad 1 : 1 :2
triplet is expected on the basis of the exact calculation^.^^.^^
Using the quadrupole data for 1, vs = 19.6 M H z for 3sCl
(16.3 MHz for 37Cl) at 4.7 T, the exact calculation gives the
following shifts of the signals relative to the isotropic I3C frequency: -12.9; -2.8; +7.8 pprn for the C 2 atom and -2.0;
-0.4; + 1.2 pprn for the C 1 atom. These values are in excellent
agreement with experimental ones: -12.2; -2.8; +7.1 ppm
for the C 2 triplet and -2.2; -0.5;
1.1 ppm for the C 1 triplet
(Fig. 1 and Table 1: values measured from the center of the
7.05 T doublets). In the case of compound 2, the expected 1 :2: I
triplet is only observed for the C 2 signal in the low-temperature
spectrum recorded at 7.05 T. The distance between adjacent
peaks ofthis C 2 triplet (8.0 ppm, Table 1) is very similar to that
observed in the corresponding doublet for 1 at the same field.
The expected long-range effect on C 1 (a 1 :2: 1 triplet with interpeak distances of about 1 ppm) may be lost within the experimental line width (Table 1). Quadrupole data are not known for
2 but information is available on the corresponding acids;['] this
shows that all three compounds have similar x ( ~ ~ Cvalues.
Thus. the I3C N M R data for the C 2 nucleus in compound 2
shown in Figure 1 and Table 1 are readily explained. Interestingly, the C 2 triplet collapses into a broad singlet at room temperature. indicating that the reorientation of the CHCI, segment
(or an overall molecular motion) may be able to induce an
efficient "C1/37C1 longitudinal relaxation which causes the l3C
nucleus to be self-decoupled.[' 'I This effect has also been reported in other cases." 21 and is very pronounced in compound 3;
thus even at low temperatures (down to 163 K) two sharp 13C
singlets (Fig. 1 and Table 1) occur. The easier rotation of the
threefold symmetric CCI, group in the solid is expected to
lead to an even faster quadrupole relaxation than in the CHCI,
In summary. I3C CP/MAS N M R spectra of chlorinated organic compounds recorded at high fields are expected to reveal
complex line shapes, if the molecular motions are slow enough
to prevent self-decoupling by fast quadrupole relaxation of the
chlorine nuclei. However, the effects can be conveniently explained by the existing theory.
E.xperimenta1 Procedure
" C NMR spectra at 75.4 MHz were recorded with a Varian VXR 300 NMR
spectrometer equipped with a Doty 7 mm probe; spectral width. -30 kHz: acquisition time, 39.4 ms ( 1 and 2). 59.7 ms (3); relaxation delay. 30 s ( 1 and 3). 300 s (2);
contact time. 3 ms ( I ) , 1 ins (2);memory, 8 K : number ofacquisitions. 1840 (Iand
2). 84 (3); spinning speed. 4200 Hz ( I ) , 2850 HL (2). 4050 Hr (3) Spectra for
compounds 1 and 2 were recorded in the CP mode. whereas compound 3 was
studied by single-pulse acquisition. The C P specti-um for 2 was recorded at
50.3 MHz on a Chemagnetics CMX 200 N M R spectrometer equipped with a Bruker 7 mm probe: spectral width. 20 kHz; acquisition time. 20 ms: relaxation delay.
30 s: contact time. 3.5 ms: memory. 8 K : number of acquisitions. 2040: spinning
$peed. 4200 Hz. The temperature was lowered by cooling the driving gas in liquid
Received: March 11. 1994 [Zh7501E]
German version: A n g t w . C'l~cni.1994. 106. 1708
[I] R. K. Harris, A. C. Olivieri. Prug. Nncl. Mugn. Reson. .Sp(u/ws<.1992.274.435.
[Z] A. C. Olivieri. L. Frydman, L. E. Diar. J. Mugn. Reson. 1987. 75. 50.
[3] A. C. Olivieri, P. Cabildo. R. M. Claramunt. J. E. Elguero. J Phr\. Chcni.
1994. 98. 5207: R. K . Harris. M. Sunnet~jo& K . S. Cameron. F. G. Riddell,
Miign. Rrson. C I I E I1993.
~ . 3/. 963; R. M. Cravero. C. Fernandel. M. GonLalez-Sierra. A. C. Olivieri, J. Chem. Soc. Chein. Coiuniuii 1993. 1253.
[4] E. M. Menger. W. S. Veeman. J. Mugn. Rrsan. 1982. 46. 257
[ 5 ] S . H. Alarcon. A . C . Olivieri, R. K . Harris. .Wid S/urt, ,+ud Mugn. RP.WI.
1993. 2. 325.
[6] A. C. Olivieri. J. Mugn. Reson. 1989, 81. 201: i i d . 1993. 101 4 . 313: S d d S i ~ i ~
Nuel. Mugn. Reson. 1992, 1. 345.
[?I E. A. C. Lucken. N u d i w r Quudrupole Coupling Con.rrnnr.~.Academic Press.
London, 1969, p. 187.
1.31 H. C. Allen. J. A m . Cl7em. Soc. 1952. 74. 6074.
[9]P. Pyykko, Z. Naturforsch. A 1992, 47. 189.
[ l o ] M. Ichikawa, Acru C' w l . B 1974. .W.651: D. HadLi. I . Leban. B.
Orel. M. Iwata. J. M. Williams, J. M u / . Sfrucr. 1979. Y. 117.
[ I l l The relevant relaxation rate in this case is the one connecting the I +3;2) ;ind
I f1'2) 3'CI states, since these are responsible for each of the components of
the symmetric doublets observed at high fields.
[I21 A. C. O1ivieri.J. Cheii~.So<.Prvkin Fun.\. 2 1990. 85; H. W. Spiess. U . Haeberlen. H. Zimmermann. J. M q n . Reson. 1977. 25, 55: R. K Harris, A. Root,
M d P h w . 1989.66.993: S. H. Alarcon. A. C. Olivieri. P. Jonsen,J. C'/iivii. Soc.
Prrkin 7i.mi.s. 2 1993. 1783.
Dissolution of Cholesterol in Water
by a Synthetic Receptor**
Blake R. Peterson and Franqois Diederich*
Dedicated to Professor C. Riichardt
on the occasion of' his 65th birthcky
Cholesterol is an essential component of animal cells. This
highly water-insoluble steroid serves as both a modulator of
membrane fluidity and as a raw material for the production of
steroid hormones and bile acids.['] The hydrophobic properties
of cholesterol, which make it an essential component of cell
membranes, also make it lethal when deposited on arterial walls.
The deposition of cholesterol-containing plaque in blood vessels, known as atherosclerosis.['] is a leading cause of death in
western industrialized societies. While current drug therapies
focus on enzyme inhibition of endogenous cholesterol biosynthesisc3] or bile acid depletion[41 to reduce blood cholesterol
Prof. Dr. F. Diederich. B. R. Peterson
Laboratorium fur Orgdnische Chemie. ETH-Zentrum
Universititstrasse 16, CH-8092 Zurich (Switzerland)
Telefax: Int. code + (1)261-3524
This uork was supported by ETH Zurich. We thank Dr. B. Jaun for his
assistance in obtaining and interpreting the 2D N M R spectra of the macrotricyclic compounds and Drs. W. Amrein and P. James for recording the mass
Без категории
Размер файла
259 Кб
solis, 13c, cpmas, sodium, couplings, dipolar, effect, 35cl37cl, nmr, variables, residual, chlorinated, temperature, spectral, acetate
Пожаловаться на содержимое документа