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Collisional Activation Mass SpectrometryЧA New Probe for Determining the Structure of Ions in the Gas Phase.

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Volume 15 - Number 9
September 1976
Pages 509 - 568
International Edition in English
Collisional Activation Mass Spectrometry-A
New Probe for
Determining the Structure of Ions in the Gas Phase[**]
New analytical
By Karsten Levsen and Helmut Schwarz“]
Organic ions with high translational energy colliding inelastically with neutral atoms or
molecules become excited electronicallyat the expense of their translational energy. The excitation
energy enables a wide range of dissociation reactions to occur and the intensity relationships
yield information about the structure of the ions concerned and also permit conclusions to
be drawn about the mechanism of their formation.
1. Introduction
Mass-spectrometric investigations are affected in many cases
by the occurrence of collision-induced processes involving
the residual gas so that often, e.g. in the study of metastable
ions, considerable effort is necessary to eliminate the influence
of these collision processes. Only during the last eight years
has it been shown by the systematic investigations by McLaf
ferry[’],Jennings[21and Beynod39 4l that these processes, originally considered a nuisance, can in fact provide a wealth
of information. The above authors concentrated on the one
hand on charge-transfer
and on the other hand
on collision-induced dissociation processes“ - 3* ‘1. In the
course of this work it became clear that these collision-induced
dissociation processes-to which the present article is confined-provide valuable information about organic ions, for
example about their physical properties, their structuresr],
and the mechanisms of their formation and decomposition.
Further, analytical applications become apparent for the determination of the structure of organic compounds and for analysis of mixtures. In what follows the theoretical principles
of collision-induceddissociation and the methods of measurement are first described, followed by presentation of the applications.
2. Theoretical Principles
2.1. Energy Transfer
5 3
[*] Priv.-Doz. Dr. K. Levsen
lnstitut fur Physikalische Chemie der Universitat
Wegeler Strasse 12, 5300 Bonn (Germany)
Priv.-Doz. Dr. H. Schwarz
Institut fur Organische Chemie der Technischen Universitat
Strasse des 17. Juni 135, 1000 Berlin 12 (Germany)
[**I Abbreviations: CA:collisional activation; CI: chemical ionization; CID:
collision-induced dissociation; DADI: direct analysis of daughter ions; EA:
electronattachment; EI: electron impact ionization; F D : field desorption; FI:
field ionization: ICR: ion cyclotron resonance; MI: metastable ions; SID:
surface-induced dissociation; MIKE: mass analyzed ion kinetic energy.
Angew. Chem. Int. Ed. Engl. / Vol. 15 (1976) No. 9
When ions having a high translational energy (a few hundred
eV or more) collide inelastically with neutral atoms or molecules, part of their translational energy is converted into excitation energy of the ions which can subsequently lead to the
latter’s decomposition. A collision-induceddissociation of this
kind can also be called a dissociative ion-molecule reaction
[in accordance with eq. (a)].
[ml]+
+ N-
Em,]+
+ [m3] + N
(a)
[‘I The term “structure” refers here solely to the bonding of atoms. Bond
lengths, bond angles, configurational or conformational differences, and electron-density distributions, are left out of account. Thus, “structure” is here
equivalent to “constitution”.
509
The whole group of fragments of a given primary ion produced by collision is known as the collisional activation (CA)
spectrum of this ion"] PI. If fragment-ions rather than molecular ions are investigated, then the CA spectrum is in some
respects the mass spectrum of a fragment.
When the translational energy exceeds 1 keV and the colliding gas atoms are small, the first process that takes place
is mainly electronic excitation of the impacting ion[']. This
electronic excitation is converted into vibrational excitation
which rapidly (within a few picoseconds) distributes itself statistically over the whole ion and can lead to the rupture of
specific bonds. Since the energy-transfer mechanism corresponds to that in the collision of an electron with a neutral
molecule in the ion source of a mass spectrometer, it can
be described by the quasi-equilibrium theory (QET)"]. The
analogous energy-transfer mechanism is also the reason why
the electron impact and the collisional-activation spectra of
a molecular ion show the same fragments['s2,6,9].
This does
not necessarily imply that the internal energy distribution
in a molecular ion must be the same after collision with
electrons and atoms, since this energy distribution is a function
of the kinetic energy of the electrons or ions. However, the
relative fragment intensities in 70 eV electron impact spectra
are very similar to those obtained in collisional-activation
spectra of the same molecular ions with 8-10 keV ions[',2391,
and this makes it prodable that the energy distributions are
often comparable. In both cases a broad spectrum of excitation
energies (0-1 0 eV) is produced; in collisional activation the
energy distribution after the collision is only slightly influenced
by the energy distribution prior to the
The similarity between the collisional-activation spectra and the 70-eV
electron impact spectra is due not only to a similar energy
distribution but also to a comparable decomposition time,
which for instrumental reasons is around
sec in both
cases.
It
lam
5'
fkactron coordinate
-
Fig. 1. Energy transfer in collision-induced dissociation. Let the primary
ion [m,]' have a mean excitation energy E A before the collision. On inelastic
collision an amount Q of the translational energy is converted into internal
energy. After overcoming an activation threshold Eo the excited ion decomposes into a fragment ion [m,]' and a neutral particle m3. The excess
("non-fixed) energy E* and the activation energy of the reverse reaction
E , are released partly as translational energy T (signal broadening). (The
kinetic shift is left out of account for the sake of clarity.)
The excitation energy that the ion gains in the collision
is of the same magnitude as the loss of its translational energy.
(The fraction of kinetic energy transferred to the target atom
['I The abbreviationCID (collision-induceddissociation) is used in addition
to CA (collisional activation) [3].
510
is negligiblep].) The decrease of translational energy, Q, can
be measured directly and thus permits a direct determination
of the excitation energy transferred. During the subsequent
unimolecular dissociation of the excited ion part of the excess
energy .E* ("nonfixed energy"), i.e. the energy not required
for overcoming the dissociation threshold, is released as translational energy Tp*] (the fragment ions and neutral particles
fly apart relative to the original center of gravity of the masses),
and this leads to unsharpness in the energy and thus to a
broadening of the collision-induced signalsI6! An analogous
signal broadening is found in the unimolecular dissociation
of metastable ions (MI spectra). Since, however, the excitation
energy and thus also the excess energy of the ions dissociating
by collision are on average much higher, the signals in the
CA spectra are generally broader than those in the MI spectrar6I. The relationship between the quantities Q and T is
shown in Figure 1.
2.2. Collision-Induced Dissociation
Three types of information can thus be obthined from a
collisional activation spectrum: 1. the relative intensities of
the secondary ions; 2. the translational energy T released
on dissociation; and 3. the excitation energy transferred, Q.
While the intensity ratios and the energy T released in a
dissociation (i. e. the peak half-width) are suitable for the
characterization of an ion structure, determination of the excitation energy by means of the value Q (together with T )
provides an insight into the thermochemical properties of
an ion. Work in which these three values were systematically
investigated will now be briefly reported.
The intensity ratios of collision-induced fragments were
studied by McLafferty et a1.r' as a function of various parameters. It was found that when the translational energy (velocity)
of the impacting ions increases there is not only an increase
in the yield of the collision-induced secondary fragments but
also a change in the intensity ratios between these fragments.
As an immediate consequence of an increase in the mean
transferred energies, the intensity of fragments having higher
activation energies increases more than does that of fragments
with lower activation energies. A similar increase in fragmentation processes with higher activation energy is found at high
pressures of the collision gas; this is explained by the occurrence of multiple collisions. The nature of the collision gas,
however, has no influence at all on the intensity ratios["]
(and also generally none on Q and TI6]), although it does
influence the yield of collision-induced fragments: as the size
of the atoms or molecules decreases, the cross section and
thus the yield increases, so that helium and H2 are particularly
suitable collision gases.
Especially important is the finding that the intensity ratio
of the collision-induced secondary fragments from a given
primary ion depends to only a very small extent on the excitation energy, i. e. on the energy distribution before the collision,
as is shown by varying the electron energy or by comparing
the collisional activation spectra of field-ionized or electronimpact-ionized molecules["* '1. Dependence on the excitation
p] This assumption is valid for ions that undergo little or no scattering,
i.e. those selected by the slit arrangement out of all the colliding ions.
p*].Not only the excess energy but also the activation energy of the reverse
reaction, E , (if present), contributes to the translational energy T released.
Angew. Chem. Int. Ed. Engl.
/ Vol. 15 (1976) N o . 9
energy is found only, if at all, with processes involving the
lowest activation energies'"].
The decrease in translational energy Q caused by energy
transfer on collision, as also the energy Treleased on subsequent dissociation, have been studied in detail by Cooks et
al. for the case of the aliphatic alcohols[6J. It was established,
taking into account the internal energy present before the
collision, that the decrease in translational energy Q in a
specific collision-induced fragment is equal to the excitation
energy predicted from the breakdown diagram, at which the
fragment in question is most probably formed. Furthermore,
the values of Q and T can be used to obtain information
on how the excess energy of a highly excited primary ion
will be distributed on decomposition between the translational
energy Tand/or the excitation energy of secondary fragments
and/or the neutral particles ejected (energy partitioning).
Finally, for diatomic and triatomic molecules it is possible
to derive, from the quantity Q, the electronic state in which
the collision-induced fragments are
collision gas (e.g. helium) is admitted through a high-vacuum
valve into the collision chamber where the pressure is
to
tom. A differential pumping system prevents all but
small amounts of the collision gas from entering the ion
source or the electric sector. (To increase the reproducibility
in structural studies of organic ions it has proved advantageous
not to carry out the investigation at constant pressure but
instead at a constant scattering cross section, which is different
for ions of different masses.) In a manner analogous to the
DADI technique, the collision induced fragments can be subjected to energy and thus mass analysis by scanning of the
electrical sector potential.
al
-
78
x 2500
L
x 50
Ll
2.3. Methods of Measurement
Since the place where collision-induced fragments are
formed in the mass spectrometer is the same as for metastable
ions (one of the field-free regions of the spectrometer), they
can be detected by the same methods. Collision induced fragments can be observed in any mass spectrometer working
with a magnetic field['"%l b , 3, 5 , 6 1 . However, double-focussing
spectrometers with "reversed' Nier-Johnson geometry are particularly suitable for this purpose; in these the ion source
is followed first by the magnetic and then by the electric
sector" '. ''1. Such an arrangement, as used in principle also
in the MIKE" l C 1o r the DADI technique["] for the detection
of metastable ions, is shown schematically in Figure 2.
'2
Fig. 3. a) DADI spectrum of the benzene molecular ion ( m / e = 7 8 ) ; b) CA
spectrum of the benzene molecular ion ( m / e = 7 8 ) ; c) CA spectrum of the
I-octene molecular ion ( m / e = 112).
Fig. 2. Schematic layout of a mass spectrometer suitable for the measurement
of collisional activation (CA) spectra.
Figure 3 b shows a collisional activation (CA) spectrum
obtained in this way for the benzene molecular ion"]. In
Out of the large number of primary and secondary fragments
formed in the ion source the ion to be examined is selected
by means of the magnetic field and passes with high translational energy (3-10 keV) into the collision chamber" 'I. The
[*] Strictly speaking, the spectrum in Fig. 3 b shows a superposition of
collision-induced and unimolecular dissociations, so that a pure CA spectrum
is obtained by subtracting the metastable ions corrected for the different
scattering cross-sections in Fig. 3a from Fig. 3b. However, this subtraction
is not necessary for most analytical purposes.
Angew. Chem. I n t . Ed. Engl.
1 Vol. 15 ( 1 9 7 6 ) N o . 9
511
Figure 3 a is shown the same spectrum, but one obtained
without a collision gas, i. e. a DADI
which contains only the unimolecular dissociations of metastable molecular ions. Comparison of Figures 3 a and 3 b shows that the
CA spectrum contains many more signals, more intense but
also markedly broader, than there are in the DADI spectrum.
The slight shift of the peak maximum in the direction of
smaller masses is to be ascribed to the decrease, discussed
above, in the translational energy by the amount Q as a
result of the collisional excitation. The varying half-widths
of the signals, which reflect the translational energy released
in the dissociation, are clearly recognizable.
The linear mass scale is an advantage. Nevertheless, the
figure also shows at once a considerable drawback of the
method. In spite of the relatively small precursor mass of
benzene (m/e=78) the signals overlap, in some cases to an
appreciable extent. Although the resolution is much better
with many other compounds because of the smaller amount
of energy released in the fragmentation, e.g. with I-octene
(m/e= 112, Fig. 3c), it is rarely possible to resolve individual
signals even partially with an accelerating potential of 10kV
and above a precursor mass of m/e=150“]. This does not
reduce the method’s value for the elucidation of the structure
of organic ions, because for this purpose it is only necessary
to determine whether two spectra are the same or different.
The limited mass resolution does, however, make it considerably more difficult to carry out an isotope-labeling analysis
in the case of the larger organic ions. What is favorable,
nevertheless, is that only partial incorporation of labeled atoms
does not disturb isotope-labeling analyses, since with the
above-described magnetic field and electrostatic analyzer
arrangement the mass analysis occurs before the collisioninduced dissociation.
Finally, mention must be made of an interesting variant
for the production of collision-induced fragments, described
by Beynon et d.[’
’1. In this “surface-induced dissociation”
(SID) method the primary ions d o not collide with a collision
gas but glance a metal surface where collision-induced fragmentation occurs, analogous to that described above“’].
2.4. Characterization of Ion Structures
According to the quasi-equilibrium theory (QET) describing
mass-spectrometric dissociation, the intensity ratio of massspectrometric fragments depends both on the energy distribution in the primary ion and on the rate constants for dissociation‘’]. The rate constants of a given ion are, further, an
unequivocal function of the vibrational and rotational frequencies as well as of the activation energies for subsequent dissociations. Ions having the same elementary composition but different structure should thus differ at least partially in their vibrational and rotational frequencies and in the activation energies
of the subsequent reactions, and consequently, with the same
energy distribution, in the intensity ratio of the subsequent
dissociation processes. Since the CA spectra are largely inde[*] The resolving power for a given precursor ion can be improved by
increasing the accelerating potential, since the signal half-width is inversely
proportional to the velocity of the primary ion.
[**I Improvements in this technique are currently being swdied in many
laboratories [20].
512
pendent of the distribution of excitation energy (see Section
2.1), they reflect the structure of the ion directly. Thus, fi
the C A spectra oftwo ions having the same elementary composition agree both in the intensity ratio and in the peak half-width
(translational energy released), then the conclusion is drawn
that the structures are identical, and vice versa. This comparative
method requires knowledge of the structure of one of the
ions. If there is no suitable reference ion, then the structures
can in some circumstances be derived from the fragmentation
behavior, which in turn can be clarified by isotopic labeling.
If, after the exclusion of the processes with the lowest activation energy“], the intensity ratio is found to depend on the
electron energy, then it can be concluded that a mixture
of non-interconverting structures is present, the latter being
formed simultaneously from the precursor ion by competing
mechanisms with different activation energies. Because of the
different activation energies the composition of the mixture
changes as a function of the excitation energy. On the other
hand, if the intensity ratio changes little or not at all with the
electron energy it cannot be definitely concluded that only
a single structure is involved; interconverting structures (isomerization) may be present before the collision, even if the
subsequent collision-induced dissociation occurs predominantly through one of them[**].
2.5. Comparison with Other Methods for the Determination
of the Structure of Ions in the Gas Phase
The collisional activation technique for the determination
of the structure of organic ions competes with several older
methods which also differ themselves in the information they
provide. If we exclude the very indirect method in which
conclusions about the structure of an ion are drawn from
the reaction mechanism, the following methods have been
used most
1. Determination of the heats of
2. Investigation of ion-molecule reactions[’‘],
3. Analysis of dissociation products
a) intensity ratios in the dissociation of metastable ions
(MI)“ ’I.
b) translational energy T released in the
and
c) collisional activation spectral’. 2!
All these methods are as a rule comparative, i.e. if the
heats of formation, the ion-molecule reactions, or the dissociation products are the same for two ions of the same elementary
composition, it is concluded that the structures are identical,
and vice versa. The methods differ in that ions with different
excitation energies and different lifetimes are investigated.
In the first two methods those ions are studied whose excitation energy does not suffice for dissociation within the mass
spectrometric time scale. These ions are therefore described
as “stable”[***1.In the determination of the heats of formation
[*] Since these processes-recognizable by the presence of intense unimolecular dissociation in the MI spectra-often depend on the energy distribution,
any structural assignments based on these peaks, if at all possible, must
be made with caution.
In rare cases the absence of an energy-dependence of the CA spectrum
may be due to presence of a mixture oi non-interconverting structures that
happen to have the same kinetic parameters,
[”‘I “Stabi1ity”means that no furtherdecompositionoccurs.Ofcourse, isomerization of the ion may perhaps take place.
[**I
Angew. Chem. Int. Ed. Engl.
Vol. 15 ( I 976) No. 9
ions with a minimum of excitation energy are present. (This
still depends on the kinetic shift and in particular on the
activation energy for the reverse reaction.) In the study of
ion molecule reactions at low pressures, e. g. by the ion cyclotron resonance technique (ICR)[l61on the average higher excitation energies are involved, reaching up to the lowest dissociation thresholds. In the determination of the structure of ions
by analysis of unimolecular decomposition of metastable ions
(methods 3 a and 3b), however, ions are studied whose energy
does suffice for further fragmentation (often also referred
to as “unstable” or “reactive” ions). From the quasi-equilibrium theory it can be deduced that these ions are characterized by a well defined excitation energy that is only slightly
higher than the lowest dissociation threshold.
The situation with the collisional activation method described here ismorecomplex owingtodouble excitation. While
it is true that most of the ions entering the collision chamber
are stable, i.e. they have a spectrum of excitation energies
that reaches up to the lowest dissociation threshold, the ions
that finally undergo collision-induced dissociation are highly
excited reactive ions. Since with such highly excited ions direct
dissociation of the original stable structure is often just as
fast (or faster) as an isomerization that is theoretically possible
at higher excitation energies, the relative intensities of the
collision-induced secondary dissociations reflect to some
extent the structure of the stable ions.
Whether the structural information obtained by the above
methods differs or not will depend mainly on the isomerization
behavior of the ion in question, as may be seen from Figure
4. If the isomerization threshold (Ei) is appreciably higher
than the lowest dissociation threshold (EoA+), then all the
methods will yield the same information (Fig. 4a). If, however,
Ei <E,,A+ (Fig. 4 b), it can be shown by means of the metastable
characteristic^"] that A + and B + dissociate via the same
structure (isomerization), whereas ion cyclotron resonance
measurements and heats of formation give different structures
for A+ and B+. The CA spectra of A + and B + should also
differ, since, although after the collision a considerable amount
of isomerization and thus decomposition by way of B+ can
occur, some of the ions A+ decompose directly. With very
low isomerization thresholds (Fig. 4c) it is still possible for different heats of formation to be found for A + and B + . However,
in all the other methods most of the ions isomerize already
before the fragmentation to a mixtureofA+ and B’. Moreover,
since in CA investigations the energy-richest ions are preferentially excited by
and since isomerization can also
occur after the collision, the CA spectra of A + and B + differ
little if at all[*]. (The collision-induced fragmentation itself
occurs both of A + and (preferentially) of B+.) Finally, the
intensity ratio of metastable ions will be the same for A’
and B’. (Here the dissociation occurs via structure B+.)
Apart from the excitation energies of the ions under study,
their lifetimes play a role in the provision of structural information. In most of the methods this lifetime is similar (lo-’
to lo-’ sec); only in ion-molecule reactions in the ICR cell
are ions with very much longer lifetimes analyzed ( 1 O - j sec).
A discussion ofthe sources of error in the methods presented
here will be of interest. Reliable determination of the heats
[*] Thus, it is not permissible to conclude from identical CA spectra for
A t and B + that there is no isomerization threshold between the two ions.
The threshold may only be relatively small ( E i g E , * + 1.
Anyrw. Chum.
Iiil.
Ed. E i y l . J Vol. 15 ( 1 9 7 6 ) No. Y
a1
W
cl
A+
El+
,
A+
A+
B’
-
B+
Fig. 4. Influence of internal energy on structural information obtained by
different methods about an ion A + that may rearrange to an ion B + after
overcoming an isomerization threshold Ei (the excitation energy region (or
decomposing metastable ions is hatched diagonally, in CA and ICR investigations vertically, and in measurements of the heats of formation horizontally).
a) E I % E o ~b*):E i < E O A * ; C ) E , < E o n - .
of formation is often impossible, because the kinetic shift
contributing thereto as well as the activation energy for the
reverse reaction can generally be obtained experimentally only
with difficulty if at all. If the structure of an ion is characterized
by means of the intensity ratio of metastable ions (method
3 a), the deductions may be false because this intensity ratio
depends on the energy distribution. In ion-molecule reactions
(ion-cyclotron resonance technique) it is at present not clear
to what extent the structure-dependent reactivity of an ion
is additionally determined by its excitation energy, the latter
being, as discussed above, variable within wide limits.
According to results available so far, the ICR method, CA
spectroscopy, and determination of the energy released in
the dissociation of metastable ions seem to be the most reliable
tools for the determination of ion structures in the gas phase.
Of these three methods CA is particularly attractive, because
a) it is much more sensitive than the other procedures and
b) it involves considerably more information (released kinetic
energy and the intensity ratios of numerous fragments). Its
only disadvantage is the poorly defined excitation energy,
due to the double excitation.
3. Ion Structures
It was shown in principle in Section 2 when an ion entering
the collision chamber will isomerize and what kinetic situation
must be present to prevent this process. Examples will now
be given for both limiting cases and for transitions between
them.
3.1. Carbeniurn Ions
The classical investigations by Meyerson et ~ l . [ ~ on
l ] the
structure of gaseous C,H? ions have provoked countless theoretical and experimental studies“], without it having been
0
[*] In this connection particular reference should be made to ICR studies
carried out very recently 1221.
513
made conclusively clear in most cases whether the species exists
as tropylium ions (1) or as benzyl cations (2)[231. The extensive CA investigations by McLafferty and W i r ~ k / e r [on
~ ~sixty
]
compounds of different structures that yield C7HY on electron
impact appear to have closed this chapter of organic mass
spectrometry. Contrary to the deductions from the earlier
work, it must now be assumed that in most of the compounds
investigated, including the [M - HIQ ion from cycloheptatriene itself, which was seen as the prototype for ( I ) , an
appreciable fraction of stable C7HY ions is present as (2).
Furthermore, the above authors showed that, besides these
structures, stable 0-,m- and p-tolyl cations (3), and even
the norbornadienyl cation ( 4 ) , can be detected when suitable
precursors are used. The isomerization threshold between the
ions ( 1 ) to ( 4 ) appears to be too large for complete equilibration. From the experimental findings it follows further that
no CTH? ion possessing only the tropylium structure could
be found. Out of all the compounds studied so far the [MCl]’ ion of (acyclic!) 2,4-heptadien-6-ynyl chloride has the
I*?
highest proportion of (1 ), namely > 95
The mutually contradictory results on the structure of
gaseous C8HF ions from phenethyl bromide (5)[”l could
also be largely clarified by CA measurements on deuterated
model compounds. Here too a mixture of several species
must be present, whose composition was determined by varying the electron energy[2y].At the lowest excitation energy
( z 11 eV) it contains ethylenebenzenium ions (6) as the sole
component, whereas in the 70-eV spectra the main proportion
consists of f -phenylethyl cations (8). In the medium-energy
region 2-phenethyl cations ( 7 ) also occur, but, in contrast
to the rearrangements of CsHF ions in the condensed phase[3o1,
these d o not act in the gas phase as precursors for the ion
( 8 ) (Scheme 1).
x
171
Scheme 1. Cs H? ions from phenethyl bromide ( 5 ). G =gas phase, FI = liquid
phase, EI =electron-impact ionization.
The problem of the structure of reactive CsHf. radical
cations (especially ionized benzene), which has been studied
by many groups and by almost every available method, has
also recently been clarified by CA measurement^['^^ 321.
Investigations of 3C-labeled 1,3-he~adien-5-yne[~’]
provide,
inter a h , the first experimental proof that the mechanism
previously postulated for the H-scrambling process[23.331 in
ionized benzene may proceed reversibly via acyclic interme3 1 3
[*I
This result confirms earlier energy measurements and investigations of
‘3C-labeled acyclic model compounds [27J, from which it also follows that
a considerable proportion of the [M -CIJQ ions from 2,4-heptadien-6-ynyl
chloride must be present as tropylium ions f J ).
514
diates (namely 1,3-hexadien-S-yne).It can be further concluded
from the CA spectra that hexadiyne radical cations participate
only to a minor extent in this e q u i l i b r i ~ m [ ~ ~
a . result
~~],
that is also in accord with the results of elegant electron
impact investigations reported by M ~ m i g n y [ ~ ~ I .
In addition to CA investigation of highly unsaturated carbenium ions such as C6H?[1y*313351,
C6Hp[19*31‘321
C6&[35I, C,HY[24-261, C,Hp[25. Id], CTH:[36l,
C8Hp[31I:
CsHQ0[311,CsH$[291, and C9H?1391 which will not be discussed here, the behavior of more saturated molecular ions
and fragment ions on isomerization has also been stud381. To a first approximation, the results obtained
so far permit the following generalization: Fragment ions
of composition C,,HYn+l (n=3-7), C,,HYn-, (n=3-7), and
C,Hfn- ( n= 6 or 7) give almost identical CA spectra, independently of the structure of the molecular ions and also independently of the excitation energy, a result that indicates almost
complete equilibration of the differently branched ions (Fig.
4c). Although ten molecules having different carbon skeletons
were used as precursors, the isomeric C.5HF3 ions, for example,
decompose largely from one structure, (9), while for C7H75
decomposition from the structure (10) was po~tulated[~’”l.
37v
In contrast to this complete isomerization of fragments
(even-electron carbenium ions), hardly any isomerization of
the carbon skeleton before the fragmentation was observed
for the molecular ions of isomeric octanes, octenes, cycloalkanes, (except for n-propylcyclopentane, which isomerizes to
1-octene before fragmentation), or CsH?; radical cations. Hydrogen shifts, however, occur so rapidly that, judged from
CA spectra, localization of the double bond, e.g. in isomeric
octenes, appears so far to be possible only to a very limited
extent.
The very different behavior of molecular ions and fragment
ions on isomerization appears to be due to the different activation energies Eo for further decomposition. From measurements of the appearance potentials it can be concluded that
the EO for further decomposition of fragment ions is generally
appreciably greater than the activation energy required for
the primary fragmentation of molecular ions. When the relationship between EOand kinetic processes, discussed in Section
2.4, is taken into account, this means that for molecular ions
Ei > E, and isomerization is suppressed in favor of fragmentation.
It must be mentioned that this behavior of molecular and
fragment ions is not generally valid but that other, still
unknown, factors play a part with C,H2,-2 compounds of
lower molecular weight[351,with a r e n e ~ [ ’ ~311,* ~ and
~ . with
alkynes isomeric with the latter[323
3y1.
However, the importance of the threshold energy ratio Ei/Eo
for the structural stability of the carbon skeleton is shown
by a study of isomeric ions containing a heteroat~m[~’].
Whereas isomeric C5HFI ions having originally the
unbranched skeleton ( I 1 ) or the iso-skeleton ( 1 3 ) isomerize
in lo-’ sec to a common structure [probably (12)[373411],
the carbon skeleton hardly isomerizes at all before the decomposition if the cation contains a charge-stabilizing group X
(Scheme 2).
Anyew. Chem. I n t . Ed. Engl.
Vol. 15 ( 1 9 7 6 ) N o . 9
direct contradiction to interpretations based only on analysis
of unimolecular dissociations (including consideration of the
kinetic energy released during the dissociation).
CA investigations of oxygen-containing radical cations of
composition C zH4O
c*l, C z H 4 0y'[481,and C 3H6O@'[491
have also shown that, depending on the primary structure,
a series of stable ionic structures exists that are interconvertible
with one another only to a limited extent if at all. The structure
of ions produced by y-hydrogen-transfer to carbonyl groups,
e . g . in (24)[501,will be briefly mentioned as an example of
non-interconvertible ions. The CA spectra show, in agreement
with other methods of investigation, e. g. ion cyclotron
resonance mass spectrometry, that the energy barrier for interconversion cannot be surmounted (Scheme 3).
@
(141
(15)
Scheme 2. Isomerization of carbon skeletons. X
NH = CH,.
= C=Oe,
8 = CH,,
Here too analysis of the energy data shows that the heteroatom group X appreciably increases the potential barrier to
isomerization, and this is certainly related to preferential localization of the charge on the heteroatom. The alternative, that
the presence of X makes possible other fast reactions that
outrun isomerization, probably does not apply here. Similar
effects have been found in the CA spectra of immonium ions
from oligopeptides containing leucine or isoleucine components. The analytical importance of these differences is discussed in Section 5.
3.2. Oxonium and Immonium Ion
The stabilizing effect of heteroatoms on carbenium centers
is well known"] and is also expressed in the CA spectra
of a number of oxonium and immonium ions. In extensive
investigations McLafferty et
43al detected the structures
(1 6)-(22) as stable species (X = 0" d, 431, X = NH[43bl). The
isomerization thresholds between the ions are so high that
rearrangements before or during the collisional activation
are almost wholly excluded.
1241
(26)
(25)
Scheme 3. y-Hydrogen transfer in radical cations of carbonyl compounds
( 2 4 ) , X = H, alkyl, OH.
In principle, several non-interconverting ions can arise
whenever the formation of the primary fragment, e. g. C3H60@
from HO(CH2)3Y,Y = C1, Br, or OH, involves several cyclic
transition states sensitively influenced by molecular parameter$ e.g. bond energies or steric effects,and when isomerization
after the fragmentation (e.g. elimination of HY) is little favored
on energetic grounds. For example, compounds HO(CH&Y
in which Y =C1, Br, OH afford C3H60'. ions of structures
(27) and (28), whereas when Y =OCH3 only (28) is prod~ced'~~'.
0.
H3C
0.
OH
HH
H
CI
Br
OH
OCHJ
Since, furthermore, by investigations on numerous precursors it could be shown which structural element of a neutral
molecule was to be correlated with which ionic structure,
in unknown compounds conclusions about the intact molecule
could be drawn from the CA spectrum in combination with
other mass-spectroscopic data. It is also remarkable that a)
a series of structures, e. g. protonated epoxides [such as ( 2 3 ) ] ,
previously considered to be stable, are relatively unstable
under CA conditions and rearrange almost quantitatively to
(16), X = O[ldl[**],and b) the CA results are here partly in
p] M O calculations show that e.g. CH,OHe and CHINH? ions are stabilized with respect to CH2He ions, by 48 and 66 kcal/mole, respectively
r421.
[**I It was recently shown by C A investigations [44] with equipment of
considerably improved energy resolution, that ions of structure ( 2 3 ) can
arise if 2-bromo- or 2-nitroethanol is used as precursor. Also, in the same
study [44], the formation of the protonated thiirane ( 2 3 ) , S in place of
0,wasdemonstrated, whereas the unbridged HS-CH,-CH:
ion is unstable
under CA conditions. SCF-MO calculations [4S] lead to the same result.
Angew. Chem. Inr. Ed. Engl.
1 Vol. 15
( 1 9 7 6 ) No. 9
60
33
50
0
:
:
:
:
40
67
50
100
4. Reaction Mechanisms
As has already been mentioned in Section 3, there is a
direct correlation between the fragmentation mechanisms and
the structures of the ions produced. While labeling studies
can provide information on the origin of transferred or eliminated atoms or groups, in the strict sense they contribute
little to our knowledge of the structure of the nonreactive
species formed. On the contrary, there are many reactions
whose mechanism remains obscure in spite of intensive labeling. Here CA spectroscopy with the use of appropriate reference spectra seems to be the method of choice for clarifying
the reaction route via a determination of the ion structures.
In what follows a few examples will be used to show both
[*] For C2H,0e' ions it could be shown also by analysis of the signal
shapes and of the translational energy released in the unimolecular dissociation
[47] that, in agreement with CA studies, only a few structures are stable.
515
the importance and the limitations of collisional activation
mass spectrometry in the clarification of such questions.
Intense ions of elementary composition C4H8Cl' are characteristic for the mass spectra of higher n-alkyl chlorides (n t 6)
(29)[5 '1. Although numerous indirect arguments indicated
a tetramethylenechloronium ion ( 3 0 ) as the structure, direct
experimental proof was first achieved by a CA investigation
of labeled n-alkyl chlorides (Scheme 4)f5'1. The finding that
( 3 2 a ) and ( 3 2 6 ) are formed in the same ratio from both
precursors ( 2 9 a ) and ( 2 9 b ) permits only the cyclic structure
(30) on the grounds of the symmetry properties of ( 3 0 )
and (31)[*1.
z:
;fixz
3
0
yH2
z
€j-
0
CA
0
Cl=CX2 + Cl=CY2
Scheme 6. Radical elimination from radical cations of z,P-unsaturated carboxylic acid piperidides ( 3 8 ) . R=alkyl, benzyl, aryl.
Reciprocal['.'] shifts, which in the limiting case of a concerted course can also be considered as electron-impactinduced dyotropic rearrangement~r~~l,
could also be confirmed
by CA investigationsf6']. For example, the ion ( 4 2 ) rearranges
completely to ( 4 3 ) . The protonated epoxide ( 4 4 ) cannot be
detected under CA conditions.
c3H4Y2 or -C3H4<
(32a)
H
132h)
(30)
Hz -R'
Hz
( 2 9 a ) , 129h:
0
YZC-(CH2)2-CXzCl
131)
129a), X
= H,
Y = D;
129b), X
= D,
CA
- C3&YZ
@
Cl-CXZ
(32a)
Y = H
Scheme 4. Structure of [M-RIm ions obtained from ( 2 9 a ) and ( 2 9 h j .
The question of the structure of [M-Alkyl]'
fragments
from a$-unsaturated carbonyl compounds (Scheme 5), which
received contradictory answers in the literature, could also
be decided in favor of the acyclic structure ( 3 5 ) by CA investig a t i o n ~ [ ~Substituted
~!
dihydropyrylium ions ( 3 4 ) , postulated
by M~L,affeerty[~~I,
could be excluded with certainty for aldehydes and ketones and with high probability for esters. The
course of the reaction is compatible only with a reaction
that starts with a y-hydrogen-transfer and gives the conjugated
oxonium ion ( 3 5 ) by allylic fission[55! It is further remarkable
that in the gas phase (35) does not undergo rearrangement
to ( 3 7 ) , which Olah et al.[561were able to observe on reaction
in a superacid medium.
Much space is devoted in mass-spectrometric literature to
hydrogen transfers occurring in alkylarenes with elimination
of alkenes, but in many cases it has not been possible to
generalize about the transfer mechanism (e.g. ring size of
the transition state if heteroatoms are incorporated into the
side chain) or to clarify the structure of the resulting [MAlkene]" ions (Scheme 7). Here again CA spectroscopy provided the answer. Whereas with compounds of the type (45),
X = CH2, the hydrogen transfer takes place exclusively to the
benzene ring [formation of ( 4 6 ) [ 2 5 7 ,ionized phenols ( 4 7 )
are formed almost exclusively from alkyl aryl ethers ( 4 5 ) ,
X = O[6'l[*l.In addition, it could be proved, by a combination
of field ionization kineticd (FIK) and CAr61bI, that the Htransfer in phenyl n-propyl ether (Scheme 7, R = CH3, X = 0)
is not preceded by H-scrambling in the side chain, but that
the hydrogen is transferred to the oxygen through transition
states of various ring sizes; on energetic grounds (lowest activation energy) a transition state with a five-membered ring is
favored (transfer of a hydrogen from the methyl group to
the oxygen).
X = CH2
7
(330)
(35)
136)
137)
Scheme 5. Alkyl elimination from radical cations of a$-unsaturated carbonyl
compounds (33 j , X = H, CH3, OCH3; R = alkyl.
The unusual fragmentation behavior of amides, especially
piperidides, of a$-unsaturated carboxylic
was also
clarified by CA investigati~ns[~*l.
According to the CA spectra,
the [M-R]'
ions of ( 3 8 ) exist partly as ( 4 0 ) ; formally
speaking the reaction route ( 3 8 ) --+ ( 4 0 ) is the first example
of an electron-impact-induced intramolecular Michael addition (Scheme 6). The [M-R]'
ions from enamino ketones
(41 ) serve as reference ions.
p] It proved possible to exclude complete H/D scrambling [52].
516
(47)
Scheme 7. Alkene elimination with hydrogen transfer from compounds ( 4 5 ) ,
X = CH,, 0 ; R = H, alkyl.
The question of the structure of the [M-COz]@' ions
from diphenyl carbonate ( 4 8 ) , and thus also of the course
of this extrusion reaction, could equally be answered by CA
mass
In agreement with the results of Beynon
et aL[64a],the CA results prove that [M-C02]'.
exists as
diphenyl ether ( 4 9 ) and not as phenylphenol (50). The differ[*] ICR investigations also show that the [M-Alkenele" ions from alkyl
phenyl ethers exist as ionized phenols [62].
Angew. Chem. Int. E d . Engl. 1 Vol. I5 (1976) No. 9
ences observed by Williams et al.[64bl in the unimolecular
dissociation of the metastable ions from ( 4 9 ) and [MCO,] *’ ions from (48) are thus not structure-determined
but result from different energy distribution functions that
are in principle to be observed when using MI characteristics.
It should also be noted here that so far it has not been
possible to differentiate and identify position-isomeric arenes
by collisional activation mass spectrometry, except for those
compounds that show pronounced ortho effects in conventional EI spectra: ( 4 9 ) and (50) give different and interpretable CA spectra, whereas the spectra of 0-,m- and p-phenylphenol are
vertible ions and the intact structure of the neutral molecules,
even mixtures of oligopeptides can be analyzed.
Derivatized
peptide
Rz
(55)
156/, mle = 6 1 (Rz= H)
(571, mje = 75 (R’ = CH3)
Scheme 8. Decomposition of immonium ions ( 5 5 ) rrom leucine ( R ’ = C H I ,
R 2= H) and isoleucine (R’ = H, R’ = CH3) fragments.
(49)
Finally it should be mentioned that CA spectroscopy in
combination with other methods (D-labeling, measurement
of appearance potentials, determination of Tki,)has contributed substantially to the clarification of the unusual elimination of ammonia from w-phenylalkylamines (51 )[651,a reaction that is of interest in connection with Meyerson’s concept
of “internal solvation of ions in the gas
. 0nly
( 5 2 ) is formed by elimination of NH3 from (51 a ) , whereas
(51 b) gives rise to several olefins, including ( 5 3 ) and (54),
in a complex reaction.
A completely new procedure for qualitative analysis of mixtures without their previous separation (e.g.by gas chromatography) has been proposed by Levsen and Schulten[”I. These
authors pyrolyzed a deoxyribonucleic acid, ionized the pyrolyzate with low electron energy (suppression of fragmentation),
and then measured the CA spectra of some selected ions.
By comparison with reference spectra they were able to identify, as pyrolysis products e. g . methanol, acetonitrile, propargyl
alcohol, furan, 2-methylfuran, furfuryl alcohol and a-angelica
lactone.
Electron impact in conjunction with collisional activation
has also been used by Kruger et al.[721for quantitative mixture
analysis. The authors claim that mixture analysis is possible
with an accuracy of +_5 % if an internal standard is used
and report a detection limit of 10- g.
Because of the multiplicity of fragmentations induced by
collisional activation and the fact that the dissociation reactions can be connected with the structure of the reactive
species, the CA method should supplement the information
from all mass-spectroscopic methods that produce exclusively
molecular or quasimolecular ions. The first encouraging works
in this direction shows that in combination with, e.g. field
ionization[’], electron-attachment mass spe~trometry[’~l
(investigation of negative ions), or chemical ionization[’d.441
it is possible to draw conclusions from fragmentation processes
about the structure of molecular ions that are stable under
the conditions of the ionization method, a feature that was
hardly ever possible from FD, FI, EA, or CI spectra alone.
’
(5Ia), n = 3
(Jilb), n = 5
0’
@H=CH-(CHZ)+~H)
-
1”’
OCH~-CH=CH-CH2-C&
153)
(54)
5. Analytical Applications
It is well known that, for example, effective and unequivocal
sequence analysis of oligopeptides by mass
is limited by the following factors: a) the vapor pressure
of the sample is too low, b) unspecific pyrolysis reactions
occur before the actual electron-impact-induced dissociation
and c) the fragmentation behavior of molecular ions and
fragment ions is ambiguous, which is a great disadvantage
particularly with leucine and isoleucine containing peptides.
Whereas the first two effects can be considerably mitigated
by use of field desorption mass spectrometry (FD-MS)[681,
the limitation due to c) can be reduced by CA spectroscopy:
the structural stability of the immonium ions and the characteristically different CA spectra (see also Section 3.2) serve as
a basis for the sequence determination in a peptide. Thus,
the CA spectrum of the ion (55) shows an intense fragment
( 5 6 ) at m/e=61 when the peptide derivative contains leucine,
whereas ifisoleucine is present the fragment (57) with m/e= 75
is formed (Scheme S)16’! In the authors’
on the
basis of the characteristic differences in the CA spectra of
other fragment ions, and of the fact that an unequivocal
correlation is possible between the CA spectra of non-interconAngew. Chem. I n t . Ed. Engl. J Vol. 15 (1976) No. 9
6. Critical Assessment and Prospects
In spite of the undisputed success of mass spectrometry
in the elucidation of the structure of organic compounds, it
is seldom possible by mass spectrometry alone to determine
unequivocally the structure of the ions formed, and then onlyif at all-with the aid of additional techniques. The collisional
activation method reviewed here constitutes a very promising
supplement to present-day techniques for structure determinations of ions in the gas phase. According to investigations
already at hand,.the method is reliable, the experimental technique is simple, and the data are easy to interpret. The method
shares with most other techniques the disadvantage of being
comparative. In the absence of suitable reference structures
recourse must therefore be taken to additional information,
e. g. fragmentation behavior, isotopic labeling, and measurement of the appearance potentials; in this connection, com517
bination of CA measurements with isotopic labeling is of
special importance.
Although at first the center of interest in CA investigations
lay in the structure of organic ions, it may be expected that
in future the method will be used more frequently to study
reaction mechanisms by clarification of the ion structures.
Although the great value of the method for solution of the
two problems (ion structures and reaction mechanisms) can
hardly be disputed, only the future will show whether the
analytical possibilities indicated above are of practical importance; for example, in view of the combination of gas chromatography with mass spectrometry, so widespread today, the
direct analysis of mixtures using the CA method will be limited
to special problems.
Finally, consider the instrumental side of the method. What
is particularly important to note is that all the conventional
magnetic instruments can be used for the measurement of
CA spectra, which is not the case with the ICR technique.
Instruments with the reverse Nier-Johnson configuration are
undoubtedly particularly advantageous. Collisional activation
mass spectrometry has a higher sensitivity than the competing
techniques (ICR and MI), but its limited resolution is a disadvantage. A decisive improvement in resolution was achieved
by most groups in 1975by increasing the acceleration potential
to 10kV. A further doubling of the acceleration potential
is conceivable and desirable. A possible alternative to instruments with the reverse configuration is offered by the “linked
'Obi, by means of which collisional activation studies
can also in principle be carried out with double-focussing
instruments of conventional geometry. Only the kinetic energy
released in a dissociation cannot be measured by this method.
The spread of a new analytical method depends further,
and decisively, on the commercial availability of the necessary instrument. Mass spectrometers with additional equipment for the determination of CA spectra have recently
been offered by two companies, so that it may be expected
that collisional activation studies will in future be carried
out routinely in organic mass spectrometry for the determination of ion structures and reaction mechanisms.
The authors should like to extend their sincere gratitude
to Professor F. W McLafferty, Cornell University ( U S A ) ,
who recognized and systematically studied the potential of collisional activation mass spectrometry for the determination of
the structure of organic ions, for his cooperation and for the
stimulating influence of his work.
Received: March 1 , 1976 [A 128 IE]
German version: Angew. Chem. 88,589 (1976)
Translated by Express Translation Service, London
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Predictive Molecular Orbital Calculations in Organic Chemistry
By Joseph J. D a n n e n b e r g [ * l
The utility and importance of predictive, rather than correlative, molecular orbital calculations
in organic chemistry are discussed. Examples of the predictive approach to organic synthesis
and the study of organic reaction mechanisms are presented.
1. Introduction
A cursory survey of the chemical literature will very quickly
demonstrate the increasing proportion dealing with theoretical
treatments of Organic Chemistry. The majority of articles
dealing with this subject can be properly described as theoretical explanations or rationalizations of known experimental
results. This type of work generally takes one of two forms:
either an article devoted to the explanation (often with great
insight) of certain reactions or categories of reactions, or what
often amounts to a theoretical section, added to an experimental paper, which purports to rationalize the results in an
acceptable manner. The latter type of discussion is necessarily
less detailed, in general, than the former, as it is rarely the
unifying force of the article. The increasing acceptance of
these theoretical organic studies testifies to the fact that increasing numbers of organic chemists are coming to accept molecular orbital theory as a part of their scientific world and as
a useful contribution to organic chemistry, in particular. Thus,
molecular orbital calculations are now generally accepted by
[*] Prof. Dr. J. J. Dannenberg
Department of Chemistry
Hunter College of the City University of New York
695 Park Avenue, New York, N. Y. 10021 (USA)
and
Centre d e Mkcanique Ondulatoire Appliquke C N R S
23, rue du Maroc, F-75019 Paris (France)
most organic chemists as being correlative with experimental
results. Nevertheless, many organic chemists continue to ask
a very reasonable question which can be stated as follows:
“How much d o we really benefit from calculations that simply
tell us that what we have already experimentally demonstrated
is correct?”
Implied in this question is the underlying suspicion that
the theoretician might not have arrived at the correct result
had he not known it at the outset. An example is shown
in Figure 1 which is the representation of a lecture slide[’]
I
0
V
I
-Theoretically derived value
Experimental value
----
Lm1910
1920
1930 1940 1950 1960
1970
Fig. I. The experimental and theoretically acceptable values for the dipole
moment of carbon monoxide m. time.
519
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