J. Phys. Chem. 1996, 100, 16817-16821
16817
Collision-Induced Dissociation of Vanadium-Carbon Cluster Cations
K. P. Kerns, B. C. Guo, H. T. Deng, and A. W. Castleman, Jr.*
Department of Chemistry, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802
ReceiVed: May 14, 1996X
Collision-induced dissociation (CID) studies of vanadium-carbon clusters were made employing a triple
quadrupole mass spectrometer system coupled with a laser vaporization source. The results reveal that the
primary dissociation channel is loss of a metal atom for all but V8C14+, which loses C3, and V9C14+, which
loses both V and VC2. These findings are in general agreement with earlier ones reported for the titanium
system, except that under single-collision conditions V8C14+ loses a C3 unit to become V8C11+, while Ti8C14+
loses Ti. Importantly, we show that both the met-car V8C12+ and V8C13+ are significantly more resistant to
dissociation than the neighboring V8C11+ cluster species. In addition to reporting the primary fragmentation
products of several VxCy+ clusters, we also present the results of studies of the multiple-collision dissociation
patterns of both V8C12+ and V8C13+, which are observed to undergo C2 and C3 loss after some metal loss has
occurred. These findings are consistent with the building patterns observed for these clusters and our proposed
structure for V8C12+. Through study of the dissociation of V9C12+, the ionization energy of V8C12 is found
to be less than that of the vanadium atom, i.e., less than 6.74 eV, in accord with theoretical predictions.
1. Introduction
In early 1992, our group reported the discovery of a stable
metal-carbon species with the stoichiometry M8C12+ (M ) Ti,
V, Zr, and Hf).1-3 These metallocarbohedrenes, or met-cars,
have generated considerable interest, and both experimentalists
and theoreticians are striving to help determine their extract
structure and properties. Following the original work, we
therafter established the stability of Nb8C12+,4 and Duncan and
co-workers observed the existence of met-cars containing iron,
chromium, and molybdenum.5 A structure resembling a pentagonal dodecahedron containing 12 faces and having Th
symmetry in its nondistorted configuration was originally
proposed to account for the findings, though various other
configurations have been predicted on the basis of theoretical
calculations;6-26 all findings point to a cagelike structure.
Many dissociation studies have been undertaken in order to
further elucidate the uniqueness of met-cars. Photodissociation,27 collision-induced dissociation,28 and metastable decay
experiments29,30 have all provided strong evidence for the unique
stability of M8C12+ species. Most recently, studies of the
reactivities of met-cars,31-34 along with the discovery of binary
metal met-cars35-37 in our laboratory, have stimulated even
more interest in this new family of molecules.
In order to provide further evidence for the physical stability
and structure of met-cars compared to proximate species with
differing carbon contents and to compare the vanadium and
titanium met-cars,28 we conducted collision-induced dissociation (CID) studies of vanadium-carbon clusters. Although an
analysis of the absolute dissociation energies for large, tightly
bound systems is always problematic because of the kinetic shift
involved, the purpose of the present studies is to further
investigate the stability of the met-car V8C12+ by studying its
dissociation properties and comparing its relative CID threshold
value with that of other vanadium-carbon clusters. Specific
comparisons are made with one that is carbon-deficient, namely
V8C11+, and another that is a carbon-rich V8C13+ species; the
latter is expected to have a completed cage structure similar to
that of the met-car V8C12+,29 while the former is expected to
X
Abstract published in AdVance ACS Abstracts, October 1, 1996.
S0022-3654(96)01397-4 CCC: $12.00
have a noncomplete cagelike structure.38 Finally, a study of
the CID of V9C12+ provides an upper limit of the ionization
energy of V8C12.
2. Experimental Section
The apparatus used in this work is a triple quadrupole mass
spectrometer coupled with a laser vaporization source, a diagram
of its construction and details of its operation are described in
another paper.28 Briefly, the 532 nm second harmonic of a Nd:
YAG laser is tightly focused onto a rotating and translating
vanadium rod. Pulses of a methane/helium mixture are directed
over the locally hot rod surface. The metal-carbon clusters
are formed upon rapid dehydrogenation of the CH4 in the gas
mixture and are cooled as they undergo supersonic expansion.
Thereafter, the cluster ions pass through the first set of ion lenses
and deflectors and into the first quadrupole mass spectrometer
where a particular VxCy+ cluster of choice is mass-selected for
further analysis by collision-induced dissociation. The second
group of ion lenses and deflectors guide the chosen cluster cation
into the second quadrupole mass spectrometer that serves as
the collision cell. After the selected ions undergo the desired
number of collisions with krypton, the selected and dissociated
ions drift out of the collision cell and enter the third quadrupole
mass spectrometer. The final product distribution is thereby
analyzed and detected with a channeltron electron multiplier.
Since the effects of cluster internal energy are potentially
critical in the CID threshold studies, we have undertaken
experiments under various source conditions. For example, we
have varied the methane concentration within a range of 1523%, source backing pressure from ca. 4 and 6 atm, and laser
power from 5 to 20 mJ/pulse. Further implications of varying
the source conditions are discussed later.
As mentioned previously,28 the CID threshold energies were
obtained at essentially single-collision conditions by keeping
the cell pressure at or below 0.10 mTorr during these experiments. Based on considerations of the hard sphere and Langevin
collision cross sections of V8C12+ with krypton, the mean free
path was found to be slightly higher than the cell length (25
cm), which greatly diminishes the probability of multiple
collisions. Some studies were purposely conducted at higher
© 1996 American Chemical Society
16818 J. Phys. Chem., Vol. 100, No. 42, 1996
Kerns et al.
TABLE 1: Collision-Induced Dissociation Products of
VxCy+
VxCy+
mass
CID product(s)
neutral(s) lost
(6,8)
(6,9)
(6,10)
(6,11)
(7,9)
(7,10)
(7,11)
(7,12)
(8,10)
(8,11)
(8,12)
(8,13)
(8,14)
(9,11)
(9,12)
(9,13)
(9,14)
402
414
426
438
465
477
489
501
528
540
552
564
576
591
603
615
627
(5,8)
(5,9)
(5,10)
(5,11)
(6,9)
(6,10)
(6,11)
(6,12)
(7,10)
(7,11)
(7,12)
(7,13)
(8,11)
(8,11)
(8,12)
(8,13)
(8,14)
(8,12)
V
V
V
V
V
V
V
V
V
V
V
V
C3
V
V
V
V
VC2
pressures to investigate dissociation under multiple-collision
conditions for purposes of comparison.
3. Results and Discussion
Vanadium-carbon clusters have been studied by collisioninduced dissociation, and observations of their dissociation
patterns are discussed herein. Table 1 lists the primary collision
products of the various vanadium-carbon clusters studied under
single-collision conditions. As seen in Table 1, the majority
of the VxCy+ clusters studied here are smaller than the metcar V8C12+, and their primary dissociation patterns mainly result
in loss of a neutral vanadium atom. These results are quite
consistent with previous metastable decay studies of VxCy+
clusters.30 Observation of metal loss in most of these clusters
provides evidence for strongly bonded carbon units in the
formation of these species, although consideration must be given
to the resulting disrupted cage structure in the multiple loss
process.
Upon increasing the pressure of the collision gas, which
serves to increase the number of collisions between the selected
cluster ion and the krypton, it is found that multiple (probably
sequential) metal atom loss from V8C12+ is the preferred
channel. Figure 1a-c shows the CID patterns of the met-car
V8C12+, all at a constant collision energy of 100 eV, but with
varying pressures of krypton. The peaks labeled x/y in these
spectra correspond to VxCy+. The reason for the dominance of
metal loss is evidently the strong carbon-carbon bonding in
the completed cage structure.3,37 It should be noted that these
fragmentation patterns are quite similar to those observed for
the met-car Ti8C12+.28
A detailed study of the primary CID patterns of the met-car
V8C12+ enabled a determination of the approximate dissociation
threshold. The purpose of the work was to enable a comparison
of the relative values for the met-car with that of the proximate
species V8C11+ and V8C13+. The dissociation threshold, or the
upper limit of the bonding energy of the met-car, is estimated
experimentally by keeping the collision gas pressure constant
at a low value (approximately 0.1 mTorr), while varying the
collision energy by small increments.28 Specifically, we study
the 50-120 eV range for V8C12+. It is determined that the
minimum lab-frame collision energy needed to fragment the
met-car V8C12+ is close to 65 eV. The lab-frame energy is
converted to center-of-mass energy by the following equation:
Ecm ) Elab[N/(N + P)]
Figure 1. Collision-induced dissociation spectra of V8C12+ at a krypton
pressure of (a) 0.11, (b) 0.34, and (c) 0.68 mTorr, all at 100 eV collision
energy.
Figure 2. Plot of collision energy vs fragment intensity of V8C11+,
V8C12+, and V8C13+. The adjustable parameters, E0 and n, are optimized
to 65 and 2.5, respectively, for both V8C12+ and V8C13+. The best
curve-fitted values are 45 and 2.5 for V8C11+. The lines represent the
best curve-fitted values over the entire range of energies studied for
V8C11+ (solid), V8C12+ (solid), and V8C13+ (dash). Note that both
V8C12+ and V8C13+ are significantly more stable than V8C11+ in terms
of their CID thresholds.
where N is the mass of the neutral collision gas and P is the
mass of the selected ion, both in units of amu. The masses of
V8C12+ and Kr are 552 and 84 amu, respectively. In order to
approximate the CID threshold, we use an extrapolation
procedure similar to the method used by Armentrout and coworkers.38 Specifically, we determine the CID fragment
intensities at a constant low pressure, but under a wide range
of collision energies.28 The fragment intensities are plotted as
a function of collision energy, and these results are graphically
presented in Figure 2. The extrapolated CID threshold value,
however, is determined by a curve-fitting procedure using the
following equation:39
σ ) σ0(E - E0)n/E
where E is the ion translational energy, E0 is the threshold
energy, and σ0 is an energy-independent scaling factor. From
this equation, the adjustable parameters are varied extensively
until a “best fit” curve of the data points is obtained. This curve
is extrapolated to zero fragment intensity by the curve fitting
Dissociation of Vanadium-Carbon Cluster Cations
program, resulting in a lab-frame CID threshold value of
approximately 65 eV for V8C12+. The corresponding centerof-mass energy is thereby determined to be slightly less than 9
eV, which is comparable to the value obtained for Ti8C12+.28
We have reported previously that this value is probably an
overestimate due to the kinetic shift; various theoretical calculations estimate the bonding energy to be close to 6 eV.9,12,19
However, the observation of a large value for the dissociation
threshold is consistent with a multiply bonded metal atom
(cagelike) structure of met-cars and provides evidence for higher
physical stability of the met-car V8C12+ compared to other
smaller vanadium-carbon clusters, such as V8C11+ whose
dissociation behavior is discussed below.
It has been mentioned previously by Armentrout40 that
measurements of the absolute CID threshold can be affected
by the internal energy of the clusters produced.41 Over the
course of these experiments, we find that excess internal energy
in the clusters is not significant, as we have widely varied our
source conditions (laser power, backing pressure, and CH4
concentration), without observing any significant changes in CID
threshold values. Our findings of clusters cooled sufficiently
to eliminate a dependence on the above variables could also be
due to one or both of the following: (1) the long time scale
between ion production and study in the collision cell and/or
(2) the source conditions. These factors are further considered
in what follows.
One parameter that influences the internal energy of the
cluster is the time between its production and its study. The
time between production and analysis differs significantly
between experiments made in a time-of-flight (TOF) apparatus,
which is on the order of tens of microseconds, and our triple
quadrupole apparatus, which can range from 300 to 500 µs.
Due largely to this time scale difference, we have observed
enhanced “magic number” stability for Ti8C12+ 1 on our apparatus, and the longer times in the present apparatus enable us
to carry out reaction and CID studies after substantial internal
energy cooling has been achieved. Further evidence of the latter
is the lack of appreciable metastable dissociation in the present
apparatus compared to that seen in TOF experiments. Other
observations also support the conclusion that the clusters studied
in this apparatus are relatively cold. In other related experiments
made with this source which employs a “waiting-room”
design,42,43 we are also able to observe weakly bound ligated
clusters, such as M+-N2 and M+-CH4.
Also, we have found previously that both cluster species and
their relative abundances on this apparatus can change by
varying the source conditions.4 Very recently, we found that
the species Ti8C12+(L) (L ) I, C6H6, CH3OH, etc.) can be
produced directly from our laser vaporization source by adding
a low concentration of L (about 1%) to the 15-20% CH4 diluted
in helium buffer gas.34 The Ti8C12+-polar molecule bonding
is expected to occur via comparatively weak electrostatic
interactions, and these clusters would not be expected to be
formed unless the clusters were already relatively cool. It is
expected that the high CH4 concentration may lead to efficient
quenching of the metal-carbon clusters, which is perhaps
another reason why Ti8C12+(L) is able to be formed in the
source. For example, Armentrout and co-workers44 and MacTaylor et al.45 have shown that CH4 can quench the excited
states in the ion formation process of transition metal species
much more effectively than buffer gases such as He and Ar
alone. Numerous collisions with the quenching gas CH4 can
occur after Ti8C12+(L) and other M8C12+ species are formed
before supersonic expansion.
J. Phys. Chem., Vol. 100, No. 42, 1996 16819
In addition, we also observe similar reaction pathways of
Ti8C12+ with reactant molecules, even weakly bonded polar
molecules, in both the second quadrupole33,34 and a highpressure ion drift cell.31 The similar reactivities observed for
the formation of Ti8C12+(L)n species in the source, the second
quadrupole collision cell, and the ion drift cell suggest that the
clusters are likely to be at nearly the same effective temperature
in all three situations. Taken together, these observations offer
strong evidence for the fact that the clusters are likely to be
relatively cool.
Further CID threshold experiments were undertaken on the
carbon-deficient cluster V8C11+ and the carbon-rich cluster
V8C13+. In past studies we have obtained some evidence
suggesting that the M8C13+ species is similar to M8C12+, but
with an endohedral carbon atom.29 The purpose of these
experiments is to gain some insight into the relationship between
the estimated dissociation energy and the predicted geometric
structure of the cluster. We have mentioned previously28 that
our absolute threshold values may be overestimated due to
kinetic shift. Despite the fact that the CID threshold values
contain some uncertainty on an absolute scale, we conducted
experiments to obtain the relative threshold values by comparing
the met-car V8C12+ to both V8C13+ and V8C11+. As presented
below, we observe a significant difference in the estimated
dissociation energies as related to predicted cluster structure.
These variations with composition are important and provide
valuable information beyond knowledge of the absolute dissociation energies which are not the focus of the present study.
Figure 2 shows the plot of collision energy vs fragment
intensity of V8C11+, the met-car V8C12+, and V8C13+. It is
observed that both the met-car V8C12+ and V8C13+ are significantly more stable than V8C11+, with their lab-frame CID
threshold values about 15-20 eV higher than that of V8C11+,
which corresponds to roughly 2-2.5 eV higher in terms of Ecm.
The lab-frame threshold of V8C12+ is only about 5 eV higher
than that of V8C13+. Some error is introduced into the threshold
calculation of V8C11+ due to the fact that signal intensity below
50 eV becomes much weaker, although we still see fragmentation to V7C11+ at 50 eV. As a result, we estimate that the CID
threshold of V8C11+ is somewhat lower than 50 eV as we
approach this experimental limitation. In addition, we can even
observe a second metal loss from V8C11+ under conditions
similar to those employed in determining the CID threshold of
V8C12+. In other words, there is enough collision energy present
under these conditions to initiate loss of two metal atoms from
V8C11+, although a single metal loss channel is more dominant.
Although the nature of the bonding in V8C11+ (and smaller
VxCy+ clusters) is not certain, from these observations we predict
that the metal atom sites adjacent to the missing carbon atom
are most susceptible to dissociation in this cluster species.
Through ion chromatography experiments,38 Bowers and coworkers have shown that the met-car Ti8C12+ and their
neighboring clusters have similar cagelike structures. The
results of the present experiments suggest that V8C13+ has the
V8C12+ structure as its “backbone”, leading it to a similar CID
threshold. Although we do not observe the same degree of
enhanced “magic” stability for V8C12+ as we do for Ti8C12+ in
the total ion distribution of these metal-carbon clusters,1 these
experimental findings provide strong evidence that the structural
stability of the met-car V8C12+ is indeed significantly greater
than V8C11+.
Figure 3 shows the consequence of a multiple CID experiment
where the mass spectrum of V8C12+ at both high collision energy
and high krypton pressure is displayed. Although loss of several
metal atoms from V8C12+ dominates in this dissociation process,
16820 J. Phys. Chem., Vol. 100, No. 42, 1996
Kerns et al.
Figure 3. CID spectrum of V8C12+ at 0.98 mTorr of Kr and 160 eV
collision energy.
Figure 4. CID spectrum of V8C13+ at 0.32 mTorr of Kr and 200 eV
collision energy.
it is seen that several carbon units are also lost. Regarding
carbon, C2 is the main loss channel observed in this spectrum,
but some C3 loss is seen as well. These results are quite similar
to the multiple dissociation process observed28 for Ti8C12+;
however, we observe that C2 loss occurs at an earlier dissociation
stage for V8C12+ than for Ti8C12+. This is evident by the fact
that V7C12+ can lose C2 to form V7C10+, whereas Ti7C10+ is
not observed as a fragment from Ti7C12+. For multiple
collisions of Kr with V8C12+, both C2 and C3 loss becomes
dominant after dissociation of V6C12+; however, for Ti8C12+,
the same pattern does not become evident until below Ti5C12+.
Concerning the fragmentation patterns of both V8C12+ and
Ti8C12+, another noticeable feature is the common occurrence
of metal-carbon cluster ratios at or near 1:2 (i.e., M6C12+,
M5C10+, M4C8+). These products are in agreement with the
results of Wei et al.,3 who first reported the abundance and
importance of MC2 units as the main building blocks in the
formation of metal-carbon clusters. The present results provide
further evidence for the met-cars being comprised of some stable
C2 units, which in turn is consistent with a structure such as
the one we originally proposed.1-3,31
Collision-induced dissociation experiments on V8C13+ are also
of interest since they could provide some insight into whether
the extra carbon attributed to the very stable V8C12+ molecule
is either outside or inside the cage. As shown in Table 1, we
observe that V8C13+ does indeed behave like V8C12+ by losing
a neutral vanadium atom to become V7C13+ as a primary
fragmentation product. Interestingly, we find that V8C13+ is
quite resistant to fragmentation, and its dissociation properties,
in terms of energetics, are more similar to the met-car V8C12+
than to V8C11+, as mentioned previously and shown in Figure
2. This is in agreement with metastable decay results on
vanadium-carbon clusters.30 This result is not surprising since
both V8C12+ and V8C13+ are predicted to have similar completed
cage structures, unlike V8C11+. In these vanadium-carbon CID
studies, however, we have also studied the dissociation process
for V8C13+ under multiple-collision conditions; see Figure 4.
Although metal loss is quite dominant for the first two steps,
both C2 and C3 loss become more evident beginning at V6C13+;
in fact, C3 loss appears to be slightly more dominant than C2
loss at this point. Interestingly, no single carbon loss is observed
during the first three steps of the fragmentation of V8C13+. The
fragmentation channels are quite similar for both V8C13+ and
the met-car V8C12+, and these multicollision CID results are
consistent with the structures proposed for these two clusters.
Vanadium-carbon cluster ion fragmentation patterns may be
compared to the metastable decay results obtained in another
study in our laboratory. The prior metastable dissociation
studies29,30 focused on the dissociation of TimCn+, VmCn+, and
NbmCn+. The results obtained by the reflection time-of-flight
mass spectrometer study of both the titanium and vanadium
systems agree quite well with the present data; however, there
are differences in the products observed upon dissociation of
the V8C11+ and V9C14+ clusters. V8C11+, along with other
metal-rich clusters studied on the present apparatus, lose a
neutral metal atom to form a Mx-1Cy+ daughter product;
however, the metastable decay product loses a C3 unit to form
V8C8+. It is possible that there may be other isomers of this
cluster ion, but the ionization process30 may also alter the
structural and energetic properties of V8C11+. The other
difference involves the fragmentation products of V9C14+. The
present experiments show loss of both V and also VC2 units to
form V8C12+; however, the metastable decay results show the
loss of a VC unit to form V8C13+. Since there may be several
isomers for these species, their fragmentation patterns are
expected to show some differences. In addition, we find that
the primary step CID processes of vanadium-carbon clusters
are quite similar to those of titanium-carbon clusters. The only
obvious exception appears to be for M8C14+, for which Ti8C14+
loses predominantly a metal atom to become Ti7C14+, while
V8C14+ loses a C3 unit to become V8C11+. It may be that the
two M8C14+ species have different isomeric structures, similar
to the case of M8C13+, but electronic structure may also be the
reason for this difference in behavior.
In the present experiments, we also conduct CID experiments
of V9C12+ with the objective of gaining insight into the
ionization energy (IE) of V8C12 relative to the vanadium atom.
Since fragmentation of V9C12+ proceeds via loss of a neutral
vanadium atom rather than a vanadium cation, we can conclude
that the IE of V8C12 is lower than the IE of the vanadium atom,
Dissociation of Vanadium-Carbon Cluster Cations
or 6.74 eV.46 We have shown that the lack of observation of
V+ is not due to low collection efficiency at low mass, which
may obscure this peak. As shown in Figure 1c for the
dissociation of V8C12+, V+ appears at higher pressures, under
which conditions collection efficiency is substantially less than
at the lower pressure. The result of a low IE for V8C12 is
consistent with theoretical studies,6,9,20 which predict IE’s of
approximately 5-6 eV for various met-cars. In addition, the
previously estimated IE of the met-car Ti8C12 in our CID
studies28 is also in accord with findings of a low ionization
energy for V8C12 in the present work. Recently, experimental
results by Duncan show that the IE of the met-car Ti8C12 is
about 5 eV.47 The observations of such low IE’s for these metcars are consistent, and further experiments to pinpoint the IE’s
of met-cars and other metal-carbon clusters would be of value.
In addition, it should be noted that several other vanadiumcarbon clusters, as listed in Table 1, also have lower ionization
energies than vanadium.
Freiser and co-workers have reported experiments involving
the reactivities of V8C12+ by FTICR techniques.48 They show
that the met-car V8C12+ is capable of inducing dehydrogenation
and breaking bonds of certain reactant molecules. We have
also observed increasing chemical reactivity of another group
V met-car, Nb8C12+, as opposed to the group IV met-car
Ti8C12+, during reactions of these species with acetone.33 Other
unpublished work from our laboratory further shows that both
V8C12+ and Nb8C12+ are quite active in inducing bond breaking,
unlike Ti8C12+, which mainly associates reactant ligands.31,34
Although both V8C12+ and Nb8C12+ appear to be more chemically reactive than Ti8C12+, the physical stability of V8C12+ does
not display much difference as supported by these CID studies.
These properties further establish the M8C12+ species as a family
of stable molecular clusters displaying rich chemistry, as
provided by their novel reactivities.
4. Conclusions
Collision-induced dissociation of various vanadium-carbon
cluster cations, including the met-car V8C12+, was studied using
a triple quadrupole mass spectrometer equipped with a laser
vaporization source. Most VxCy+ clusters dissociate by loss of
a neutral vanadium atom during the primary step dissociation.
Metal loss also dominates under multiple-collision conditions
for both V8C12+ and V8C13+, but fragmentation by loss of C2
and C3 units occurs as well. The dissociation threshold of
V8C12+ is found to be slightly below 9 eV, which is comparable
to the corresponding value for Ti8C12+, and provides further
evidence regarding the stability of V8C12+. More importantly,
from these studies, both the met-car V8C12+ and V8C13+ are
shown to be appreciably more stable than V8C11+ as revealed
via their display of a much higher CID dissociation threshold.
The value for V8C12+ and its multiple-collision fragmentation
patterns provide further evidence that met-cars are quite stable
and have multiply bonded metal atoms, and contain C2 units,
such as those in our proposed structural model. Finally, the
observation of a low ionization energy for V8C12 is in accord
with previous theoretical studies.
Acknowledgment. Financial support by the Air Force Office
of Scientific Research, Grant F49620-94-1-0162, is gratefully
acknowledged.
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