Collision-Induced Dissociation of Vanadium−Carbon Cluster Cations

J. Phys. Chem. 1996, 100, 16817-16821

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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

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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,

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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. References and Notes (1) Guo, B. C.; Kerns, K. P.; Castleman, A. W., Jr. Science 1992, 255, 1411. (2) Guo, B. C.; Wei, S.; Purnell, J.; Buzza, S. A.; Castleman, A. W., Jr. Science 1992, 256, 515.

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