J. Phys. Chem. 1996, 100, 8175-8179
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Formation, Structure, and Stabilities of Metallocarbohedrenes S. F. Cartier,† B. D. May, and A. W. Castleman, Jr.* Department of Chemistry, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed: NoVember 22, 1995; In Final Form: February 21, 1996X
Time-of-flight mass spectrometric investigations of the formation of metallocarbohedrenes and binary metal metallocarbohedrenes under both direct laser vaporization and laser vaporization/molecular beam conditions were conducted. A wide range of Ti-to-carbon ratios and in other experiments a range of Ti-to-Zr molar ratios were explored. These new studies provide further insight into the formation process of Met-Car clusters and, moreover, serve to eliminate certain suggested formation mechanisms such as one involving the attachment of metal atoms to a preexisting core consisting of a 12-atom carbon cage. There is no evidence in the experiments to suggest that Met-Cars produced via direct laser vaporization exist in a higher energy configuration than those formed in a laser plasma and then subjected to cooling in a molecular beam expansion. Mass spectral distributions of binary metal metallocarbohedrenes produced under widely varying conditions provide evidence that there is one set of equivalent metal sites in the structures produced under both low- and high-energy conditions. Finally, the stability and bonding of metallocarbohedrenes are interpreted in terms of known chemistry for early transition metals.
Introduction The discovery of metallocarbohedrene clusters (Met-Cars) by our group in 19921 has been followed by a number of experimental2-5 and theoretical6-14 reports which suggest that Met-Cars have a caged structure. While the exact nature of this structure is yet to be determined, the general consensus is that Met-Cars are a family of caged species composed of a metal-carbon network. Among these proposed structures are the pentagonal dodecahedron structure of Th symmetry originally proposed by Guo et al. upon the discovery of Met-Cars.1 Observation that the M8C12+ species associatively “take up” a maximum of eight polar molecules such as water, ammonia, and methanol provided strong evidence that Met-Cars have eight equivalent, exposed metal sites. More recently, this structure has received considerable experimental substantiation.5,15 In the proposed dodecahedral structure, the cage is comprised of 12 five-membered rings, with each ring comprised of an M2C3 unit. Alternatively, the cage can be considered to consist of a cube defined by the eight metal atoms with one ethylene-like C2 unit sitting above each face. Each metal atom is σ-bonded to three carbons, and there is no metal-metal bonding. Thus, the metals are formally in the M(III) oxidation state with a coordination number of 3. An alternative structure which has received considerable theoretical attention, and limited experimental substantiation, is the tetracapped tetrahedron of Td symmetry (and the closely related structure of D2d symmetry). Originally proposed by Dance,14 Be´nard and co-workers have recently conducted a series of studies on this structure.16 The tetracapped tetrahedron structure can be deduced from the pentagonal dodecahedron by a (45° rotation of the opposite C2 units with a butterfly folding of each face of the Ti8 cube. From this rearrangement, two distinct sets of metal atoms arise. One set of metal atoms forms an “outer” tetrahedron (THN) of metal atoms, each σ-coordinated to three acetylene-like C2 units (coordination number 3). The remaining four metal atoms assemble into an “inner” tetrahedron (thn) in which each metal is surrounded by three
C2 units displaying side-on coordination (coordination number 6). The electronic structure of these “inner” metal atoms is such that they take an active part in the back-donation interaction toward the π* orbitals of the acetylenic C2 units. Several other potential structures have been suggested for the metallocarbohedrenes as well. In particular, Pauling10 proposed a cubic structure with eight metal atoms at the vertices of the cube and six C2 pairs in the cube faces. This structure is a variant of the originally proposed structure of Th symmetry in which the C2 units are sitting aboVe the faces of the metal cube. Also, Kahn has reported the results of theoretical studies suggesting the structure of Ti8C12 is a metal decorated cage (MDC) in which eight titanium atoms are arranged around a 12-atom carbon cage.17 Stabilities and bond energies were reported for two different isomeric arrangements for the eight metal atoms. The cubic isomer, consisting of eight metal atoms located at the corners of a distorted cube, and the noncubic isomer, which is comprised of eight titanium atoms forming a bicapped trigonal antiprism, are the proposed metal shell structures which encapsulate the C12 cage. Among the various experimental investigations of Met-Cars, mass spectrometric studies have provided evidence for their structure,5,15,18 formation mechanism,19 and stabilities.20 In particular, Wei et al.19 conducted time-of-flight mass spectrometric investigations of the formation of Met-Cars and reported evidence which suggests that the formation of a pentagonal dodecahedron structure occurs via the successive addition of TiC2 units. Herein, results are presented which further rule out the possibility that Met-Cars are formed via attachment of metal atoms to a C12 cage as has been suggested recently.17 As part of the present work, binary metal Met-Car species have been produced from a range of molar mixtures of titanium carbide and zirconium powders. The implications of these results for the possible Met-Car structure are discussed. Lastly, the stabilities and bonding of Met-Cars are considered in terms of both the new findings for the Ti/Zr boundary species reported here and our previously reported work on other binary systems.21 Experimental Section
†
Present address: Chemistry Department, Colgate University, Hamilton, NY 13346. X Abstract published in AdVance ACS Abstracts, April 15, 1996.
S0022-3654(95)03438-1 CCC: $12.00
The instrument employed in these investigations of the production of both single metal metallocarbohedrenes (Met© 1996 American Chemical Society
8176 J. Phys. Chem., Vol. 100, No. 20, 1996 Cars) and binary metal metallocarbohedrenes consists of a cluster source coupled to a reflectron-equipped time-of-flight mass spectrometer. Although the ion detection scheme is the same for all the experiments described here, several cluster sources have been employed. The production of metallocarbohedrene clusters from metal/graphite mixtures and titanium carbide/metal mixtures is accomplished by the direct laser vaporization of compressed powder targets. In addition, binary metal Met-Cars are generated in a laser-induced plasma reactor and subsequently undergo supersonic expansion into the extraction region of the time-of-flight mass spectrometer. Direct Laser Vaporization. The source in the direct laser vaporization experiments consists of a boron nitride sample holder containing the loosely packed powder mixture, which is placed below the center of the TOF extraction region. Samples used include Ti (-325 mesh, 99%)/graphite (1-2 µm) and TiC (2.5-4 µm, 99%)/Zr (-325 mesh, 99%), from which mixtures of desired composition were prepared. The vaporization is accomplished by continually rastering across the sample the second harmonic output (532 nm) of a Nd:YAG (Spectra Physics DCR-1) laser operated at approximately 7 mJ/pulse. This laser is directed perpendicular to both the sample surface and the applied electric fields and focused to a spot size of slightly less than 1 mm onto the target by a 1 m focal length lens. During the analysis of ions created during the vaporization, fast rising (∼80 ns), high-voltage pulses are applied to the repeller and extraction electrodes. These pulses are generated by a fast, high-voltage transistor switch (Behlke HTS 81) that provides both short rise time and very stable output pulses. Laser-Induced Plasma Reactor. Alternatively, a laserinduced plasma reactor, which has been described in detail elsewhere,22 is mounted externally to the vacuum chamber. It consists of a stainless steel block intersected at its center by three perpendicular channels: the target rod channel, the laser irradiation channel, and the reaction channel. The target rod channel houses a 1/4 in. metal rod which translates and rotates under the control of a 1 rpm motor. The target rod in these studies is a 10:1 molar mixture composite rod of titanium and zirconium. During the experiments, the focused output of the vaporization laser is directed through the laser irradiation channel and impinges upon the rod’s surface. Under typical operating conditions, the vaporization laser (Nd:YAG, second harmonic, 532 nm), which provides 25 mJ/pulse, is focused to approximately 1 mm with a 1 m focusing lens. Pure CH4 (70 psi backing pressure) is used as the reactant/carrier gas. Continual translation and rotation of the rod assures a fresh surface with every laser shot. A General Valve, Series 9 pulsed valve is mounted on the end of the source block. The appropriately timed carrier/reactant gas pulse from this valve expands into the reaction channel and flows over the surface of the rod. As the gas pulse flows over the rod, the vaporization laser is fired, and reaction occurs amidst the gas and metal vapor. Following the vaporization/reaction event, the generated plasma cools as it travels through the reaction channel and expands through a 15° half-angle nozzle into the first of two differentially pumped chambers. Following expansion into the vacuum chamber, and upon entering the ionization region of the TOF lens, the neutral clusters are ionized with the third harmonic of a second Nd: YAG laser. The ionization laser beam is collimated with a 2 mm pinhole for the experiments reported here. The pulse energy is measured by a power meter placed before the Brewster’s angle input window, both prior to and after collecting a file at a particular pulse energy. The laser power is adjusted by varying
Cartier et al. the lamp energy, and the laser pulse is monitored with a photodiode which triggers the oscilloscope. Detection Scheme. Following extraction of the clusters within the TOF lens, any misalignment of the source and detector is compensated with a pair of deflection plates. After being focused with an Einzel lens, the clusters drift field free for approximately 2 m where they are reflected by a reflectron (reflection angle of 4°). The reflectron is mounted in a diffusion pumped chamber on a variable position stage. For the experiments reported here, the reflectron is operated in the focusing mode by placing voltages on the second (2000 V) and last (3400 V) rings, while maintaining the first at ground potential. Finally, the cluster signal is detected by an MCP (Galileo) placed 1 m from the reflectron. The MCP is suspended in a chamber pumped by a cryogenic pump which maintains an operating pressure of 2 × 10-6 Torr. Signal is collected on a 175 MHz digital oscilloscope (Lecroy 9400A) which is GPIB interfaced of a 486 PC. Timing and control are accomplished with a locally built digital pattern generator. Results and Discussion Production of Ti8C12+ from Titanium/Graphite Mixtures. Parts a-e of Figure 1 are mass spectra obtained from the direct vaporization of 1:1, 4:1, 6:1, 8:1, and 10:1 molar ratio mixtures of titanium and graphite, respectively. These spectra show a progression of increasing Met-Car intensity as the molar concentration of titanium in the mixture is increased. For the 1:1 molar ratio sample, the assignment of Ti8C12+ at 528 amu is somewhat speculative due to the unusual presence of cluster species at higher masses. Fullerenes such as C60 and C70 are not observed, and thus it is unlikely that the species indicated is a pure carbon cluster, i.e., C44. The mass degeneracy of four carbons and one titanium, in addition to the weak signal intensity, precludes definitive identification of this mass. However, for the titanium-rich samples, the mass spectra are strikingly similar to those of the pure carbide,23 and the definitive assignment of Ti8C12+ can be made. (Note that the identity of this cluster has been definitively established through the use of isotope substitution.1) The production of Met-Cars upon vaporization of mixtures of titanium and graphite powders further supports our previous claim21a for the formation of Met-Cars in the plume above the vaporized sample. Additionally, these results suggest that the proposed formation of Met-Cars via the formation of a C12 cage followed by the successive addition of metal atoms to this cage17 is incorrect. The spectra shown here, and all of those previously reported by our group, have never displayed any evidence of C12 formation. Under high-resolution conditions, Wei et al.19 showed that the only species present at this mass (144 amu) is Ti2C4, which is an important building block of the Ti8C12 cluster. Moreover, since the conversion of the metal-carbon species to Met-Cars is not complete, if Met-Cars were forming via the attachment of metal atoms to the C12 cage, we would expect to obtain some evidence of intermediate products from this mechanism in the mass spectra corresponding to the successive addition of metal atoms, i.e., TiC12, Ti2C12, Ti3C12, etc. Even when the samples are 10 times richer in titanium than carbon (Figure 1e), there is no observation of this suggested growth pattern. Rather, we continue to observe the typical broad distribution of titanium/carbon clusters, and the spectra can be readily explained using the concepts originally proposed by Wei et al. Binary Metal Met-Cars of Titanium and Zirconium. Direct laser vaporization of 2:1, 4:1, 8:1, and 12:1 molar mixtures of titanium carbide and zirconium results in the
Formation of Metallocarbohedrenes
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Figure 1. Mass spectra of TixCy+ clusters from the direct laser vaporization of (a) 2:1, (b) 4:1, (c) 6:1, (d) 8:1, and (e) 10:1 molar mixtures of titanium and graphite powders.
formation of binary metal Met-Car species, TixZryC12 (x + y ) 8), Figure 2a-d. As noted originally, if there were nonequivalent sites (i.e., higher vs lower coordinated), we would not expect the purely statistical substitution of Ti by Zr which is clearly observable from the data, especially under 4:1 molar ratio conditions. Structural models displaying nonequivalent sets of metal bonding sites (tetracapped tetrahedron and metal decorated carbon cage) would predict certain particularly unfavorable stoichiometries based on symmetry considerations. For example, in the tetracapped tetrahedron structure, metals favoring higher coordination such as Zr would preferentially substitute into the more highly coordinated sites, of which there are four. Ti5Zr3C12 and Ti3Zr5C12 would be expected to be relatively unstable compared to Ti4Zr4C12. We would not necessarily expect a pure magic number to “dominate” this mass region, but we would expect a discontinuity in the distribution corresponding to selective occupation of the favored higher coordinated metal sites by the zirconium. As seen in Figure 2b, there is not a discontinuity at any mass corresponding to Zr occupation of the more highly coordinated nonequivalent metal sites of the
tetracapped tetrahedron or metal decorated cage structures. Even at a 2:1 molar ratio of Ti to Zr the same general distribution is maintained, and we still do not observe any discontinuity corresponding to the occupation of the predicted more highly coordinated metal sites. [It should be noted that under these conditions the onset of formation of ZrxCy+ clusters is observed; i.e., the additional broadening and intensity in the Ti5Zr3C12+ channel are attributable to a mass degeneracy with Zr6Cy+ clusters, and the broad peak at approximately 800 amu corresponds to a series of Zr7Cy+ clusters (see ref 23).] At mixing ratios richer in Zr than 2:1, only ZrxCy clusters are detected. It has been suggested17 that perhaps in the direct laser vaporization production of Met-Cars the lowest energy may not be forming. In order to address question, we have produced binary metal Met-Cars under laser vaporization/molecular beam conditions. Figure 3 shows the distribution of binary metal MetCars produced in the laser-induced plasma reactor source followed by supersonic expansion into the ionization region of the spectrometer. Obviously, under direct laser vaporization conditions the clusters are not as cool as under molecular beam
8178 J. Phys. Chem., Vol. 100, No. 20, 1996
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Figure 3. Mass spectrum of TixZryC12+ clusters produced under laser vaporization/molecular beam conditions. Labels (x,y) indicate number of titanium and zirconium atoms for each metallocarbohedrene species.
Figure 2. Mass spectra of TixZryC12+ (x + y ) 8, 0 e y e 5) species obtained under direct laser vaporization conditions with the number of substituent zirconium atoms and the Ti:Zr molar ratio of the target mixture indicated.
conditions, but the striking similarity of Figures 2a and 3 suggests that even under direct laser vaporization conditions we are observing the same low-energy structures. Furthermore, the apparent statistical substitution of Ti by Zr implies that, under both sets of conditions, we are in fact observing clusters with similar structural motifs comprised of eight equivalent metal sites. Metallocarbohedrene Stabilities. The existence of binary metal metallocarbohedrenes of titanium with zirconium, hafnium, yttrium, niobium, tantalum, molybdenum, and tungsten has previously been reported.21 Interestingly, among these metals, yttrium, tantalum, and tungsten have not previously displayed a propensity for Met-Car formation. Upon consideration of the chemical and electronic nature of these transition metals, the experimental observations are readily explainable in terms of the pentagonal dodecahedron structure of Th symmetry. MetCars of yttrium (Y8C12) have never been observed. Furthermore, yttrium does not replace Ti atoms in Ti8C12 to an appreciable extent, nor do the binary TixYyC12 species that it does form appear to be particularly stable. As yttrium readily forms +3 ions in solids, it would, at first thought, be expected to exhibit an enhanced propensity for the equivalent metal sites in the Th structure. However, this is not the case. The explanation lies in the fact that yttrium is relatively electropositive and has a d3 electronic configuration leading to the instability of yttriumcontaining Met-Cars. Typically, in metal-carbon bonding a vacant metal orbital of proper symmetry can accept electrons from a ligand. In a complementary, synergetic process, the lowlying π* orbitals of a C2 unit accept electrons from a metal d orbital which strengthens the M-C σ bond, a phenomenon known as back-bonding. However, the electropositive nature of yttrium precludes the donation of an electron into suitably directed hybrid metal orbital, and the absence of a metal d electron to be donated to a vacant C2* orbital eliminates any possibility of back-bonding. Thus, stable Y-C σ bonds do not form due to the electronic nature of yttrium, and yttriumcontaining Met-Cars are unstable. Preliminary electronic calculations24 performed in our group also point to the destabilization of the Met-Car Th structure upon continued substitution of increasing numbers of Y atoms. However, calculations on the Th structure by Hay11 have shown a more localized character for remaining d electrons on the transition metals in the Met-Car clusters. This may preclude a strong influence from a back-bonding scheme. Further, considering the group VA metals vanadium, niobium, and tantalum, the effect of the d5 metal valence electronic configuration can be addressed. Vanadium does form MetCars,20b although recent experiments25 suggest they may not be as stable as the titanium homologues. Niobium represents a
Formation of Metallocarbohedrenes true “transition metal” in Met-Car chemistry. Depending on the formation conditions, cubic NbxCy or Met-Cars may form.26 The final congener of this group, tantalum, is observed to exclusively form cubic TaxCy clusters. The group VA metals exhibit the transition from Met-Car to cubic carbide chemistry. This trend is attributable to several factors. Vanadium is the most electronegative of the group (but only slightly more so than Nb and Ta), and its d orbitals are relatively contracted compared to those of Nb and Ta. Vanadium can accept electrons from a C2 unit but at the same time destabilize the C2 unit by back-donating two valence electrons to a π* antibonding orbital. However, as these π* orbitals are low-lying, their occupation does not render the Met-Car completely unstable. Nb and Ta may both form M-C σ bonds in accompaniment with the synergetic back-donation of a pair of electrons. Although in the case of these metals, the d orbitals are more diffuse and, combined with their lower electronegativities, may destabilize the C2 units to a greater extent than does vanadium. Finally, the instability of niobium- and tantalum-containing Met-Car species is compounded by the enhanced propensity of Nb and, to a greater extent Ta, to form metal-metal bonds. This propensity, in conjunction with the destabilized C2 units, renders the Nb Met-Car, Nb8C12 (which does form to some extent26), and, to a greater extent, the Ta Met-Car, Ta8C12, much less stable than Met-Cars of their congener vanadium. So much so that, in the case of tantalum, cubic TaxCy clusters are observed exclusively. The metals of group IVA exhibit the perfect balance of properties which enable stable Met-Car formation. Met-Cars have been observed for each of these metals (Ti, Zr, and Hf), and experimentally, titanium and zirconium have always displayed a particular propensity for Met-Car formation. These metals are less electropositive than those of group IIIA, but more so than those of group VA. Furthermore, they have the d4 electronic configuration which enables the formation of three M-C σ bonds, with one remaining electron to participate in back-bonding. Because Zr and Hf are more likely to form metal-metal bonds, Met-Cars containing these metals are less stable than those of Ti, but more stable than those of Nb and Ta. As the group IVA metals have only one valence electron available for back-bonding, the C2 units are not destabilized to the same extent as for Nb and Ta. This outweighs any propensity for metal-metal bonding and maintains the integrity of the Met-Car structure. Thus, the Met-Cars of group IVA in general, and titanium in particular, are characterized by an ideal balance of properties which confers upon them an enhanced propensity for Met-Car formation. Conclusion Metallocarbohedrenes have been produced by the direct laser vaporization of titanium/graphite mixtures. Interpretation of the mass spectral distributions obtained here and previously suggests that the growth process of Met-Cars does not proceed via the formation of a C12 cage, followed by the successive addition of metal atoms. Furthermore, the production of binary metal MetCars of Ti and Zr from a range of mixing ratios provides further evidence that Met-Cars are comprised of one set of equivalent metal sites. The observation of very similar mass spectral distributions of binary metal Met-Cars, produced under widely different formation conditions, suggests that under direct laser vaporization conditions we are observing species with the same structural motif as under supersonic molecular beam conditions. Although under direct vaporization conditions the clusters are “hotter”, there is no evidence to suggest that they consist of a higher energy geometrical configuration. Finally, the focus of
J. Phys. Chem., Vol. 100, No. 20, 1996 8179 most recent theoretical attention applied to Met-Cars has been the stability of the tetracapped tetrahedron structure of Td symmetry. Contrary to the predicted stability of this structure, the observations of Met-Car formation among a range of transition metals can be explained in terms of accepted transition metal chemistry and the Th symmetry structure. However, until spectroscopic confirmation of the Met-Car structure is provided, these arguments serve mainly to further stimulate consideration and discussion of the properties of these novel metal-carbon clusters. Acknowledgment. Financial support by the Air Force Office of Scientific Research, Grant F49620-94-1-0162, and an AASERT Grant from the Air Force Office of Scientific Research, Grant F49620-95-1-0353, is gratefully acknowledged. References and Notes (1) Guo, B. C.; Kerns, K. P.; Castleman, Jr., A. W. Science 1992, 255, 1411. (2) Guo, B. C.; Wei, S.; Purnell, J.; Buzza, S.; Castleman, Jr., A. W. Science 1992, 256, 515. (3) Wei, S. J.; Guo, B. C.; Purnell, J.; Buzza, S.; Castleman, Jr., A. W. J. Phys. Chem. 1992, 96, 4166. (4) Purnell, J.; Wei, S.; Castleman, Jr., A. W. Chem. Phys. Lett. 1994, 229, 105. (5) Gotts, N. G.; von Helden, G.; Bowers, M. T. Science 1995, 267, 999. (6) Dance, I. J. Chem. Soc., Chem. Commun. 1992, 1779. (7) Chen, H.; Feyereisen, M.; Long, X. P.; Fitzgerald, G. Phys. ReV. Lett. 1993, 71, 1732. (8) Lin, Z.; Hall, M. B. J. Am. Chem. Soc. 1993, 115, 11165. (9) Grimes, R. W.; Gale, J. D. J. Phys. Chem. 1993, 97, 4616. (10) Pauling, L. Proc. Natl. Acad. Sci. U.S.A. 1992, 89, 8175. (11) Hay, P. J. J. Phys. Chem. 1993, 97, 3081. (12) Reddy, B. V.; Khanna, S. N.; Jena, P. Science 1992, 258, 1640. (13) Lou, L.; Guo, T.; Nordlander, P.; Smalley, R. E. J. Chem. Phys. 1993, 99, 5301. (14) Dance, I. J. Chem. Soc., Chem. Commun. 1992, 1779. (15) Deng, H. T.; Kerns, K. P.; Castleman, A. W., Jr. J. Am. Chem. Soc. 1996, 118, 446-450. (16) (a) Rohmer, M.-M.; De Vaal, P.; Be´nard, M. J. Am. Chem. Soc. 1992, 114, 9696. (b) Rohmer, M.-M.; Be´nard, M.; Henriet, C.; Bo, C.; Poblet, J. M. J. Chem. Soc., Chem. Commun. 1993, 15, 1182. (c) Rohmer, M.-M.; Be´nard, M.; Bo, C.; Poblet, J. M. J. Am. Chem. Soc. 1995, 117, 508. (d) Bernard, M.; Rohmer, M.-M.; Poblet, J. M.; Bo, C. J. Phys. Chem. 1995, 99, 16913. (17) Khan, A. J. Phys. Chem. 1995, 99, 4923. (18) (a) Wei, S.; Guo, B. C.; Purnell, J.; Buzza, S. A.; Castleman, A. W., Jr. Science 1992, 256, 818. (b) Wei, S.; Castleman, A. W., Jr. Chem. Phys. Lett. 1994, 227, 305. (19) Wei, S.; Guo, B. C.; Purnell, J.; Buzza, S. A.; Castleman, A. W., Jr. J. Phys. Chem. 1992, 96, 4166. (20) (a) Wei, S.; Guo, B. C.; Purnell, J.; Buzza, S. A.; Castleman, A. W., Jr. J. Phys. Chem. 1993, 97, 9559. (b) Purnell, J.; Wei, S.; Castleman, A. W., Jr. Chem. Phys. Lett. 1994, 229, 105. (21) (a) Cartier, S. F.; May, B. D.; Castleman, A. W., Jr. J. Chem. Phys. 1994, 100, 5384. (b) Cartier, S. F.; May, B. D.; Castleman, A. W., Jr. J. Am. Chem. Soc. 1994, 116, 5295. (c) Deng, H. T.; Guo, B. C.; Kerns, K. P.; Castleman, A. W., Jr. Int. J. Mass Spectrom. Ion Processes 1994, 138, 275. (22) Guo, B. C.; Wei, S.; Chen, Z.; Purnell, J.; Buzza, S.; Kerns, K. P.; Castleman, A. W., Jr. J. Chem. Phys. 1992, 97, 5243. (23) Cartier, S. F.; May, B. D.; Toleno, B. J.; Purnell, J.; Wei, S.; Castleman, A. W., Jr., Chem. Phys. Lett. 1994, 220, 23. (24) Cartier, S. F. Ph.D. Thesis, The Pennsylvania State University, University Park, PA, 1995. (25) Yeh, C. S.; Afzaal, S.; Lee, S. A.; Byun, Y. G.; Freiser, B. S. J. Am. Chem. Soc. 1994, 116, 8806. (26) Wei, S.; Guo, B. C.; Deng, H. T.; Kerns, K.; Purnell, J.; Buzza, S. A.; Castleman, A. W., Jr. J. Am. Chem. Soc. 1994, 116, 4475.
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