8170
J. Phys. Chem. 1996, 100, 8170-8174
Chemistry in Clusters: Synthesis of NO+(N2O3)n and NO2+(N2O3)n Species M. Z. Martin, S. R. Desai,† C. S. Feigerle, and J. C. Miller* Chemical and Biological Physics Section, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6125, and Department of Chemistry, UniVersity of Tennessee, KnoxVille, Tennessee 37996 ReceiVed: NoVember 13, 1995; In Final Form: February 20, 1996X
Mixed clusters were produced by expansion of an NO/CH4/Ar/H2O mixture (5%, 5%, 90%, trace) in a supersonic jet. Ionization of the clusters with 30 ps, 266 nm laser pulses resulted in the virtually 100% conversion to NO+ (N2O3)n and NO2+(N2O3)n cluster ions subsequently detected in a time-of-flight mass spectrometer. Some incomplete conversion was evidenced by the detection of (NO)m+(N2O3)n ions with m > 1. The identity of the product ions was confirmed by the use of isotopically substituted nitric oxide and the role of the water was investigated using 18O-substituted H2O. Possible photochemical mechanisms are discussed.
Introduction The chemistry of compounds composed of differing numbers of nitrogen and oxygen atoms is extremely rich. About 25 neutral or charged species of the form NxOy are known.1,2 Many are stable in the gas phase while others have been investigated in low-temperature, matrix-isolation environments. Study of these compounds is frequently complicated by facile interconversion leading to situations where several different NxOy species coexist at equilibrium. For instance, germane to the present study, NO + NO2 a N2O3 is a well-known reaction. Additionally, many other chemical interconversions may be photoinduced or occur in the presence of oxygen or water.3 Consequently, studies of any given species are complicated by the presence of other chemically similar molecules. Several notable errors in the literature can be traced to the effects of interconverting NxOy species. In several recent papers we have studied the clustering of nitric oxide with itself and with other common molecules.4,5 This was accomplished by mixing NO, or NO and a “reactant” species, with argon in a supersonic expansion and detecting the resulting clusters by picosecond multiphoton ionization followed by mass spectrometry. Typically, species of the form (NO)m+(Y)n were observed where Y represents NO, Ar, N2O, NO2, H2O, CO2, SO2, SF6, or CS2. Since NO2 and N2O are always present as impurities in nitric oxide samples, ions such as NO2+, N2O+, and (NO)m+NO2 and (NO)m+N2O were usually observed as small peaks in the mass spectrum. For the case of nitrous oxide the deliberate addition of N2O resulted in a full range of (NO)m+(N2O)n ions being formed. However, doping the expansion mixture with nitrogen dioxide did not produce large mixed cluster ions. That is, we have never observed (NO)m+(NO2)n ions with n greater than 1. Absence of these species in the mass spectra has been attributed to reaction of the neutrals prior to the expansion or perhaps to photodissociation of NO2 which occurs readily with visible and ultraviolet light. It should be noted here that the heterocluster NO-NO2 is indistinguishable from N2O3 in a mass spectrometer. The dimeric heterocluster NO-CH4 is well-known and its spectroscopy has been previously reported.6 Attempts to produce larger (NO)m(CH4)n clusters by coexpansion of nitric † Pacific Northwest Laboratory, Environmental and Molecular Sciences Laboratory, P.O. Box 999, MS K2-14, Richland, WA 99352. X Abstract published in AdVance ACS Abstracts, April 15, 1996.
S0022-3654(95)03337-5 CCC: $12.00
oxide and methane in an argon beam yielded ions of mass 106, 182, 258, 334, ... amu. These ions were initially assigned to cluster species of the form (NO)m+(CH4)n analogous to the (NO)mYn species studied previously. Two preliminary accounts of this experiment outlined this interpretation.7,8 However, later experiments using deuterated methane failed to shift the mass peaks in the expected manner. The realization that the mass of a methane molecule and that of an oxygen atom are the same (16 amu) led to the consideration of NO+(N2O3)n clusters as the origin of the peaks. Experiments using 18O-substituted NO confirmed this unexpected assignment. The present paper details these studies and discusses possible photochemical mechanisms for the conversion of nitric oxide clusters to dinitrogen trioxide clusters. Laser-induced processes in clusters are currently of much interest and have been observed by several groups.9 We have previously investigated laserinduced polymerization in carbon disulfide clusters10 and attributed the results to intracluster polymerization through a radical cation chain mechanism. The generation of NO+(N2O3)n and NO2+(N2O3)n clusters from ionization of dilute NO/CH4 expansions represents another example where photochemistry in a cluster is highly selective at producing certain types of cluster ions. Experimental Section Mixtures of nitric oxide and methane, diluted in argon, were expanded in a pulsed supersonic jet at a backing pressure of 100 psi. The expanding gas was skimmed and intersected about 15 cm downstream with the fourth harmonic of a focused 30 ps Nd:YAG laser beam. At this wavelength two photons are required to nonresonantly ionize nitric oxide or nitric oxidecontaining clusters. The resulting photoions were extracted into a 1 m, linear time-of-flight (TOF) mass spectrometer. The laser (Quantel YG571C) and the pulsed valve (R. M. Jordan Co.) were triggered by adding appropriate delays to a 10 Hz master pulse generator. The mass spectrum was recorded with a digitizing oscilloscope (Tektronix 11402) and traces from 1 to 4000 laser shots were added to improve the signal-to-noise ratio. In some experiments fully or partially deuterated methane or isotopically labeled nitric oxide was used in the gas mixture to unravel mass coincidences. A trace amount of water was required to observe the product ions. This was accomplished by exposing the vacuum manifold and mixing chamber to water © 1996 American Chemical Society
Chemistry in Clusters
Figure 1. Laser ionization mass spectrum of a 5% NO/5% CH4/90% Ar/trace H2O expansion mixture. The inset shows on an expanded scale the peaks in the vicinity of NO+(N2O3)n for n ) 1.
(or isotopic variants) at its room-temperature vapor pressure of 24 Torr and then pumping out the system for a brief time. The water which adhered to the surfaces was sufficient to induce the formation of the product ions. Several different sources of normal and/or isotopic nitric oxide were used. Samples of 5% NO in argon (Matheson), neat NO (Matheson) and 15N18O (Isotec) were used without purification. It is well-known that these cylinders may develop appreciable amounts of NO2 and N2O from slow disproportionation reactions which take place at high pressure. Low-temperature slush baths can eliminate the N2O and reduce the amount of NO2 as we have shown previously.5 No such slush baths were used in the present experiments. It should be noted that low-pressure cylinders (such as the isotopic samples) contain lesser amounts of these compounds. Further details of the apparatus and techniques can be found elsewhere.5 Results The TOF mass spectrum produced by two-photon ionization (266 nm) of a 5% NO/5% CH4/90% Ar/trace H2O mixture is shown in Figure 1. In contrast to what might be expected from such a complex mixture, the spectrum is remarkably simple, consisting primarily of a single cluster-ion series. Beginning at a mass of 106 amu, a series of peaks spaced by 76 amu with monotonically decreasing intensities is observed. Twelve members of the series are seen in the figure but the series can be followed out to 1246 amu (16 members) in other spectra. Because all binary cluster combinations previously studied had led to the observation of heteroclusters of the form (NO)m+(Y)n a similar assignment of the peaks of Figure 1 was sought. All of the peaks could tentatively be assigned as (NO)m+(CH4)n ions. With this interpretation the repeating unit of 76 amu was attributed to an (NO)2CH4 species. Subsequent experiments using deuterated or partially deuterated methane, however, yielded anomalous results. That is, peaks attributed to methane-containing species did not shift to higher mass as required. Furthermore, when methane was replaced by ethane in the gas mixture, the mass spectrum exhibited a similar cluster ion distribution with peaks again spaced by 76 amu. The absence of H-D isotope shifts when using CD4 and the puzzling results obtained when using C2H6 suggested a reinterpretation of the data. A CH4 molecule and a O atom both have a mass of 16 amu and represent a common isobaric interference in mass spectra. An alternate assignment to the repeating unit of mass 76 amu is then to the N2O3 molecule, a
J. Phys. Chem., Vol. 100, No. 20, 1996 8171
Figure 2. Laser ionization mass spectrum of an expansion mixture similar to that of Figure 1 using 15N18O instead of 14N16O. Again the inset shows the mass region including NO+(N2O3). The peaks at 115 and 117 amu are due to NO+(N2O3) with three and four 18O atoms, respectively.
well-known chemical species (again note that NO-NO2 is equivalent to N2O3 in a mass spectrum). In this scheme the peaks are reassigned as NO+(N2O3)n clusters. This identification was confirmed by using isotopically labeled 15N18O in the gas mixture and Figure 2 shows the resulting mass spectrum. The use of 18O removes the ambiguity between O and CH4 and analysis of the isotopic shifts of the peaks provides proof of the identity of the ions. For instance, the NO+(N2O3) peak is seen to split into several peaks in the figure, representing differing numbers of 18O and 16O atoms. The highest-mass peak at 117 amu corresponds to NO+(N2O3) containing only 15N and 18O atoms. The peak at 115 amu represents replacement of one 18O with an 16O atom and the 113 amu peak represents the replacement of two 18O atoms. The highest-mass peak possible with the methane-containing cluster would have occurred at 115 amu for the (NO)3+(CH4) species formed from 15N18O. These results, coupled with the absence of mass shifts using deuterated methane, provide unambiguous proof of the assignment to clusters containing dinitrogen trioxide. The isotopic purity of the nitric oxide, which was initially 98.6% 18O and 99.9% 15N, was reduced by exchange reactions within the gas manifold which led to the presence of large amounts of 16O-containing species. In general, for an NO sample that is predominantly N18O, the probability of inclusion of an 16O atom will increase with the number of oxygen atoms in the cluster in a manner which reflects the isotopic purity of the initial sample. Experimentally we find that by the time seven oxygen atoms are present in the cluster the most intense peak has one 16O atom in the cluster which suggests an 85% 18O sample after some isotopic mixing. The exchange reactions which dilute the 18O concentration probably involve trace amounts of either H2O or O2 which may be present in the mixing vacuum manifold. Two additional cluster series, corresponding to inclusion of one fewer or one more oxygen atom in the cluster relative to NO+(N2O3)n, are observed in the spectrum shown in Figure 1. The second major series of peaks is assigned to NO2+(N2O3)n clusters and the intensity of these peaks is about 25% of the corresponding NO+(N2O3)n peak. A weaker third series corresponding to (NO)3+(N2O3)n-1 is observed 16 amu to lower mass than each of the first five main peaks. In previous experiments,5 cluster ion series built on the (NO)3+ core were found to be especially stable relative to other cluster series with different numbers of nitric oxide molecules.
8172 J. Phys. Chem., Vol. 100, No. 20, 1996
Martin et al.
4NO a 2N2O + O2
(5)
3NO a NO2 + N2O
(6)
In the condensed phase, several of the various NxOy species exist primarily in the form of dimers. The odd-electron molecules NO and NO2 are nearly completely dimerized in liquids and solids and are usually written as N2O2 and N2O4 to reflect this fact. Likewise, mixtures of NO and NO2 may condense as N2O3 giving a blue color to the condensed phase samples.11 In the gas phase, dinitrogen trioxide is mostly dissociated. So in the condensed phase reactions 1 and 6 may be written as
Figure 3. Laser ionization mass spectrum of an expansion mixture similar to that of Figure 1 using H218O instead of H216O. Again, the relative numbers of 16O and 18O atoms are noted for each peak.
Although the cluster ions observed contain only nitrogen and oxygen atoms, all of the observed ions required methane and trace amounts of water for their formation. If either (or both) were left out of the expansion mixture, the spectrum obtained upon ionization was similar to that of a 5% NO/95% Ar mixture published previously.5 The requirement for a trace of water was discovered upon baking out the vacuum system prior to the isotopic experiments. Spectra containing N2O3 could not be obtained until water had been deliberately added to the mixture. Thereafter reproducible spectra were regularly obtained when trace amounts of water were present. Water was added by “dosing” the vacuum manifold and mixing chamber with water vapor for a brief time and then pumping out the system prior to mixing the other gases. Presumably the water adsorbed on the surfaces of the vacuum system was sufficient to allow the reactions to proceed. The addition of more than a trace amount of water did not lead to N2O3 clusters and only peaks due to (NO)n clusters were observed. Furthermore, the quality of the resulting spectra was degraded. In previous studies of heteroclusters we have also noted that certain product species are only observed within a rather narrow range of concentrations of the minor substituent. The role of the water was further investigated by dosing the system with water containing 18O (Aldrich 95 atom %). The results are shown in Figure 3 where it can be seen that one 18O atom has been + incorporated into the NO+ 2 (N2O3)n series but not the NO (N2O3)n series. Discussion The N2O3 molecule was first identified by Gay-Lussac in 1816.11 It forms readily in mixtures of nitric oxide and nitrogen dioxide where its concentration is governed by the following equilibria:
NO + NO2 a N2O3
(1)
2NO2 a N2O4
(2)
N2O3 + H2O a 2HNO2
(3)
3HNO2 a 2NO + HNO3 + H2O
(4)
Most cylinders of nitric oxide contain NO2 and N2O due to disproportionation reactions13,14 such as
N2O2 + N2O4 a 2N2O3
(7)
3N2O2 a 2N2O + N2O4
(8)
respectively. The latter reaction has been studied by Agnew et al.14 in cold nitric oxide (176 K) at high pressure (1.5 GPa). Similar reactions are expected in cold clusters. Consequently, in any experiment involving nitric oxide one must also contend with the presence of other NxOy species and their mutual equilibria. Also, in photolysis,15,16 pulsed radiolysis18-20 and ionization5,21 experiments involving nitric oxide, where the addition of energy may yield atomic nitrogen and oxygen along with various ions and excited states, many similar reactions come into play, again yielding NO, NO2, N2O, and N2O3 species and their ions. With this background it is not surprising to see N2O+, NO2+, and N2O3+ ions (both isolated and bound in various heteroclusters) in multiphoton ionization experiments on NO clusters. Previous work from this laboratory has amply demonstrated this; however, in that case, the peaks were relatively small and were attributed chiefly to impurity amounts of NO2 and N2O. Several other studies have also seen similar small peaks in both positive5,22 and negative23 ion studies. It is surprising, however, to observe predominantly NO+(N2O3)n ions in such an experiment. Furthermore, the role of the methane and water, which are required for the observation of the dinitrogen trioxide clusters, is perplexing at best. The present results are, however, surprisingly similar to those reported from three other distinctly different experiments. In the first, Goulomb and Good21 reported the same cluster ions observed in the present study following 90 eV electron impact ionization of pure nitric oxide expansions. That is, they observed (NO)n+ clusters only to n ) 3, but detected NO+(N2O3)n ions to n ) 7. This contrasts with previous studies from this laboratory5 on laser ionization of pure NO and NO/ Ar expansions where (NO)m+ clusters up to n ) 40 were seen and no dinitrogen trioxide clusters were observed. The second experiment, by Michl and collaborators,24 involved secondary ion mass spectrometry (SIMS) on frozen samples of NO. In these experiments keV ions bombarded the solid samples and the secondary ions were analyzed. The positive ion spectra obtained using 4 keV Ar+ as the primary beam is virtually identical to that of Figure 1. In contrast, the negative ion SIMS from a solid NO target yielded only O-, NO-, O2- and NO2-. Finally, in Rydberg charge exchange experiments on nitric oxide expansions, Kondow25 reported the negatively-charged clusters NO2(N2O3)n-, NO3(N2O3)n-, and N2O4(N2O3)n-. Later work by Carman23 with purer nitric oxide samples did not, however, see such ions. NO+(N2O3)n Clusters. Clearly there is ample precedent for the observation of dinitrogen trioxide clusters in experiments involving nitric oxide in the condensed phase or in clusters.
Chemistry in Clusters
J. Phys. Chem., Vol. 100, No. 20, 1996 8173
While the present experiments are unable to establish a complete, detailed mechanism which accounts for the observed NO+(N2O3)n species, the following factors are expected to play a role. The first step in the process is the formation in the supersonic expansion of nitric oxide clusters, nNO ) (NO)n, with varying amounts of Ar, CH4, and H2O also present in the cluster. In experiments with NO/Ar expansions, pure (NO)n clusters as big as n ) 40 are readily formed and detected in our apparatus. Furthermore, the odd-even alternation observed in these experiments indicates that the nitric oxide is extensively “dimerized” in the cluster, just as it is in other condensed phases. Temperatures of such clusters are extremely difficult to ascertain and even the distinction between a “solid” or “liquid” cluster is blurred. Where they can be measured or modeled, temperatures of clusters containing similar species produced under similar expansion conditions are between 50 and 100 K. Once formed, the neutral clusters must be ionized in order to be detected. The laser beam which achieves this will initiate various excitation and ionization steps which put excess energy into the cluster with the potential for bond breaking and rearrangement. Photolysis of NO can provide a pathway for the formation of N2O3 through reactions such as
NO + hν a N + O
(9)
N + NO f N2 + O
(10)
O + NO a NO2
(11)
NO + NO2 a N2O3
(12)
Each NO which is photolyzed within this sequence can lead to the production of two oxygen atoms and ultimately to two N2O3 molecules. Reactions similar to eqs 9 to 12 are known to occur involving dimers which is the form of nitric oxide expected to predominate in clusters. For instance, photochemical reactions such as (NO)2 + hν ) NO2 + N and 2N2O4 + hν ) N2O3 + N2O5 have been observed17 or postulated14 in lowtemperature, condensed phase experiments. Furthermore, the cluster environment has the potential for enhancing the fundamental reactions in the pathway and indeed there is evidence for this in the O + NO ) NO2* reaction.21 The laser pulse will, of course, also ionize the molecule. The charge on the resulting ion probably resides on the nitric oxide molecule which is the species with the lowest ionization potential. Thus the ion formed after ionization is of the form NO+(NO)m-n-1(N2O3)n with, again, various number of solvating argon, methane, or water molecules possibly present. Following ionization, clusters must rid themselves of excess energy and this usually occurs through evaporation. That is, the weakest (usually van der Waals) bonds break and the leaving molecules cool the cluster by carrying off kinetic and internal energy. In the present case, the NO+-N2O3 bond must be stronger than the NO+-NO bond and is certainly stronger than any bonds with Ar or CH4. This is reasonable as N2O3 is more polarizable and has a larger dipole moment than the other species present in the cluster. The final form of the cluster is thus NO+(N2O3)n as observed. This mechanism is similar to those proposed by Kondow25 and Orth et al.24 to explain their Rydberg charge exchange and SIMS experiments, respectively. In Figure 1 the cluster series (NO)3+(N2O3)n is also present as a set of small peaks which precede the major peaks by 16 amu. The presence of this series may indicate incomplete evaporation of nitric oxide molecules. Alternately it may reflect the unusual stability of the nitric oxide trimmer ion which has been observed by several groups.5,26
There are two deficiencies in the above picture. First, the roles of methane and water are not addressed, yet experimentally, they are required in order to observe the products. As the product ions contain no carbon, hydrogen, or oxygen from the added methane or water, these molecules most likely serve as a specialized solvent or catalyst, respectively. Second, unless some of the excess energy is channeled into also producing N2O3, each photolysis of NO would yield only two N2O3 molecules. Experimentally, clusters with as many as 16 N2O3 groups have been observed, which would require many more photons if the above mechanism is invoked. A process which involved the absorption of so many photons would be expected to yield a wider distribution of m and n in NO+(NO)m(N2O3)n clusters. In addition, the spectrum would be expected to exhibit a high-order laser power dependance, which is not observed. The high product selectivity, i.e., near-exclusive formation of NO+(N2O3)n for n up to 16, would appear to be more consistent with a chain type mechanism. Such a mechanism has been proposed to explain the formation of large sulfur clusters following ionization of carbon disulfide clusters.10 It is possible that water is involved in such a chain mechanism, maybe through formation of acidic species as shown in reactions 3 and 4. The species HNO2-NO+ has been observed with similar intensity to that of N2O3 in some experiments. Also the 18O experiments demonstrated that the oxygen atoms from the water were not incorporated into the NO+(N2O3)n clusters but were, however, incorporated into the NO2+(N2O3)n clusters. This observation suggests that the water might be catalytically involved in the formation of these clusters. Although the exact mechanism is not detailed, previous radiolysis studies have indicated that use of moist samples of NO led to enhanced production of NO2.20 In the present experiments this enhanced production of NO2 could occur in the preexpansion gas mixture or during the excitation-ionization stage. At least two possible roles can be imagined for the methane. The temperature of the cluster probably plays an important role in the chemistry which takes place within it. The temperature is a function of expansion conditions as well the heat capacities, mole fractions, and aggregation tendencies of the sample gases. Possibly the methane provides the proper mix of conditions to yield an expansion temperature which is optimized for the chemistry. Another possible role of the methane could be in providing a proper chemical environment. It is well-known in matrix-isolation studies that matrix effects can dramatically change the rates of photochemical and photophysical processes. The stability of different isomers such as cis or trans nitric oxide dimers or sym or assym N2O3 molecules may also be dependent on details of the host microenvironment. In particular, the photochemistry of nitric oxide dimers is considerably different in nitrogen matrices than in atomic hosts like neon. Legay and Legay-Sommaire17 have shown that NO dimers photolyze to NO2 in neon but convert to N2O in nitrogen. Furthermore, the NO2 comes solely from the cis NO dimer and N2O from the trans form. These data make plausible a suggestion that the microenvironment in a heterocluster of nitric oxide and methane influences the later photochemistry, perhaps by influencing the stability of various isomers. If this is true, the microenvironment must be very sensitive to the choice of solvent, since none of the additives used in previous experiments (Kr, Xe, SF6, CO2, CS2, N2O, etc.)5 to form heteroclusters with nitric oxide have led to dinitrogen trioxide clusters as observed when methane is used. Finally, the previous studies from other laboratories21,24,25 which have observed the same product ions have not required any additives; typically they have used neat nitric oxide samples.
8174 J. Phys. Chem., Vol. 100, No. 20, 1996 NO2+(N2O3)n Clusters. This cluster series is observed 16 amu to higher mass than the main series in Figure 1 and is observed to n ) 12. It is reasonable to assume that these clusters form in a similar manner to the main series clusters but undergo an extra oxidation step, converting NO+ to NO2+. Unlike the main series, however, the nitrogen dioxide clusters do incorporate an oxygen-18 atom from the trace water in the gas mixture. We believe that those clusters (or cluster ions) which have a water molecule within the cluster possibly undergo this final oxidation step, transferring an oxygen atom from the water to form NO2. Conclusions The photochemical conversion of nitric oxide clusters into clusters containing dinitrogen trioxide has been investigated. The completeness and selectivity of the conversion suggests a chain-type mechanism. Solvation of the reactant clusters by methane appears necessary and traces of water are also required for the reactions to proceed. Several possible roles for the methane and water have been advanced. Acknowledgment. Research is sponsored by the Office of Health and Environmental Research, U.S. Department of Energy under contract DE-AC05-84OR21400 with Lockheed Martin Energy Systems, Inc. Useful discussions with C. E. Klots are acknowledged. References and Notes (1) Laane, J.; Ohlsen, J. R. Prog. Inorg. Chem. 1980, 25, 465. (2) Me´len, F.; Herman, M. J. Phys. Chem. Ref. Data 1992, 21, 831. (3) See for instance: Nour, E. M.; Chen, L.-H.; Laane, J. J. Phys. Chem. 1983, 87, 1113. Fateley, W. G.; Bent, H. A.; Crawford, B. J. Chem. Phys. 1959, 31, 204. (4) Smith, D. B.; Miller, J. C. J. Chem. Phys. 1989, 90, 5203; J. Chem. Soc., Faraday Trans. 2 1990, 86, 2441.
Martin et al. (5) Desai, S. R.; Feigerle, C. S.; Miller, J. C. J. Chem. Phys. 1992, 97, 1793; Z. Phys. D 1993, 26, 220; Z. Phys. D 1993, 26, S183; J. Chem. Phys. 1994, 101, 4526. (6) Miller, J. C. J. Chem. Phys. 1987, 86, 3166. Akiikee, M.; Tsuji, K.; Shibuya, K.; Obi, K. Chem. Phys. Lett. 1995, 243, 89. (7) Miller, J. C. Multiphoton Ionization Studies of van de Waals Molecules and Clusters. In Linking the Gaseous and Condensed Phases of Matter; Christophorou, L. G., Illenberger, E., Schmidt, W. F., Eds.; Plenum Press: New York, 1994. (8) Desai, S.; Feigerle, C. S.; Miller, J. C. Reson. Ionization Spectrosc. 1994 1994, 329, 179. (9) See for instance: Garvey, J.; Peifer, W. R.; Coolbaugh, M. T. Acc. Chem. Res. 1991, 24, 48. El-Shall, M. S.; Shriver, K. E. J. Chem. Phys. 1991, 95, 3001. (10) Desai, S. R.; Feigerle, C. S.; Miller, J. C. J. Phys. Chem. 1995, 99, 1786. (11) Beattie, I. R. Prog. Inorg. Chem. 1963, 5, 1. (12) Vosper, A. J. J. Chem. Soc., Dalton 1976, 1976, 135. (13) Melia, T. P. J. Inorg. Nucl. Chem. 1965, 27, 95. (14) Agnew, S. F.; Swenson, B. I.; Jones, L. H.; Mills, R. L. J. Phys. Chem. 1985, 89, 1678. (15) Kajimoto, O.; Honma, K.; Kobayashi, T. J. Phys. Chem. 1985, 89, 2725. (16) Treinin, A.; Hayon, E. J. Am. Chem. Soc. 1970, 92, 5821. (17) Legay, F.; Legay-Sommaire, N. J. Chem. Phys. 1995, 102, 7798. (18) Hochanadel, C. J.; Ghormley, J. A.; Ogren, P. J. J. Chem. Phys. 1969, 50, 3075. (19) Boyd, A. W.; Miller, O. A. Can. J. Chem. 1978, 56, 1075. (20) Tokunaga, O.; Suzuki, N. Radiat. Phys. Chem. 1984, 24, 145. (21) Golomb, D.; Good, R. E. J. Chem. Phys. 1968, 49, 4176. (22) Beneventi, L.; Cassavecchia, P.; Rusin, L. Yu.; Volpi, G. G. In The Chemical Physics of Atomic and Molecular Clusters; Scoles, G., Ed.; North-Holland: Amsterdam, 1990; p 579. (23) Carman, H. S., Jr. J. Chem. Phys. 1994, 100, 2629. (24) Orth, R. G.; Jonkman, H. T.; Michl, J. J. Am. Chem. Soc. 1981, 103, 1564; 1982, 104, 1834. (25) Kondow, T. In Electronic and Atomic Collisions; Lorents, D. C., Meyerhof, W. E., Peterson, J. R., Eds.; Elsevier Science: Amsterdam, 1986; p 517. (26) Winkel, J. F.; Jones, A. B.; Woodward, C. A.; Kirkwood, D. A.; Stace, A. J. J. Chem. Phys. 1994, 101, 9436.
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