1220
J. Phys. Chem. 1989, 93, 1220-1 225
formed in these studies at concentrations below that at which its ESR signal could be seen. A qualitative comparison of the A1-H stretching mode absorption intensities of A1H2 and CH3AlHS indicates that the signal strength of A1H2 should have been less than 5% of that of CH,AlH formed under similar conditions. Therefore, an ESR spectrum of much lower intensity than that of CH3A1H would be expected for A1H2, which probably accounts for the fact that no spectrum was observed. Future attempts to synthesize AlH2 involving either a 4.2 K cryostat with solid H2 as a matrix support or a much thicker sample of AI in a H,-doped
rare gas matrix should result in its observation. Acknowledgment. The generous financial assistance of the Natural Sciences and Engineering Research Council of Canada's Operating and Strategic Grants programs is gratefully acknowledged. J.M.P. thanks NSERC for a postgraduate scholarship. Registry No. AI, 7429-90-5; H2, 1333-74-0; Kr, 7439-90-9; D2, 7782-39-0; AI,, 32152-94-6; AIH2, 14457-65-9; AIH, 13967-22-1; H, 12385-13-6; AID2, 92952-44-8.
Photochemical Reactions of Matrix-Isolated Aluminum Atoms with Methane and Molecular Hydrogen. 3. Structure, Bonding, and Reactivity J. Mark Parnist and Geoffrey A. Ozin* Lash Miller Chemical Laboratories, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 1A1 (Received: March 3, 1988; I n Final Form: June 23, 1988)
The structure, bonding, and photochemistry of CH3AIH and A1H2 are considered with regard to the known properties of AlH2 as well as the differences between optical absorptions associated with AlH2 and CH3AlH. The Al/CH4 and A1/H2 reactions are discussed with regard to the established experimental and theoretical data available for A1H2,AIH, and A1CH3. Several approaches to interpreting the difference between the observed chemistry of 2SAI atoms with CH4 and H, are discussed. The ground-state insertion reaction of AI atoms with CH4 and H2is shown to be symmetry-forbiddenand therefore not feasible in spite of the estimated 10-12 kcal/mol exothermicity of the reaction.
Introduction Reactions between metal atoms or ions and small covalently bonded molecules such as CHI and H2are of importance as models for metal-centered catalysis systems, since the electronic structures of metal atoms and ions as well as those of small, unsaturated organometallic molecules are well-defined. As a result, there exists the possibility of developing criteria for understanding reactivity trends for ground- and excited-state metal atoms based upon differences in the electronic properties of the reactive species in question. Group 13 metals such as B, Al, and Ga offer an excellent opportunity to extend our understanding of such reactivity trends due to their unique ground- and excited-state electronic configurations. Most metal atoms known to react in an excited state with methane or molecular hydrogen have their outermost electron in an s orbital (for example: Cu, 3d1°4s1; Fe, 3d64s2;Mg, 3s2) such that ground-state interactions with covalent molecules such as CHI and H 2 are highly repulsive. Furthermore, the excited states of these metals which show reactivity with CH4 and Hz are formed through promotion of this s orbital electron into a p orbital of the same principal quantum number (for example: Cu, 3dI04p1; Fe, 3d64s'4p1; Mg, 3s13p1). As a result, the generally accepted mechanism for such reactions involves the overlap of the occupied p orbital with the unoccupied u* orbital of the quencher, typically H2, during a side-on insertion process.'" By this criterion, ground-state group 13 atoms such as 2P B (2s22p'), AI (3s23p1), and Ga (4s24p1)should react in the ground state with both CH4 and HZ, if the reaction is exothermic. Furthermore, the lowest excited state of all group 13 metals has the electronic configuration ns2(n+l)s' (e.g. A1 2S, 3s24s'), such that it should not show reactivity with either CHI or H, since overlap of an s orbital with the unoccupied u* orbital of the covalent molecule should be negligible for side-on approach. At best, an excited-state abstraction process might be expected following end-on approach. 'Present address: Laser Chemistry Group, Division of Chemistry, National Research Council Canada, 100 Sussex Dr., Ottawa, Ontario. Canada KIA OR6.
0022-3654/89/2093-1220$01.50/0
In fact, ground-state A1 atoms are not found to be reactive with , A1 is found to react either CH4 or H,, while excited-state S efficiently with CH4 but extremely inefficiently with H2.7-9 Moreover, an insertion product is formed exclusively in the *S AI + CHI system, while both A1H2 and AlH + H atoms are found in the S , A1 H2 system, again through what is believed to be an insertion process. An analogous reactivity trend has been observed recently in the gas phase,I0 where ,S Ga atoms exhibit a gas kinetic rate for quenching by CHI but an extremely low cross section for quenching by H2 (3 orders of magnitude smaller). These results indicate that occupation of a valence p orbital is not always an essential factor in the quenching of excited-state metal atoms, particularly when an insertion mechanism is involved. The difference in reactivity between 2S A1 or Ga atoms with CHI and H2 also suggests a fundamental difference in the way these latter two molecules interact with group 13 metal atoms, since both have similar bond strengths (CH4, 105.1 kcal/mol;" H,, 104 kcal/mol12). Such a difference must clearly be of importance in other metal atom reactions involving these two quenching
+
( I ) Botschwina, P.; Meyer, W.; Hertel, I. V.; Reiland, W. J . Chem. fhys. 1981, 75, 5438. (2) Adams, N.; Breckenridge, W. H.; Simons, J. Chem. f h y s . 1981,56,
327. (3) Blickensderfer, R. P.: Jordan, K. D.; Adams, N.; Breckenridge, W. H. J . Phys. Chem. 1982, 86, 1930. (4) Pokier, R. A,: Peterson, M. R.; Menzinger, M. J . Chem. Phys. 1983, 7 8 , 4592. (5) Novaro, 0.;Garcia-Prieto, J.; Poulain, E.; Ruiz, M. E.; J . Mol. Struct. 1986, 135, 79. (6) Ruiz, M. E.; Garcia-Prieto, J.; Poulain, E.; Ozin, G. A,; Pokier, R. A,; Mattar, S. M.; Czismadia, I. G.; Gracie, C. G.; Novaro, 0. J . f h y s . Chem. 1986, 90, 219. (7) Parnis, J. M.; Ozin, G. A. J . A m . Chem. SOC.1986, 108, 1699. (8) Parnis, J. M.; Ozin, G. A., next to preceding paper in this issue. (9) Parnis, J. M.; Ozin, G. A., preceding paper in this issue. (10) Mitchell, S. A.; Hackett, P. A,; Rayner, D. M.; Flood, M. J . Chem. Phys. 1987,86, 6852. (11) Baghal-Vayjooee, M. H.; Colussim, A. J.; Benson, S . W. J. Am. Chem. SOC.1978, 100, 3214. (12) Huber, K. P.; Herzberg, G. Molecular Specira and Molecular Structure 4 : Constants of Diatomic Molecules; Van Nostrand: Princeton, NJ. 1979; p 24.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1221
Structure of CH3A1H and A1H2 molecules and may be indicative of factors influencing reactivity that were previously thought to be unimportant. In this paper, we examine some general aspects of the reactions of 2Sand 2D A1 atoms with CH4 and H2. The energetics of the ground- and excited-state reactions are considered as well as the structure and bonding characteristics of CH3A1H and A1H2. The electronic structure of these divalent AI(I1) species is considered with regard to the observed photochemistry of CH3A1H and A1H2. The reactivity of ground and excited states of A1 atoms with CH, and H 2 is then discussed with regard to known and calculated properties of other related metal atom systems. Several proposals are made to explain the differences in reactivity between CH, and H 2 with zS Al, all of which involve interactions other than those involving metal p orbital overlap with the u* orbital of the quencher. Details of the experimental design and results for the photochemical reactions of A1 atoms with methane and molecular hydrogen are given in parts 1 and 2 of this ~ e r i e s . ~In, ~order to facilitate discussion of these systems, a brief summary of the experimental results is given here: 2S zP or 2D 2P excitation of A1 atoms at 348/368 or 305 nm in methane results in the formation of CH3A1H. This molecule exhibits UV-visible, infrared, and ESR spectra that are consistent with a bent Al(I1) molecule. CH3A1H exhibits two optical absorptions at 270 and 525 nm. Photolysis at 270 nm results in fragmentation to form secondary products, while photolysis at >450 nm results in highly efficient regeneration of AI atoms. 2S zP or 2D zP excitation of A1 atoms in Hz-doped krypton matrices leads to inefficient formation of AIHz and AlH H atoms, characterized by IR and ESR spectroscopy (no ESR spectra of AIHz were obtained). Both extensive photoagglomeration of A1 atoms to form A12 and 2S 2P Stokes-shifted emission are observed as major contributors to relaxation of excited-state A1 atoms in Hz/Kr mixtures. Production of A1H is independent of AIHz growth, indicating that the former arises from excited-state fragmentation of AlH2 following its formation. Preferential production of H atoms following excitation of A1 atoms in a HD-doped Kr matrix is consistent with an insertion mechanism, as is the observation of CH3A1H as the exclusive photoproduct in the AI/CH4 system.
- -
- -
+
-
Discussion A. Energetics Considerations. Aluminum forms strong bonds with both hydrogen and carbon, such that generation of stable organoaluminum hydrides is expected to be a favored process. The bond dissociation energy (BDE) of diatomic AlH in its l2+ground ~ ~ , ~recent ~ state has been reported to be about 70 k ~ a l / m o l ,while calculations have found the BDE to be 71.3 (SCF 6-311G/p,d)I5 and 70.3 kcal/mol (ab initio CI).16 The only value available for which was obtained with the BDE of A1-CH3 is 67.8 k~al/mol,~' full CI using a double-zeta basis set with polarization functions, although it has been estimated that the true value could be as much as 10 kcal/mol greater than that given above. Therefore, it seems reasonable to assume that Al-CH3 and AI-H bonds in comparable chemical environments will have roughly the same bond strength. Information about the bond strength of second and third hydride or methyl bonds to AI is much more scarce. The most useful calculated bond dissociation energy available is that of the first hydride bond of A1H2, for which Do(HAl-H)lS has been estimated to be 45.6 kcal/mol. This same work has reported Do(H2A1-H) = 85.2 kcal/mol, and the total binding energy for AlH3 was found to be 202.1 kcal/mol. This latter value is in excellent agreement with the value of 205 kcal/mol calculated for AlH3 by other workers,l* and therefore it is felt that the BDE value quoted above for HAl-H is reasonably accurate. The weakness of this second (13) Cade, P. E.; Huo, W. M. J . Chem. Phys. 1967, 47, 649. (14) Wilkinson, P. G. Astrophys. J . 1963, 138, 778.
CH3+ H+AI(2P)
100 -
-
A I ( ~ D )+ C H ~
80 AI(2S)
CH4
-
CH3AI + H
=(
CH3+ AIH
20 1
01
AI(2P)
CH4
-(
)
CH3AIH
-20 -
Figure 1. Illustration of the relative energy of various reactants and products that could be involved in the reaction of AI atoms with methane, based upon the values discussed in the text. Lines in parentheses indicate uncertain energy values.
bond of AlH2 is due to the promotion energy required to "break" the filled 3sz shell of Al, such that chemical bonding can occur. This effect is expected to equally affect the formation of an A1-CH3 bond between AlH and CH3, such that the bond dissociation energy of both the CH3Al-H and HAl-CH3 bonds of CH3AlH is likely to be about 45 kcal/mol. Therefore the total bond dissociation energy of CH3AlH is likely to be close to the value calculated for AIHz of 116.9 kcal/mol. These values clearly indicate that the ground-state reaction of Al (2P) atoms with either methane or Hz is 10-1 1 kcal/mol exothermic, and therefore the absence of ground-state chemistry in the present study implies a significant energy barrier to the ground-state insertion process. The excitation energy corresponding to the 2S zP transition in the gas phase is about 72 kcal/mol, while that of the 2D 2P transition is about 93 kcal/mol. Therefore, the insertion of a 'S or 'D A1 atom into CHI or H2 is about 84 and 105 kcal/mol exothermic with respect to the ground states of CH3AlH and AlH2. The abstraction reaction to form AIH for these two states is 37 and 58 kcal/mol exothermic with respect to the ground state of A1H. Thus, both abstraction and insertion are highly favored reactions in a thermodynamic sense. As the exothermicity of the insertion of either 2Sor 2D states of AI into CH4 or H2 is greater than the first bond dissociation energy of both CH3A1H and A1H2, respectively, an insertion/fragmentation reaction could also occur. The relative energy values discussed above for reactants and products in the AI/CH4 system are illustrated in Figure 1 . B. Structure and Bonding in Methylaluminum Hydride and Aluminum Dihydride. Discussion of the structure and electronic characteristics of CH3AlH is conceptually simpler if preceded by an examination of the characteristics of AIH2. The lowest-energy configuration of AlH2 has been found both e ~ p e r i m e n t a l l yand ~~*~~ t h e o r e t i ~ a l l y to ' ~ be ~ ~ bent ~ with an HAlH bond angle of 119' and 118O, respectively. The experimentally derived bond length are of 1.59 A and the theoretical values of 1.60 and 1.61 in excellent agreement as well. The AI-H bond length of AIHz is considerably shorter than that of diatomic AIH, for which an experimental value of 1.648 A has been f o ~ n d . ' ~Calculated -~~ values of the AIH bond length are 1.652 and 1.645
-
-
(15) Pople, J. A.; Luke, B. T.; Firsch, M. J.; Binkley, J. S . J . Phys. Chem. 1985, 89, 2198. (16) Meyer, W.; Rosrnus, P. J . Chem. Phys. 1975.63, 2356. (17) Fox, D. J.; Ray, D.; Rubesin, P. C.; Schaeffer, H. F. J . Chem. Phys. 1980. 73. 3246. --- -.
--. -.
(18) Ahlrichs, R.; Keil, F.; Lischka, H.; Kutzelnigg, W.; Staernrnler, V. J . Chem. Phys. 1975,63, 455.
(19) Herzberg, G. Molecular Spectra and Molecular Structure 3 Electronic Spectra and Electronic Structure of Polyatomic Molecules; Van Nostrand: Princeton, NJ, 1979. (20) Herzberg, G. Adu. Photochem. 1968, 5 , 1. (21) Nestrnann, B.; Peric, M. Chem. Phys. 1984, 89, 257.
1222 The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
Therefore, a decrease in bond length of about 0.06 8, occurs on going from AlH to AIH2. Interestingly, the 211ulinear excited state of A1H2 (2B2in C2, symmetry) has been found to have an AI-H bond length of 1.53 .&.19,20 This trend in bond lengths undoubtedly reflects the changes in the A1 hybridization present in these molecules. Thus, bonding in AlH is likely to be mainly through AI 3p orbital/" 1s orbital interactions with an essentially filled 3s orbital shell on A1 that does not participate in the bonding. Evidence for the nonparticipation of the 3s2 pair can be seen in the bond strength variation between AlH (71.3 kcal/mol) and HAl-H (45.6 kcal/mol) discussed above, where the weakness of the second AIH bond is due to the promotion energy required to break the lone pair in forming an sp2-hybridized A1 atom. This hybridization for AlH2 should result in a bent geometry with an HAlH bond angle close to the sp2 angle of 120'. The inclusion of s orbital character in the u bonding framework results in a shortening of the AI-H bonds due to the increase in overlap energy and the more contracted nature of the 3s orbital of Al. This trend is also evident in the structure of the linear 2Q,excited state of AIH2, which is expected to exhibit sp hybridization. The increase in s orbital character in the u bonding framework of this excited state results in a further 0.06-A shortening of the AI-H bond length. Hybridization considerations also suggest that the unpaired electron of AlH2 should occupy an orbital of mainly A1 sp2 character and should show a significant isotropic contribution to the AI nuclear hyperfine splitting. As well, the only low-lying unoccupied orbital of AlH2 is expected to be the nonbonding orbital of mainly A1 3p orbital character that is perpendicular to the HAlH plane. The two lowest electronic states of A1H2 have been found to be 'A, and 2B1,19-21 which correlate with the 2rIustate of the Renner-Teller active linear geometry. Therefore, while the bending potential gradient for the bent state is positive for both closing and opening of the HAlH angle, it is negative for angle opening in the 2Bl state, for which the linear geometry is the most stable configuration. As a result, electronic excitation from the 2Al state to the first excited state (2B1)should not result in fragmentation of the molecule, as the u bonding framework retains four electrons and the excited-state geometry will only be expected to be unstable with respect to opening of the HAlH bond. Nestmann and Perk2' have found the 2Bl state to be dissociative only at angles less than 80°, which is considerably more bent than the estimated 118' angle of the ground state. These workers have also given data indicating that the first allowed electronic transition of AlH2 should occur with a maximum probability at 680 nm in the gas phase, with vibrational fine structure between 600 and 850 nm. The nondissociative nature of this transition is supported by the observation of A1H2stability during broad-band irradiation with visible light over long periods of time.9 Nestman and Peric have also calculated the energy difference between the minimum energy configurations of the ground (bent) and first excited (linear) state to be 21.7 kcal/mol (full CI). Therefore the energy difference between these two excited states is sufficiently large such that stabilization of the linear geometry is not expected to occur in the matrix. No other low-lying excited states have been calculated or observed for AIH2. The At2B1) X(2Al) transition of AlH2 is the only one of reasonably low energy involving electronic excitation between nonbonding orbitals. Thus, all other low-lying excited states are expected to involve promotion of a bonding electron from the bonding u orbitals or promotion of the unpaired electron into an antibonding u* orbital resulting in partial fragmentation or reductive elimination. The structure and electronic characteristics of AlH2 can be considered through examination of the well-known Walsh diagram for AH2 molecules,22 which shows the qualitative variation in orbital energy with bond angle (Figure 2). The ground state of AlH2 has been found to exhibit a valence orbital occupation of 4aI22b?5al1. The occupation of the 5al orbital results in a driving force for significant bending of the HAlH bond. Note that the +-
(22) (a) Walsh, A. D. J . Chem. Soc. 1953, 2260; see also ref 19a, p 319. (b) Herzberg, G.; Johns, J. W. C. Proc. R. Soc. London, A 1967, 298, 142.
Parnis and Ozin
90" 180' Figure 2. Qualitative Walsh-type diagram for A1H2.Illustrated are the valence orbital energy variations with bond angle, after ref 22.
molecule MgH2, in which the 5al orbital is unoccupied, is known to be linear.23 Both the 5al and 2bl orbitals correlate with the degenerate 27ruorbital pair of the linear molecule, and therefore electron excitation from the 5al to the 2bl orbital is expected to lead to opening of the HAlH bond angle since the 2bl orbital is insensitive to bending. The only other electronic excitation of comparable energy, based upon the Walsh diagram of AlH2 (Figure 2), is the (*B2) 2b2l5aI2
+ -
(2A1) 2b25a11
transition. The 2B2excited state of AIH, should be unstable with respect to increased bending of the AlH2 bond angle, and the 2B2 2Al transition is expected to lead to dissociation to form AlH + H, due to the loss of one u bonding electron. The structural characteristics of CH3AlH are expected to be similar to those of AlH2, due to the common character of the methyl and hydride groups with respect to A1 single-bond formation. As well, the absence of low-lying filled or empty orbitals on the CH, group suggests that the electronic states of CH,AlH and A1H2 will be similar in energy and character. The values for the AlH bond length (1.598 A) and the C-AI-H angle (1 18.4') calculated for CH3AlH in this study8 are in excellent agreement with the experimental and theoretical values for AIH2 discussed above, as well as recent ab initio calculations on the structure and bonding characteristics of CH3AlH by Quelch and Hillier.24 Chernier et a1.2shave calculated a value of 1.997 A for the AI-C bond of a puckered, five-membered metallocycle formed from A1 and but- 1,3-diene, for which a value of 95' was found for the C-AI-C angle. The AI-C bond length of AI-CH3 has been calculated to be 2.013 A by ab initio methods,17 which is somewhat longer than the AI-C bond found here for CH3AlH. This effect is probably analogous to that which is observed on going from A1H to AlH2 as noted above. In the case of CH,AlH, a decrease of about 0.03 8, is found, which is half that found for AlH2 and may reflect the inability of the sp3 hybrid orbital on methyl to benefit from the change in hybridization at A1 when compared with the 1s orbital of the hydride. The A1-C bond length calculated here is therefore thought to be a reasonable value, and it appears that the general bonding and structural characteristics of CH3AIH are closely analogous to those of A1H2. Methylaluminum hydride, if it were a pseudolinear species, would be expected to be unstable in the ground state with respect + -
(23) McCaffrey, J. G . ;Parnis, J. M.; Ozin, G. A,; Breckenridge, W. H. J . Phys. Chem. 1985,89, 4945. (24) Quelch, G. E.; Hillier, I. H. J. Chem. SOC.,Faraday Trans. 2 1987, 83, 2287. (25) Chernier, J. H. B.; Howard, J. A.; Tse, J. S . ; Mile, B. J . Am. Chem. SOC.1985, 107, 7290.
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1223
Structure of CH3AIH and A1H2 to bending due to the degeneracy of the e symmetry orbital (AI 3pJ. in which the unpaired electron would reside and should therefore be a Jahn-Teller active m o l e c ~ l e . ’ The ~ ground and first excited states of the bent (C,) form of the molecule are expected to be of A’ and A” symmetry. The first allowed electronic transition of CH3AlH should therefore be 2A’’ .+ zA’, which should lie in the visible region, by analogy with the closely related transition of A1H2. It would therefore seem reasonable to assign the visible absorption of CH3AlH between 450 and 600 nm to this transition. This conclusion is supported by the calculations of Quelch and Hillier,z4who predict the position of this transition to be 579 (RHF) or 644 (CI) nm. At the same time, the highly efficient yield of A1 atoms following excitation at this energy indicates that this transition is dissociative. This is in contradiction with what is expected based upon the calculated and observed properties of AIHz, for which the analogous A(2Bl) .+ X(2Al) visible transition is not dissociative. As it is well-established that thermal reductive elimination of methyl hydride species involves a significantly higher barrier than is found for analogous dihydrides,26CH3AIH should be expected to eliminate CH4 much less readily than H2 from A1H2. This apparent contradiction raises several possibilities. The first is that the observed visible absorption of CH3A1H may correspond to a transition other than the ZA” .+ zA’, one which involves excitation of an electron in the a bonding framework. The most likely is the ZA’.+ zA’ transition analogous to the 2B2.+ 2Al (a* .+ nb) transition of AlH2. It is conceivable that both these transitions occur in the visible region and that the observed optical absorption of CH3A1H is due to an overlap of both. This possibility could be tested through studies of the wavelength dependence of AI atom recovery over the range of the absorption. This may, however, be complicated by wavelength-dependent cage effect such as the one proposed for fragmentation of C H 3 C ~ H . z 7 We feel it is much more likely that the properties of CH3A1H are different from those predicted by Nestmann and Perkz1for AIHz with respect to the bending potential of the 2A’’ excited state (ZBIfor AIHz). That is, the 2A” state could be dissociative with respect to reductive elimination at much greater angles (Le., 118’) than the 34 kcal/mol if the reaction is taken as 10-1 1 kcal/mol exothermic. This, in itself, is not unreasonable, but channeling over 35 kcal/mol, about half the total excitation energy, into a single A1-H2 interaction following overall cage excitation does seem to be. This would clearly imply a fairly specific A1-H2 or CH4 interaction that would lead back to the direct insertion approach given above. With regard to the reactivity following excitation to the 2D state of Al, note that no evidence has been f o ~ n d for ~ - differences ~ in reactivity with respect to the 2S state. The established fluorescence behavior of A1 atoms in Ar and Kr indicates that both 2D- and 2S-state excitation leads almost exclusively to Stokes-shifted 2S 2P emission.8 Therefore, it would appear that a highly efficient electric-dipole-forbidden transition from the 2D state to the 2S state (7100 cm-’ energy difference) occurs in rare gas matrices and that a similar effect may occur in CH4 as well. It is concluded that all chemistry observed here originates from the same surface traversals except possibly for an initial transfer from the 2Dsurface to the 2S surface following 2D 2P excitation.
+
-
-
ConcIusion In the foregoing discussion, we have endeavored to gain insight into the dynamics of the ground- and excited-state reactivity of group 13 metal atoms with the small, covalently bonded molecules methane and hydrogen. It is hoped that through detailed theoretical and gas-phase dynamical studies of these systems the full mechanistic picture of the AI CH4 and A1 + H 2 reactions can be clearly understood. It seems certain that some sort of attractive excited-state interaction must be present to account for the large cross section for Ga atom (2S)quenching by methane in the gas phase. The absence of Stokes-shifted A1 2S 2P emission in CH4 matrices also indicates that the reactive quenching process is very efficient. As the only significant differences between methane and hydrogen that favor methane are the greater polarizability of CHI and its higher density of vibrational states, it would appear that a successful explanation of the reactivity of group 13 metals with CH4 and H 2 must incorporate one of these two properties as an important aspect of the mechanism.
+
-
Acknowledgment. The generous financial assistance of the Natural Sciences and Engineering Research Council Canada’s Operating and Strategic Grants programs are gratefully acknowledged. J.M.P. thanks NSERC for a postgraduate scholarship. Helpful discussions with Prof. W. H. Breckenridge are acknowledged. Registry No. AI, 7429-90-5; CH4, 74-82-8; H1, 1333-74-0; CH,AIH, 100993-96-2; AIH,, 14457-65-9.
