J. Phys. Chem. 1989, 93, 1215-1220 favored cage geometry. Long-range order could result in different site geometries being most stable in different regions of the matrix. The possibility of different phases of methane in the presence of AI atoms cannot be substantiated here but is thought to be the most attractive explanation for the observed site effects for A1 and CH3A1H in solid methane. The AI/CH4 reaction is also unique among metal atom reactions with methane since it involves a 2P ground-state and a 2S excited-state metal atom. This electronic configuration is “inverted” with respect to most metal atoms such as group 2 and 12 and most p orbital excitations are transition-metal atoms, for which s normally the lowest energy valence electronic transitions. The lack of reactivity of 2PA1 as well as the facile 2S A1 reaction with methane raises several questions with regard to the accepted mechanism for covalent u-bond insertion reactions of metal atoms, in which the presence of an electron in a p orbital is usually a major if not central aspect of the insertion mechanism. This point is discussed in detail in the subsequent two articles in this journal,”-42 in which the AI/H2 photochemical reaction is also described. The latter system also exhibits unusual reactivity that challenges the accepted mechanism for metal atom insertion into small covalently bonded molecules.
-
Computational Details Self-consistent field (SCF) ab initio calculations were done by using the MONSTERGAUSS family of programs available at the University of Toronto Department of C h e m i ~ t r y Calculations .~~ utilized the STO-3G basis function set and the 3-21G and 6-31G split-valence basis function sets available with the package.4648
1215
The latter was also used with polarization functions on all nonhydridic atoms (6-31G*) or on all atoms including hydrogen (6-31G**). All geometry optimizations were initially done by using the BFGS technique49with the STO-3G basis set, and then at higher levels by using the STO-3G result as an initial guess. Geometries were calculated within the restricted Hartree-Fock (RHF) formalism in which spin delocalization of unpaired electrons over all doubly occupied molecular orbitals is not allowed. Spin density calculations were done within the unrestricted Hartree-Fock (UHF) formalism in which delocalization of unpaired electron spin over all occupied molecular orbitals is permitted. U H F calculations are known to yield more realistic spin density values due to this feature.40 ESR spectral simulation was done by using the SIM14A program2’ as modified by D. F. McIntosh and J. M. Parnis. All spectra were fit through visual assessment of the goodness-of-fit following variation of the spin-Hamiltonian parameters. 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; CH4, 74-82-8; CD4, 558-20-3; 13CH4, 6532-48-5; CH3AIH, 100993-96-2; Kr, 7439-90-9; CD3A1D, 11716173-6. (47) 3-21G: Binkley, J. S.; Pople, J. A.; Hehre, W. J. J . Am. Chem. SOC. 1980, 202, 939.
(48) 6-31G*, 6-31G**: Hariharan, P. C.; Pople, J. A. Chem. Phys. Lett. 1972, 16, 217; Theor. Chim. Acta 1973, 28, 213.
(46) STO-3G: Hehre, W. J.; Stewart, R. F.; Pople, J. A. J . Chem. Phys. 1969, 51, 2657.
(49) Powell, M. J. D. Subroutine VA13AD, AERE Subroutine Library, Harwell, Didcott, Oxon. U.K. The algorithm is described in Fletcher, R. Comput. J . 1970, 13, 317.
Photochemlcal Reactions of Matrix-Isolated Aluminum Atoms with Methane and Molecular Hydrogen. 2. Molecular Hydrogen J. Mark Parnist and Geoffrey A. Ozin* Lash Miller Chemical Laboratories, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 1 A1 (Received: March 3, 1988; In Final Form: June 23, 1988)
- -
2S *Por 2D 2P excitation of A1 atoms in H2-dopedkrypton matrices is shown by UV-visible, IR, and ESR spectroscopies to lead very inefficiently to formation of A1H2 and AIH + H atoms. Extensive photoagglomeration, yielding mainly All, and 2S 2P AI atom fluorescence were also observed as major contributors to the quenching of excited-state A1 atoms. Infrared spectra of AlH2 are reported, with values of uI = 1766, u2 = 760, and u3 = 1799 cm-I. Preferential production of H atoms following excitation of A1 atoms in a HD-doped matrix was observed and is discussed with regard to an insertion mechanism for A1H2 formation. No ESR signals due to AIH2were observed, which is presumed to be due to the low efficiency of the Al/H2 reaction. The production of A1H is shown to be independent of AlH, formation, which is taken as indicating that the former arises from excited-state fragmentation of AlH2 immediately following its formation.
Introduction Many metallic elements that are currently known to photochemically activate methane in low-tempera ture matrices show similar reactivity with molecular hydrogen.’+ The similarity between the orbital morphology and dissociation energy of the C-H and H-H bonds of CH4 and H2 is often invoked as an explanation for this analogous chemical behavior. Therefore, it seemed reasonable to assume that, in light of the facile reaction of AI atoms with CH4,5the analogous reaction with H2 should lead efficiently to the formation of AIH2. In spite of its apparent ‘Present address: Laser Chemistry Group, Division of Chemistry, National Research Council Canada, 100 Sussex Dr., Ottawa, Ontario, Canada K I A OR6.
0022-3654/89/2093-1215$01.50/0
simplicity, AlH2 has only been synthesized as a gas-phase species whose characteristics are known only to the extent of the groundand first excited-state spectroscopic and structural properties.6 (1) Parnis, J. M.; Mitchell, S. A,; Garcia-Prieto, J.; Ozin, G. A. J . A m . Chem. SOC.1985, 107, 8169. (2) Ozin, G. A.; Mitchell, S . A.; Garcia-Prieto, J. Angew. Chem. Suppl. 1982, 785.
(3) Ozin, G. A,; McCaffrey, J. G. Znorg. Chem. 1983, 22, 1397. Ozin, G. A.; McCaffrey, J. G.; McIntosh, D. F. Pure Appl. Chem. 1984,56, 111. Ozin, G. A,; McCaffrey, J. G.; Parnis, J. M. Angew. Chem., Int. Ed. Engl. 1986, 25, 1072. (4) McCaffrey, J. G.; Parnis, J. M.; Ozin, G. A.; Breckenridge, W. H. J . Phys. Chem. 1985,89, 4945. ( 5 ) (a) Parnis, J. M.; Ozin, G. A. J . Am. Chem. SOC.1986, 108, 1669. (b) Parnis, J. M.; Ozin, G. A . J . Phys. Chem., preceding article in this journal.
0 1989 American Chemical Society
1216 The Journal of Physical Chemistry, Vol. 93, No. 4 , 1989
Parnis and Ozin
AIIKr
t w
0 2
6
m E
0,m
6
200
200
300
400 nm
WAVELENGTH
Figure 1. UV-visible absorption spectra of AI atoms in solid krypton (A) on deposition, (B) following photoexcitation at 390 nm (10 nm) for 23 min, (C) following subsequent photoexcitation at 368 nm (20) nm for 40 min, and (D) following subsequent photoexcitation at 303 nm (20 nm) for 220 min. Arrows indicate the photoexcitation wavelength positions.
There is no reason to believe that A1H2 is particularly unstable as an isolated molecule, and recent ab initio calculations predict it to be a stable bent molecule' in support of the above experimental study. Since little is known about the properties of AlH2 and as it promised to exhibit a well-defined ESR spectrum, qualitatively similar to that of CH3AlH,5a study of the reactivity of photoexcited A1 atoms with H2 was undertaken. Further discussion of the structure and bonding of AlH2 as well as comparison between the reaction of A1 atoms with CHI and H2 may be found in part 3 of this series.*
Experimental Section The experimental design was essentially that described in the preceding article in this journal for the A1 + CH4 reaction.5b Research grade H2 and Kr as well as C P grade D2 were supplied by Matheson of Canada (Toronto). H D of 98% isotopic purity was supplied by Merck, Sharpe & Dohme (Pointe Claire, Quebec). Results A . UV-Visible Absorption Studies. The photochemical reaction of A1 atoms with H2 was studied by UV-visible absorption spectroscopy. All studies were done in Kr matrices due to its higher efficiency for H2trapping with respect to Ar at 12 K and its favorable light-scattering properties compared with those of Xe. Therefore, as a prelude to H2 reactivity studies, A1 atoms were studied in solid Kr. The spectra obtained were essentially identical with those previously reported for A1 in Kr.9-'2 In (6) (a) Herzberg, G. Molecular Spectra and Molecular Structure 3: Electronic Spectra and Electronic Structure of Polyatomic Molecules; Van Nostrand: Princeton, NF, 1979. (b) Herzberg, G. Adu. Photochem. 1968, 5, 1 . (7) Nestmann, B.; Peric, M. Chem. Phys. 1984, 89, 257. (8) Parnis, J. M.; Ozin, G . A.; J . Phys. Chem., subsequent article in this
journal. (9) Abe, H.; Kolb, D. M. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 523. (IO) Douglas, M. A.; Hauge, R. H.; Margrave, J. L. J. Phys. Chem. 1983, 87, 2945. ( 1 I ) Ammeter, J. H.; Schlosnagle, D. C. J . Chem. Phys. 1973, 59, 4784.
400 nm
300 WAVELENGTH
Figure 2. UV-visible absorption spectra of AI atoms in a H,-doped krypton matrix (1:12) (A) on deposition, (B) following photoexcitation at 362 nm (10 nm) for 45 min, and (C) following subsequent photoexcitation at 283 nm (10 nm) for 45 min. Arrows indicate the photoexcitation wavelength positions.
-
particular, absorptions due to A1 atoms were observed at 389,369, and 364 nm (2S 2P) as well as at 31 1, 303, and 298 nm (2D 2P) and 225 nm. As noted in part l,5bthe latter has been assigned to A12 by Abe and Kolbqbut has been found in the present study to behave as other A1 atom bands. Bands due to A12 were observed at 283 and 400/404 nm as well as at 680 nm, the latter being a broad absorption similar to that reported by Douglas et a1.I0 Also present on deposition and previously unreported was a pair of weak bands at 546 and 558 nm with an intensity ratio of about 1:6 (546558). These unidentified bands disappeared after brief photolysis at >550 nm and are undoubtedly due to an impurity generated on deposition, which did not appear to correlate with any other absorptions. Figure 1A illustrates the deposition spectrum of a highly dilute sample of A1 atoms in Kr. Weak bands due to A120 at 268 and 273 nm are present as well as a small amount of A12 at 283 nm. Brief narrow band photoexcitation a t 390 nm caused the band at this wavelength to disappear completely (Figure 1B). Subsequent photoexcitation at 368 nm caused this band to reappear at about 35% of its original intensity (Figure lC), which indicates that site interconversion occurs during atomic photoexcitation. Photoexcitation at 368 nm also caused some decay of all other A1 absorptions with slight A12 and A120 growth and growth of new bands at 235 nm. Some pale blue emission was apparent during this photoexcitation. Subsequent photoexcitation at 303 nm (20 nm) caused decay of the 389-nm band, strongly supporting the assignment of this band to atomic Al. This follows since there must be a corresponding absorption in the atomic A1 2D 2P region near 303 nm. Photoexcitation at 303 nm caused preferential loss of the 293-nm band with respect to those at 303 and 310 nm, as well as loss of the 364-nm band to a lesser extent. This clearly shows that the splitting on the 2D 2P absorption of A1 in the site exhibiting the 369-nm absorption is at most twofold (303, 310 nm) as was found for AI in CH4. This contradicts the conclusion of Abe and K ~ l bwho , ~ have stated that the threefold splitting of this absorption is evidence for a site symmetry lower than axial. The present work clearly shows that the absorptions at 298 and 303/310 nm cannot be due t o the same site. Photoexcitation at
-
-
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(12) Grinter, R.; Singer, R. J. Chem. Phys. 1986, 213, 87.
The Journal of Physical Chemistry, Vol. 93, No. 4 , 1989 1217
Reactions of AI with H 2
1
W
V Z
6
a a
%a
6
~
250 250
350
450 nm
WAVE L E NG T H
Figure 3. UV-visible absorption spectra of AI atoms in a H2-doped krypton matrix (1:12) (A) on deposition, (B) following photoexcitation at 362 nm (10 nm) for 10 min, (C) following subsequent photoexcitation at 302 nm (10 nm) for 40 min, and (D) following subsequent photoexcitation at 362 nm (20 nm) for 20 min. Arrows indicate the photoexcitation wavelength positions.
303 nm also caused further A12 and A120 growth, as well as growth of unidentified bands at 235 nm. Pale blue emission was also observed, as with 368-nm photoexcitation. This undoubtedly corresponds to the visible emission observed for A1 in Kr during fluorescence studies described in part l:b which has been ascribed to Stokes-shifted 2S 2P emission. Therefore, it is concluded that photoexcitation of A1 atoms in Kr at either the *D 2Por 2S *P atomic transitions leads to A1 atom visible emission, photoinduced dimerization, and site interconversion. Co-condensation of AI atoms in Kr containing H2(1 :12) resulted in a UV-visible spectrum that is similar to that of A1 in neat Kr as seen in Figure 2A. All bands were considerably broader, and the 2S 2Pabsorptions were blue-shifted from their positions in neat Kr. As well, a new absorption at 350 nm was also present. AI atom absorptions appeared at 381, 365, 360, 350, 310, 303, and 298 nm where the 310-nm band was a poorly resolved shoulder. AI2 absorptions appeared at 282, 3991403, and 680 nm, close to the positions at which they appear in Kr. Photoexcitation at 362 nm (10 nm) for 45 min (Figure 2B) caused about a 60% drop in intensity of all A1 atom absorptions, while causing growth of AI2 bands at 283 and 399/404 nm. Some growth of the 680-nm band was also observed. Photoexcitation at 302 nm (10 nm) (Figure 3C) caused further loss of AI atoms with growth of A12 a t 399/404 nm. Considerable depletion of the 350-nm band following 302-nm photoexcitation confirms that this absorption is due to A1 atoms in a site that is not present or populated in neat Kr. As in neat Kr, pale blue emission was noted during both 302- and 362-nm excitation. Photoexcitation at >450 nm with broad-band light caused all A12 absorptions to drop significantly in intensity while causing A1 atoms to grow by about the same amount (Figure 4). Therefore the band at 680 nm is most likely due to a dissociative absorption of AI2. Photoexcitation at the dimer band at 283 nm (10 nm) also caused the 680-nm band to diminish (Figure 5C), thereby confirming this assignment. However, 283-nm photoexcitation did not regenerate a significant amount of A1 atoms (Figure 2C), which may be indicative of some A12 reactivity with H 2 following excitation at this wavelength. Conversely, A1 atom regeneration following photoexcitation at >450 nm could conceivably have been due to an A1/H2 product dissociation. Indeed, a change in the base line following 368-nm A1 atom photoexci-
-
-
+-
-
350 WAVEL E NG T H
450 nm
Figure 4. UV-visible absorption spectra of AI atoms in a H2-doped krypton matrix (1:12) (A) following extensive photoexcitation at 362 and 302 nm and (B) following subsequent photoexcitation with broad-band
visible light (>450 nm) for 20 min.
AI I H 2 : K r
1 I
W
V
z
6
m
8
v,
m
6
400
600 WAVELENGTH
800 nrn
Figure 5. UV-visible absorption spectra of AI atoms in a Hz-doped krypton matrix (1:12) (A) on deposition, (B) following photoexcitation at 362 nm (10 nm) for 45 min, (C) following subsequent photoexcitation at 283 nm (10 nm) for 45 min, and (D) following subsequent photoexcitation at 558 nm (20 nm) for 15 min. Spectra A-C correspond to the same in Figure 2.
tation consisting of a small rise between 450 and 800 nm was noted, which could have been due to a product absorption. However, its breadth and shape suggest that it could also have been due to a base-line change following photoexcitation, sometimes observed following extensive photolysis of matrix samples due to photoinduced matrix annealing. Full clarification of the source of A1 atoms following >450 nm broad-band photoexcitation could be made by photoexcitation at >450 nm prior to A1 atom photoexcitation at 362 or 302 nm, at which point no A1H2 products should be present. As in neat Kr matrices, absorptions were observed on deposition at 546 and 558 nm in H2:Kr matrices containing AI atoms, which disappeared following brief photoexcitation at 558 nm, as illustrated in Figure 5D.
1218 The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
It is clear from these results that photoexcitation of A1 atoms in Kr matrices containing H2 leads to a combination of visible A1 2S 2P emission as well as A1 dimerization. There is little evidence for the formation of a stable dihydride intermediate analogous to CH,AlH, and it is clear from the rate of loss of A1 atoms during atomic photoexcitation that the quantum yield for loss of AI atoms is much lower in H2/Kr that in neat methane matrices. Note that in other matrix systems in which facile reaction between photoexcited metal atoms and methane or hydrogen is ob~erved,l-~ rates of depletion of metal atoms during photoexcitation in neat methane or 1:12 H2:rare gas matrices are comparable. This implies that the low rate of reactivity observed here for AI atom excitation in 1:12 H2:Kr is not due to a lower concentration of reactive molecules in the matrix cage with respect to neat methane samples. B. ESR Studies. The reaction of A1 atoms with H2 in Kr was studied by ESR spectroscopy. The deposition spectrum consisted only of a poorly defined but highly reproducible spectrum of AI atoms that was unchanged in either neat Kr or H2:Kr (1:12). Some of the features of this spectrum correspond to those of the published spectra of AI in Kr,I3 although the latter are much more well-defined spectra that obtained in this study. Photoexcitation of AI atoms in H2:Kr matrices at 302 nm for 12 h resulted in extensive depletion of A1 atom features and growth of moderately intense signals due to H atoms. No new features attributable to any other species were detected after either 362- or 302-nm photoexcitation. Thermal annealing of extensively photolyzed samples caused only depletion of A1 and H atom signals with no growth of any new bands. Photoexcitation of A1 atoms at 362 nm in D2:Kr (1:12) caused D atom growth on a similar scale as observed for H2, as estimated by peak height comparison. Also present were bands of lower intensity due to H atoms, which indicates that some activation of other species such as H 2 0or H2 impurity has occurred. Subsequent broad-band photoexcitation of this sample at >450 nm caused slight growth of both H and D atoms as well as marginal growth of A1 atoms. When A1 atoms were photolyzed in a 1:l mixture of H2 and D2 in Kr (1:1:24), approximately the same amount of H and D atom growth (1.2: 1, H:D) was found, while photoexcitation of A1 atoms in HD:Kr (1:12) resulted in an approximately 3:l yield of H:D. The latter result may reflect a tendency to produce H atoms preferentially in the Al/HD reaction. A small contribution to the H atom yield may result from activation of other hydrogen-bearing molecules such as traces of hydrocarbons or water. It is likely that this contribution is responsible for the slight preference for H atom production in H2:Dzmatrices and probably occurs to the same degree in the H D experiment. Therefore, it is concluded that the H:D yield of 3:l in the H D experiment is a real effect and that a preference for H atom formation from A1 H D does exist in solid Kr. There is no doubt that activation of H2 (D2) is occurring in the A1/H2 system. Production of significant quantities of D atoms can only be due to D2 cleavage and cannot be attributed to any hydridic impurities. The most logical and reasonable source of H or D atom production is formation of A1H2 followed by fragmentation to form AIH + H, either through decomposition of AlH2 immediately following its formation or through secondary photolysis. Note that the former reaction sequence has been proposed for the photochemical reaction of Cu H2 to form CuH H atoms,2 while the latter has been proposed for the Cu CH4 system.' C. Infrared Studies. The reaction of A1 atoms with H2 in Kr was studied by IR spectroscopy. Deposition spectra normally showed only trace absorptions due to matrix-isolated impurities such as H 2 0 (1620 cm-') and C 0 2 (2346, 667 cm-I). In a few cases, a small absorption at 1588 cm-l was observed on deposition, which is close to the known frequency for AIH in argon (1593 cm-').I4 Absorptions a t 2140 cm-' due to trace matrix-isolated
Parnis and Ozin AI/H,Kr
1
-+
+
+
+
+
(13) Knight, L. B.; Weltner, W. J . Chem. Phys. 1971, 55, 5066. (14) Wright, R. B.; Bates, J. K.; Gruen, D. M. Inorg. Chem. 1978, 17, 2275.
I
AIH,
AlH
I
1900
I
I
1700
I
1500
h
"
I
900
700 c r n - '
WAVENUMBER
Figure 6. Portions of the infrared transmission spectra of a H2-doped krypton matrix containing AI atoms (A) on deposition and (B) following photoexcitation at 302 nrn (20 nm) for 12 h.
carbon monoxideI5 were present in some cases. Photoexcitation at either 302 or 362 nm for long periods of time (2-12 h) caused the growth of several bands, as shown in Figure 6. All experiments showed reproducible growth of absorptions at 1799 (w), 1766 (vw), 1588 (m), 836 (vw), and 760 (m) cm-I. Very weak bands at 779, 740, and 692 cm-' were also sometimes seen. None of the above bands showed any differential growth behavior; that is, all grew in together, which suggests that neither secondaryproduct formation or primary-product deterioration due to secondary photolysis was occurring. Broad-band photoexcitation at >450 nm caused the bands at 1799 and 1766 cm-' to disappear and new bands at 1815 and 1763 cm-' to appear. Subsequent photoexcitation at 305 nm regenerated the bands at 1799 and 1766 cm-' with no loss of the other two new bands. N o detectable change was noted in the 1588- and 760-cm-' bands during >450-nm photoexcitation, while both bands grew substantially following subsequent 305-nm photoexcitation. The very weak bands at 740 and 690 cm-' were unaffected by both photolyses, and the very weak band at 779 cm-' was not affected by >450-nm light but diminished on 305-nm photoexcitation. Photoexcitation of A1 atoms at 305 and 360 nm in.D2:Kr matrices led to similar results as with H2. Bands at 1320 (w), 1275 (vw),1156 (w), and 560 (m) cm-' were reproduced in two experiments, and only the band at 560 cm-' showed a reasonable intensity. In one case, production of small amounts of AIH is suspected due to the presence of a weak absorbance at 1588 cm-' observed on deposition, which grew slightly following photoexcitation. The spectral intensity of all bands with respect to those in the H2:Kr experiments performed under similar conditions appears to roughly correspond to the anticipated decrease in intensity on deuterium substitution,16 but could also conceivably reflect an energy barrier to insertion of A1 into H2. The absorptions at 1588 cm-' in H2/Kr and 1156 cm-' in D2/Kr undoubtedly correspond to the stretching modes of AlH and AID, respectively. Absorptions due to these species in Ar matrices14 have been reported at 1593 and 1 158 cm-I, while their gas-phase values" are 1624.4 and 1181.7 cm-l. The observed isotope shift ratio of 1.374 is virtually identical with the values of 1.376 and 1.375 obtained from Ar matrices and the gas phase, respectively, and is typical of a diatomic hydride stretch. Therefore, AIH is identified as a major IR-active product in the A1/H2 reaction in Kr. (15) Hinchcliffe, A. J.; Ogden, J. S.; Oswald, D. D. J . Chem. Soc., Chem. Commun. 1972, 338. (16) Wilson, E. B.; Decius, J. C.;Cross, P. C . Molecular Vibrations;
McGraw-Hill: New York, 1955. (17) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure 4: Constants of Diatomic Molecules; Van Nostrand: Princeton, NJ, 1979; p 24.
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1219
Reactions of A1 with H2
TABLE I: Predicted and Observed Vibrational Frequencies for AlHl and AID, in em-’
AIH2
AID,
VI
v.)
V1
1766 2240 1954 1739 1778 1275 1265
760 750 820 739 746 560 540
1799 1760 1976 1759 1798 1320 1620
obsd this work calcd ref 7 calcd ref 19 ref 19 X 0.89 ref 19 X 0.91 obsd this work calcd ref 7
Corresponding with the formation of AIH is the observation of hydrogen atom production as manifest by the weak absorption at 836 crn-’. H atoms are known to exhibit a vibrational mode in Kr at 852 cm-I, which is thought to be due to a “cage vibration” involving motion of the H atom in the trapping sitesi8 A similar but much more intense band at 838 cm-’ has been observed in the study of the Cu H2 CuH H photochemical reaction,2 which proceeds with much greater efficiency that the present reaction. The frequency shift in the A1 and Cu systems with Hz:Kr (1: 12) with respect to the value for H2:Kr (1 :400) is believed to be due to the disruption of the Kr lattice structure due to the presence of Hz as a concentrated lattice constituent instead of as a trace impurity, as is the case for the latter experiments. It is also conceivable that the frequency shift in the 1:12 HsKr matrices arises from a H atom interaction with unreacted Hz. This possibility has not been further investigated here. The three absorptions at 1799, 1766, and 760 cm-I can be assigned to the three anticipated vibrational modes of AlH2. The isotope shift ratios for the three corresponding bands on D2/Kr are 1.363, 1.385, and 1.357, which indicates that all modes involve a large amount of hydrogen nucleus motion. Recent calculations on the structure and properties of AIHz by two groups have led to the prediction of its vibrational frequencies. Table I gives the observed frequencies for A1H2 from this study, as well as predicted frequencies for AlH2 from calculations by Nestmann and Peric’ and by Pople et aLi9 The former workers have calculated values for A1D2,which are also given. The values of Nestmann and Peric agree remarkably well with the present experimental values with the exception of the symmetric stretch, which is very much higher in their theoretical calculations. Pople et al. recommend scaling their values by a factor of 0.89, which yields values (Table I, ref 19 X 0.89) that are then in reasonable agreement with the present study. Scaling with a factor of 0.91 results in an excellent fit (Table I, ref 19 X 0.91). This scaling practice, which is essentially an anharmonicity correction, can be justified by pointing out that a factor of 0.90 is required to make the calculated frequency of AIH given by Pople et al. (1771 cm-’) match the known frequency (1 593 cm-I) of A1H in the gas phase. Therefore, in light of the results of these two calculations, it seems reasonable to assign the three bands seen in the present study at 1766,760, and 1799 cm-’ to the vibrational modes of AIH2. The intensity difference between the bending mode v2 and both stretching modes is typical of the dihydrides such as MgHz,4 MnHz, and FeH23where the former is always broader and more intense. The frequency shift of the proposed u1 and v3 bands following >450-nm photoexcitation is unusual, especially since this photoexcitation caused no noticeable change in the bending mode absorption at 760 cm-l. It is suggested that visible light causes a minor rearrangement of AIH2 in the matrix, which results in a shift of the two stretching modes but not of the bending mode. AIHz is known to exhibit an electronic abscrption in the visible region? It is expected to have a maximum intensity of absorption around 680 nm with a range of 600-900 nm, based upon the calculations of Nestmann and P e r i ~ .The ~ lack of A1H growth during this photoexcitation suggests that the excitation does not lead to fragmentation. Note also that CH3AIH is known to occupy two sites in CH4,Swith the AI-H stretching mode appearing at 1746 and 1764 cm-l. N o other modes of CH3A1H show such
+
-
+
(18) Bondybey, V. E.; Pirnentel, G. C. J . Chem. Phys. 1972, 56, 3832. (19) Pople, J. A.; Luke, B. T.; Frisch, M. J.; Binkley, J. S. J . Phys. Chem. 1985,89, 2198.
splitting in CH4. Therefore a shift of 16 cm-I such as that observed here for AlH2 upon visible photoexcitation is not without precedent, nor is the lack of shifting observed for the AlH, bending mode.
Discussion The results of this study of the reaction of A1 atoms with molecular hydrogen in Kr lead to the following conclusions: (1) UV-visible, infrared, and ESR spectroscopies show that loss of AI atoms is much less efficient in Hz-doped Kr matrices than in neat methane. ( 2 ) Significant production of A12 and visible emission as well are competing routes for deactivation of photoexcited Al. (3) Both AIHz and AlH + H are formed in low yields during the initial photoexcitation of A1 atoms, and neither is significantly affected by subsequent irradiation at >450 nm. (4) Growth of both AIHz and AIH appears to be simultaneous during the photoexcitation of A1 atoms, and growth of the latter does not appear to be dependent upon the concentration of the former. (5) ESR studies in HD-doped Kr matrices suggest that H atom production is favored over D by a factor of 3:l. The reaction is presumed to proceed via an insertion of A1 atoms to form A1H2, some of which fragments due to excess internal energy immediately following its formation. This conclusion is reached since AlH grew in independently of AIHz and did not show significant growth during extensive photoexcitation of A1H2-containing samples in which no A1 atoms remained. The reaction is expected to be highly exothermic, since the thermal reaction of ground-state AI atoms with H2 is nearly thermoneutral. Pople et aI.l9 have estimated the total dissociation energy of AlH2 to be 116.9 kcal/mol, which suggests that the ground-state reaction could be exothermic by >12 kcal/mol. Therefore, addition of about 80 kcal/mol in photoexcitation energy could provide sufficient energy to rupture one AIH bond of AlH,, the strength of A which has been estimated as Do(HA1-H) = 45.6 kcal/m01.~~ photochemically induced abstraction process involving the formation of AlH + H could be about 47 kcal/mol exothermic and therefore is a possible alternative to insertion on thermodynamic grounds. If this mechanism were operative, one would be forced to invoke a matrix-cage-mediated recombination of A1H and H atoms to form A1H2. The high mobility of H atoms in rare gas matrices and the exothermicity of the abstraction process are both expected to lead to efficient isolation of the A1H and H atom fragments following an abstraction process and therefore do not favor such an interpretation. This point is discussed further in the following article in this journaL8 The preference for H atom production in the ESR spectra of A1 + HD/Kr following photoexcitation of A1 may be suggestive of a fragmentation involving a vibrationally excited insertion product.20 Similar fragmentation has been observed for Cu + H D where production of H atoms was favored over D atoms by a factor of about 2 and where CuH2 has been postulated as an unstable transient intermediate. Similar H / D ratios are implied by the M D / M H yield ratios of 4:l and 2.5:l obtained in the gas-phase reactions of 3P, Hg and 3Pl Cd with H D in which insertion mechanisms have been proposed.21,22 However, as Breckenridge has noted,z3 there are several examples of atom reactions with molecular hydrogen in which insertion mechanisms are proposed, but in which MH/MD ratios of about 1:l are found. Thus, conclusions about the reaction mechanism based upon H / D ratios do not appear to be useful. The observed preference could simply reflect the greater tendency for H atoms to escape the surrounding cage, and therefore a definitive conclusion about the mechanism cannot be made here. The absence of an ESR spectrum attributable to AIH2 in all experiments attempted is somewhat perplexing in light of the observation of a weak but detectable IR spectra of AIHz. The most reasonable explanation for this seems to be that AlH2 was (20) Tsukiyami, K.; Katz, B.; Bersohn, R. J . Chem. Phys. 1985.83, 2889. (21) Bras, N.; Butaux, J.; Jeannet, J. C.; Perrin, D. J . Chem. Phys. 1986, 85, 280. (22) Breckenridge, W. H.; Urnernoto, H.; Wang, J.-H. Chem. Phys. Lett. 1986, 123, 23. (23) Breckenridge, W. H.; Wang, J.-H. J . Chem. Phys. 1987, 87, 2630.
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
