J . Phys. Chem. 1989, 93, 4023-4029
4023
Photochemistry of Methylcopper Hydride, CH,CuH: Wavelength Dependence of the Product Distribution J. Mark Parnist and Geoffrey A. Ozin* Lash Miller Chemical Laboratories, University of Toronto, 80 S t . George Street, Toronto, Ontario, Canada M5S 1A1 (Received: July 18, 1988)
The photochemistry of matrix-isolated CH3CuH is examined in the range 27C-700 nm, over which it exhibits an unstructured absorption. Photolysis of CH3CuH between 270 and 500 nm leads mainly to CH, + CuH production, while Cu atom production is dominant between 500 and 700 nm. No production of hydrogen atoms is observed between 450 and 700 nm. The photochemistry of CH3CuH is considered with regard to the known and calculated properties of CuH,. A wavelength-dependent effect on the dissociation of a single electronic transition of CH3CuH is proposed to account for CH, radical and Cu atom yield variations during CH,CuH photolysis.
Introduction The photochemical reaction of matrix-isolated copper atoms with methane has been studied Ozin et al?.) have detailed the growth and decay behavior of the various products and fragments involved. This work established that 2P-2S excitation of Cu atoms in CH4 leads to CH3CuH formation. Tail-end absorption by CH3CuH during photoexcitation of Cu causes its decomposition to form CH3 + CuH, CH3Cu H , and Cu + CH,. Thermal annealing of product fragments results in CH3CuH reconstitution. The growth and decay behavior of Cu, CH3, and CH3CuH during Cu atom photoexcitation is consistent with a two-step process involving formation of CH3CuH as a photochemically active intermediate, a conclusion that is supported through reaction simulation. Previous studies3 of the photochemistry of CH,CuH focused upon confirming the dissociative nature of the broad, visible ab350 nm) as well as on determining sorption of CH3CuH (A,, the types of fragments formed following its dissociation and their relative distribution. Photolysis of CH3CuH at 350 nm led to the reappearance of UV and ESR absorptions due to Cu atoms and extensive bleaching of the broad UV-visible absorption band a t 350 nm and the characteristic ESR spectrum due to CH3CuH. IR spectra showed complete disappearance of peaks due to CH3CuH (1697 and 101 1 cm-I), accompanied by growth of CuH and C H 3 absorptions as well as those due to CH3Cu to a lesser extent. ESR spectra showed growth of absorbances due to methyl radicals and hydrogen atoms. No other absorptions were observed after complete elimination of CH3CuH. The branching ratio for Cu atom formation was estimated at 25% for photolysis at 350 nm, based upon estimation of the maximum theoretical yield expected under these conditions. The remaining photodecomposition was predominantly by partial fragmentation to form CH3 + CuH, which were the dominant products observed in the IR and ESR following 350-nm photolysis of CH,CuH. Jnitial studies of the photochemistry of CH3CuH were done at 350 nm in an attempt to determine the photochemical behavior of CH3CuH in the region of Cu atom a b ~ o r p t i o n .At ~ the time, the absorption of CH3CuH was thought to extend only to 400 nm, and therefore, photolysis was confined to the range 330-400 nm. When an initial study of the relative yields of CH3, H atoms, and Cu atoms did not appear to reveal a wavelength-dependent trend, the photolysis was then confined to 350 nm, the point of maximum absorbance in the band. Therefore, the incorrect assumption was made that photolysis throughout the 330-400-nm range did not lead to significantly different product distributions. Subsequent reexamination of the initial data as well as further experiments indicated that a definite trend toward reduced H atom yield is present in this range, an effect that had originally been ascribed to H atom diffusion during the relatively lengthy photolysis of
+
-
'Present address: Laser Chemistry Group, Division of Chemistry, National Research Council of Canada, 100 Sussex Dr., Ottawa, Ontario, Canada KIA OR6
CH,CuH. We have therefore reexamined the photochemistry of CH3CuH over a wide wavelength range and report here a wavelength dependence in the branching ratio for fragmentation of CH3CuH in solid methane.
Experimental Section The methods used were essentially identical with those described in an earlier study of the Cu/CH, ~ y s t e m .Briefly, ~ copper atoms, generated by heating a tantalum filament wrapped with O F H C copper wire (99.95%), were cocondensed with U H P methane at 12 K on a CsI window (IR) or a sapphire rod (ESR). IR spectra were recorded on a PE-180 spectrometer in absorbance mode. ESR spectra were recorded with a Varian E4 X-band spectrometer. All photolyses of ESR samples were done in the microwave cavity through a small aperture, such that movement of samples was not required. Photolyses were done with a 450-W xenon arc lamp in an Oriel lamp housing equipped with a IO-cm watercooled, IR-filtering quartz water cell and an f l l . 0 condensing lens. Wavelength selection was done with use of either an Oriel Model 7240 grating monochromator or Corning glass cutoff filters, as noted in the text. Samples containing high concentrations of CH3CuH were formed through partial photolysis of copper atoms in methane at 305-315 nm with narrow-band light (5-8 nm). This procedure was found to reduce the extent of secondary photolysis, presumably due to a reduction in the extinction coefficient of CH3CuH at these wavelengths. In ESR experiments, where copper atom absorptions could be monitored, Cu atom photoexcitation was typically carried out until about 20% of the initial absorbance remained. This was found during studies of the growth and decay behavior of CH3CuH and Cu atoms to be the point at which the maximum concentration of CH3CuH was present. In order to eliminate complications due to further reaction of Cu atoms during CH3CuH photolysis between 330 and 400 nm, two UV cutoff filters (C330 nm) were placed in the beam path before the sample. CH3CuH was photolyzed at 25-nm intervals between 300 and 500 nm (20 nm) as well as at 550 nm and at wavelengths greater than 600,650, and 700 nm with cutoff filters. Relative yields for Cu atoms, methyl radicals, and H atoms were obtained through measurement of ESR signal peak-to-peak amplitudes, A,. This value is linearly proportional to the number of spins in the sample N , by4 N = kH,;A,, where H,,represents the peak-to-peak width of the ESR signal (1) Billups, W. E.; Konarski, M. M.; Hauge, R. H.; Margrave, J. L. J . Am. Chem. SOC.1980, 102, 7393. ( 2 ) Ozin,G. A.; McIntosh, D. F.; Mitchell, S. A,; Garcia-Prieto, J. J . Am. Chem. SOC.1981. 103. 1574. (3) Parnis, J. M.; Mitchell, S . A.; Garcia-Prieto, J.; Ozin, G.A. J . A m . Chem. SOC.1985, 107, 8169. (4) Jen, C. K.;Foner, S . W.; Cochran, E. L.; Bowers, V. B. Phys. Rev. 1958, 112, 1169.
0022-365418912093-4023$01.50/0 0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 10, I989
4024
I
I
, .
3C3
t--
40C
*
cu
*
.
Cd3
! i
I
>-
500
60C
700 n m
PHCTCLYSIS WAVELENGTH
Figure 1. Graphical representation of the variation in yields of Cu atoms, methyl radicals, and H atoms following photolysis of CH,CuH at wavelengths between 300 and 550 nm as well as at wavelengths greater than 600,650, and 700 nm. All data except that from 300- and 325-nm photolysis were obtained following incomplete photoexcitation of Cu atoms in methane at 312 nm. Data for 300 and 325 nm have been normalized as discussed in the text. H atom data are corrected for
thermal diffusion losses. Data for each species have been scaled such that the curves do not represent relative concentrations of one species with respect to another. that is related to the absorbance bandwidth. Comparison of peak-to-peak heights in arbitrary units allows for the measurement of relative concentrations of any single species, provided that the bandwidth of the absorbance, and therefore Hm,remains constant throughout the photoexcitation and from sample to sample. These conditions were met in the present study. As has been noted previ~usly,~ comparison of the total peak height of the ESR signals due to H atoms and methyl radicals can be used as a direct measurement of the relative concentrations of these two species, since both exhibit superimposible line shapes in solid methane matrices.
Results Samples containing large amounts of CH3CuH were photolyzed over the range 270-700 nm. Photochemical activity began a t about 270 nm and continued beyond 700 nm. Figure 1 illustrates in graphical form the yields of Cu, CH,, and H atoms in arbitrary units following complete photolysis of CH,CuH between 300 and 700 nm. H atom yields have been corrected for small losses due to diffusion, through addition of the estimated loss of H atoms that occurred during CH3CuH photolysis a t wavelengths >450 nm, where H atom production does not occur. For all wavelengths above 340 nm, the photolysis of CH3CuH was preceded by photoexcitation of Cu atoms a t 312 nm such that 20 i 1% of the C u absorption present on deposition remained. Each sample contained approximately the same amount of copper on deposition such that direct comparison of product yields could be made between runs following a small correction for slight variations in the initial Cu atom content. For photoexcitation a t 300 and 325 nm, reactions were carried to completion since photoexcitation of C u atoms is unavoidable a t these wavelengths. These two data points have therefore been scaled by factors of 0.525 (300 nm) and 0.51 (325 nm) in order to give yields of CH, and H atoms that are comparable with other data points in which C u atom consumption is not complete. Evaluation of these scaling factors requires corrections for the fact that secondary photolysis a t wavelengths greater than 340 nm began with -20% Cu atoms unreacted as well as -15% CH, and H atom yields already present, while photoexcitation at 300 or 325 nm requires complete consumption of all Cu atoms. Moreover. Cu atoms formed during decomposition of CH,CuH a t 300 and 325 nm are recycled, thereby further increasing the yield of C H , and H atoms. The wavelength-dependence curves for product formation during photolysis of CH,CuH given in Figure 1 clearly reveal several ( 5 ) Poole, C. P.; Farach, W. A. The Theory of Magnetic Resonance; Wile): New York. 1972; Chapter 16.
Parnis and Ozin trends in product formation. Perhaps most striking is the absence of H atom growth a t wavelengths greater than about 425-450 nm. As a result, the CH3:H ratio increases dramatically on passing from 300 to 400 nm. This effect was not evident during primary photoexcitation of Cu atoms, since the change over the range 305-325 nm appeared to be within the error of the measurement.3 Also evident is the tendency for the yield of Cu atoms to increase slowly on going to longer wavelengths and the methyl yield to decrease gradually over the same range. In both cases, the change in yield between 300 and 600 nm is 3-fold. Thus, while the recovery of C u atoms has been reported above as about 25% at 350 nm, it rose to about 70% a t >600 nm (when corrected for the expected amount of CH3CuH present in the sample a t the time of photolysis). It is therefore clear that Cu atom production is the dominant decomposition route a t wavelengths above 500 nm in solid methane. Not evident from the data given in Figure 1 is the change in the rate of depletion of CH,CuH over the range 300-700 nm. Rates of CH,CuH loss were comparable between 300 and 450 nm, while photolysis at longer wavelengths required increasingly long exposure times such that photolysis a t 550 nm required a 5-fold increase in irradiation time. This effect is probably due to a decrease in the extinction coefficient of CH3CuH a t longer wavelengths and may not reflect an inherent decrease in the quantum yield for fragmentation. It does not reflect the intensity profile of the xenon arc lamp, which increases in intensity with longer wavelengths. The yield of CH, appears to show a maximum in the region of 375 nm, and its subsequent decrease may correlate with the increase in H atom production below 400 nm. The validity of this maximum is dependent upon the assumption that C u atoms are generated during photolysis of CH3CuH at wavelengths below 330 nm and that only CH,CuH fragmentation leads to CH3 and H atom production. As C u atom generation was detected during photolysis of CH,CuH a t 275 nm, the former assumption seems reasonable. Furthermore, modeling of the Cu/CH, photochemical reaction at 305-325 nm3 has shown that greater than 90% of the C H , produced a t this wavelength can be attributed to CH3CuH decomposition. As similar growth behavior was observed for H atoms, the second assumption is also reasonable. At the same time, it should be noted that the estimated error for the data points on the CH, curve is &IO% based upon repeated trials a t the same wavelength, and therefore, the maximum at 375 nm may not be as pronounced as it appears in Figure 1. In order to establish whether the decrease in H atom production at longer secondary-photolysis wavelengths corresponded with a decrease in the production of CH,Cu, IR studies were done,in which CH,CuH was photolyzed in the range 450-700 nm. In all cases, the growth of CH,Cu was similar to that of other species such as CuH or CH,. In a more elaborate experiment, a methane matrix containing copper atoms was simultaneously irradiated light and high-intensity, broad-band, red by weak 3 15-nm (5-nm) light (>600 nm). This was effected through the use of two lamp and monochromator assemblies with collinear beams propagating in opposite directions through the sample. It was thought that, under these conditions, any CH,CuH formed following C u atom absorption a t 3 15 nm would then be subject to intense red light causing secondary photolysis. In this way, all the Cu in the sample should have been effectively converted into the partial-fragmentation products of CH3CuH photolysis a t >600 nm through extensive recycling of the C u atoms formed following CH,CuH fragmentation. Thus, the final product distribution for partial fragmentation should have been amplified significantly, since no CH,CuH should have been permanently converted to Cu atoms. Surprisingly, this experiment produced a CuH:CH,Cu ratio that was similar to that found with 315-nm photolysis. A possible explanation for this effect is that the difference in the extinction coefficients for CH,CuH a t 315 and >600 nm is such that, even under the conditions described here, the rate of secondary photolysis a t 315 nm greatly exceeds that a t >600 nm. This is reasonable. since reaction rates a t >600 nm were extremely slow with respect to rates at 300-400 nm. Therefore, the distribution
The Journal of Physical Chemistry, Vol. 93, No. 10, 1989 4025
Photochemistry of Methylcopper Hydride
CH3+ H +CU(’S)
CU (2P) + CH4
+
-
5 60 C U ( ~ D ~ CH4 / ~ )
20
CH~CU + H CH3+ CUH
-
c
-1 C U ( ~ S ) CH4
If a BDE of 15-25 kcal/mol is used for the HCu-CH3 bond, then the reaction is estimated to be 63-73 kcal/mol exothermic. (The use of this low value for the Cu-C BDE of CH3CuH is discussed in detail below.) Thus, the formation of CH3CuH from 2S Cu C H 3 H results in a recovery of 80-90 kcal/mol. Formation of CH,CuH is therefore estimated to be at least 15-25 kcal/mol endothermic with respect to the ground-state reactants. Siegbahn et al.’ have calculated the formation of CuH2 from ’S Cu and H, to be about 30 kcal/mol endothermic, which should be comparable to the degree of endothermicity in the 2S Cu + CH4 reaction, since C u H and CuCH3 bond strengths are similar (65 and >59 kcal/mol, respectively)6,8as are the C-H and H-H bond strengths of methane and hydrogen. An understanding of the photochemistry of CH3CuH requires some knowledge of the electronic configurations of the ground and excited states involved in the excitation. SCF-ab initio calculations have been done on the C u / C H 4 system by Poirier et aL9 and Quelch and Hillier.Io The former concluded that the ground state (2Al)has an unusual geometry, being linear and C3,, but exhibiting an unusually long Cu-C bond (2.38 A; cf. CH3Cu calculated there at 1.99 A). The geometry at the methyl group is very similar to that of a free methyl radical associated with a closed-shell C u H molecule in a fashion that has been proposed for CH3-.LiI interactions following cocondensation of Li metal with CH31 in rare-gas matrices.” The ground state of CH3CuH is predicted by Poirier et aL9 to have a dl0us2up’electronic configuration, which is formally a Cu(1) system. This type of configuration should show an ESR spectrum dominated by the 13C (in 13CH4)and ‘ H hyperfine interaction of the methyl group and would not be expected to show the significant copper and hydride coupling observed e~perimentally.~ Thus, it is felt that the state calculated by Poirier et aL9 does not represent the true ground state of CH3CuH. This conclusion is supported by calculations by Quelch and Hillier,Io who have found the 2A, “ground state” of Poirier et al. to be 15 kcal/mol higher in energy than the 2Al state that they find to be the ground state of CH3CuH. The latter state exhibits geometrical parameters (e.g., Cu-C = 1.997 A) and a charge distribution that are more reasonable for a Cu(I1) molecule. Note that the *Al ground state proposed by Poirier et al. may be (one of) the excited state(s) involved in the photofragmentation of CH3CuH, discussed below. Excitation of a bonding u-orbital electron into the nonbonding d,z orbital should result in fragmentation to form C H 3 + CuH, since the weak “one-electron’’ C-Cu bond could easily be broken by the excess internal vibrational energy that would be released following a Franck-Condon transition between two states with such different geometries. Ab initio calculations have been done on the CuH2 molecule, which should show bonding characteristics similar to CH3CuH since the bond strength of CuCH3 and CuH are very similar (Do(CuCH3) 1 59 kcal/mol; D,(CuH) = 65 kcal/mol). Siegbahn et al.’ and Nguyen et a1.12 have concluded that the ground state of CuH, is linear and has a 2Z, d9us2up2configuration with equal Cu-H bond lengths, corresponding to the *Al state of Quelch and Hillier.Io Both have found the 2Z, state, which corresponds to the CH3CuH *Al ground state of Poirier et al., to be 27.6’ and 30.8 kcal/molI2 above the 2Z, ground state. The Cu-H bond lengths were found to be somewhat elongated (0.07-0.09 A) for the 2Z, state of CuH,, presumably due to the decrease in the bonding order on going from the d9us2up2to the dI0us2up1electronic configuration. The above discussion clearly supports the assertion that CH3CuH is a linear Cu(I1) molecule with no unusual geometrical
CH~CUH
-
O I
Figure 2. Illustration of the relative energy of various reactants and products that could be involved in the reaction of Cu atoms with methane, based on the values discussed in the text. Braced lines indicate uncertain energy values.
observed here may be representative of a 3 15-nm photolysis of CH3CuH and not of wavelengths greater than 600 nm. The possibility also exists that the decrease in H atom production is not related to CH3Cu growth patterns but is due to an additional wavelength-dependent effect, perhaps one that enhances H atom diffusion rates at longer wavelengths. Since methane does not exhibit any absorptions in the visible or near-UV, such an effect would probably be related to variations in the efficiency of energy transfer from the products of the photodissociation to the surrounding matrix cage at different wavelengths.
Discussion The following conclusions are drawn in the present study of the photochemistry of CH3CuH: Photolysis of CH3CuH at 270-700 nm gives rise to CH3 + CuH, CH3Cu + H, and Cu + C H 4 by a combination of partial fragmentation and what is formally reductive elimination. Production of Cu + CH4 is favored during photolysis at longer wavelengths and becomes the major decomposition route a t wavelengths greater than about 500 nm. Production of C H 3 C u H is dominant at C500 nm. H atom production is seen to decrease rapidly between 300 and 400 nm and is not observed at wavelengths >400 nm. I n what follows, the structural and electronic properties of methylcopper hydride are discussed with an aim to develop an understanding of its observed photochemistry. This is followed by a consideration of the anticipated photochemical behavior of CH3CuH and an explanation for the observed wavelength dependence in the product distributions. Finally, a comparison is made between CH3CuH and CuH, based upon the similarities and differences between the photochemical systems in which these species are observed or proposed as primary insertion products. Structure and Bonding in Methylcopper Hydride. A consideration of the energetics involved in the reaction of 2P Cu atoms with methane leads to the following conclusions, as illustrated in Figure 2. The 2P states of Cu lie about 88 kcal/mol abovt the 2S ground state in the gas phase. The C-H bond dissociation energy (BDE) of methane is 105 kcal/mol so that insertion of a 2P-state Cu atom would be exothermic if a t least 17 kcal/mol was recovered in the formation of product bonds. The formation of CuH alone results in a recovery of 65 kcal/mol,6 which indicates that the 2P Cu insertion reaction is at least 48 kcal/mol exothermic.
+
(6) Rao, V. M.; Rao, M. L. P.; Rao, P. T. J . Quant. Spectrosc. Radial. Transfer 1981, 25. 521. Squires, R. R . J . A m . Chem. Sor. 1985, 107. 4385.
+
( 7 ) Siegbahn, P. E. M.; Blomberg, M . R. A.; Bauschlicher, C. W. J . Chem. Phys. 1984, 81, 1373. (8) Weil, D. A.; Wilkins, C. L. J . A m . Chem. SOC. 1985, 107, 7316. (9) Poirier, R. A.; Ozin. G.A,; McIntosh, D. F.; Csizmadia, I. G.; Daudel, R . Chem. Phys. Letr. 1983, 101, 221. ( I O ) Quelch. G. E.; Hillier, I. H . Chem. Phys. 1988, 121. 183. ( I I)Tan, L. Y . ; Pimentel, G . C. J . Chem. Phys. 1968, 48, 5202. ( 1 2 ) Nguyen, M. T.; McGinn. M. A.; Fitzpatrick. N. J. J . Chem. Soc., Faraday Trans. 2 1986, 82, 69
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T h e Journal of Physical Chemistry, Vol. 93, No. IO, 198'1
characteristics, as predicted on the basis of IR and ESR spectral evidence., At the same time, there exists a fair amount of indirect evidence that suggests that the Cu-C bond dissociation energy of CH,CuH is quite low. The strength of the metal-hydride or metal-alkyl bonds in metal dihydrides as well as monohydride and monomethylmetal cations has recently been related to the promotion energy required to place the metal in an electronic state with the necessary electronic configuration to form the bond(s) in question. Armentrout et al.I3 have proposed such a correlation to account for the observed bond strength of metal-alkyl and metal-hydride cations formed from metal ion/molecule reactions. With respect to metal-methyl bond strength, bond dissociation energies ( D o ) have been found to be largest for those monomethylmetal ions whose ground states have a 3d"'4s1 electronic configuration (Fe', Mn'), while those with 3dn configurations (Cr', Co', Ni') have lower bond strengths. The extent to which the M+-CH3 bond is weakened very closely correlates with the energy required to promote the metal ion from a d" to a d"-'s' state. This trend has been attributed to the fact that the ground state of d"-type metal ions does not correlate with the ground state of the molecular ion in question, since the metal atom's contribution to the bonding is almost exclusively from the 4s' electron of the 3d"'4s1 configuration. A more recent publication by Elkind and Armentrout14 presented a similar correlation between metal-hydride ion bond dissociation energies and promotion energies from a d" to d"%' configuration but included the energy required to spin-decouple the s-orbital electron from the remaining d-orbital electrons. This work also reported the bond dissociation energy of CuH+ as 21.8 kcal/mol, the lowest of all first-row transitionmetal ion hydrides, as predicted earlier by Armentrout et al.', due to the exceptionally large promotion energy of 63 kcal/mol (69.9 kcal/mol, spin decoupled) required to achieve the requisite 3d94s' excited state of Cu+. The promotion energy considerations that relate to the formation and strength of metal-carbon and metal-hydride bonds of copper monomethyl and monohydride ions should also be applicable to the formation of two such bonds in either C u H z or CH3CuH, since both processes involve the formation of what is formally a Cu(I1) oxidation state. Consideration of the predicted ground-state orbital occupancy of CuH, discussed above indicates that the formation of the two o-bonds of CuH2 or CH3CuH will require promotion of a d-orbital electron from the filled d" shell of Cu to achieve a mixture of d9s2and d9sLp'configurations. As with monomethylmetal ions, it is likely that the HCu-CH, and CH3Cu-H bonds are weakened by an amount similar to the d" d9 promotion energy. The 2D *S (d9s2 d"s') atomic transition energy for the copper atom (about 35 kcal/mol) provides a reasonable estimate of the lower limit for this promotion energy, since it is the lowest excited state of Cu having a d9 configuration. The actual value is expected to be somewhat higher. however, since some mixing of the d9s'p' configuration ( 1 14 kcal/mol above the *S ground statei5) would certainly be required to allow for some 4p-orbital participation in the bonding op-orbital. By analogy with A r m e n t r ~ u t , ' ~the . ' ~total bond energy of CH,CuH is predicted to be that of C u H (65 kcal/mol) and CH,Cu (>59 kcal/mol) less the required promotion energy to form a copper(l1) molecule. Therefore, the total bond energy of CH,CuH should be 59 kcal/mol.8 The unobserved Cu-C mode is now expected at about 600 cm-'. The feature at 350 cm-' remains unassigned, although it appears to be hydridic in nature. ( I 9) Huber. K . P.: Herzberg, G. Molecular Structure and Molecular Spectra 4 : Constant.\ of Diatomic Molecules: Van Nostrand Reinhold: New York, 1979. (20) Herzberg. G .Molecular Spectra and Molecular Structure. 3: Elwrronic Specirrr and Electronic Structure of Po/>,aromic.Molecules; Van Noairand: Scu York. 1966; Chapter 4.
The Journal of Physical Chemistry, Vol. 93, No. 10, 1989 4027
Photochemistry of Methylcopper Hydride CuH vibration would not be enough to cause its dissociation at wavelengths longer than about 320 nm. The observation of Cu atom growth at wavelengths as low as 700 nm (about 40 kcal/mol of excitation energy) clearly indicates that excitation at energies far below that of the estimated total bond dissociation energy of CH3CuH can lead to formation of Cu atoms. It is clear, therefore, that a total fragmentation process need not be considered as a major source of Cu atoms during the photolysis of CH3CuH between 300 and 700 nm. The exclusion of a total fragmentation route leads to the conclusion that the photodecomposition of CH3CuH involves either a combination of partial fragmentation and reductive elimination or exclusive partial fragmentation followed by secondary reactions. Reductive elimination is expected to involve a significantly bent excited state, while partial fragmentation could arise from either a linear or a bent excited state. There are several electronic transitions expected for CH3CuH that could lead to some form of photodissociation. If a d9us2ap2 ground-state electronic configuration and a linear C3, nuclear configuration (as discussed above) are assumed, then the lowest 'A, energy transition possible should involve a d-d-type *E transition in which an electron is promoted from either the d,+9Jyor dx,,y,-orbital pairs into the half-filled dzz orbital. These transitions should not significantly affect the bonding of CH,CuH, since they do not change the occupancy of the bonding u-orbital framework. This is confirmed by calculations on CuH2I2that show that the geometries of the excited states associated with the are very similar analogous transition in CuH2 ('II,, 'A, to the calculated CuH2 ground-state geometry. Moreover, the calculated energy difference between the '2, ground state of CuH2 and these two lowest lying excited states corresponds to excitation in the near-IR at 16500 and 14400 nm. Since no major changes in the minimum-energy geometrical configuration following both excitations are expected, it is likely that these transitions will appear as weak (dipole-forbidden) and relatively narrow absorptions at wavelengths close to these values. Finally, these states of C u H 2 have been predicted to have repulsive potentials with respect to bending along both Renner-Teller-active coordinates,'I making reductive elimination from these states unlikely. Thus, excitation to either of these states of CuH,, as well as the analogous states of CH,CuH, is not expected to lead to significant photochemistry. The first allowed transition for CH3CuH that involves a bonding 2Al (C,,) transition (dl0us2op' a-orbital electron is a ,Al d9as2up2)in which an electron is promoted to the d,z orbital in the copper d-orbital shell. This state, having only three electrons in the a-orbital framework, should be only weakly bound or dissociative and would be expected to lead to partial fragmentation of CH3CuH to form either CH, + C u H or CH3Cu H. The geometry of the excited state associated with the analogous 'E,, has been redicted to have a transition for CuH2, ' 2 , significantly longer CuH bond (1.63 vs 1.54 for the 22,ground state), which no doubt arises from a significant weakening of the CuH b0nds.l' The corresponding excitation energy has been estimated by two calculations to lie 27.7' and 30.912 kcal/mol above the ground state, which would involve a transition in the area of 900 nm if a vertical transition between the two states in their equilibrium geometries were possible. However, such a transition, in which the upper-state equilibrium geometry differs significantly from that of the ground state, should exhibit a transition at considerably higher energy than that required to cleave the bond, in accordance with the Franck-Condon principle. Whether an excitation at energies of 40 kcal/mol in excess of the HCu-CH, BDE is reasonable for such a process is not clear. Few examples of similar processes involving metal alkyls are known, the best being the dissociation of Hg(CH,), in which the cleavage of the first Hg-CH, bond (about 53 kcal/mol)" is believed to be the initial photochemical step in a process for which the ab-
-
+
-
-
+
+
1
(21) Lee, T.J.; Fox, D.J . ; Schaeffer, H . F.. 111; Pitzer. R. M . J. Chrm. Phys. 1984, 81, 357. (22) Baughcum, S. L.: Leone, S. R. C'hem. Phys. Lerr. 1982. 89. 183.
SCHEME I
CH,CuH ('AI)
A CH3CuH* ('E)
-
CH,
CH3CuH (*Al)
-
+ CuH, CuCH, + H
CH3CuH* ('Ai) CH3CuH** (2A', bent)
-
CH.,
+ Cu
sorption is centered at 200 nm. The excitation energy of about 143 kcal/mol is about 90 kcal/mol in excess of the CH3Hg-CH3 BDE due to the highly repulsive nature of the upper state. Similar excitation energies have been observed for the analogous transition in Cd(CH3)' and Zn(CH3),.', Therefore, an excitation energy of about 40 kcal/mol in excess of the CH3-CuH BDE is not without precedent, and this transition is a probable source of partial fragmentation of CH3CuH. The 'A, excited state of CH3CuH may also lead to reductive elimination through a bent 'Af configuration involving s-d hybrid bonding and an essentially nonbonding unpaired p electron. Computation studies of several transition-metal dihydrides have predicted stable bent molecular configurations involving such bonding for NiHz (49°)24and CuH2 ( 1 1 1.5°).25 A barrier to interconversion between the linear ground state and the bent minimum for CuH, of 8 kcal/mol has been p r e d i ~ t e d . ' ~As the linear 2E, state of CuH2 is found to be a potential m i n i m ~ m , ' * ~ ~ - ~ ~ a barrier should exist to the formation of the bent 'A' state of 'A, excitation. If this barrier were CH3CuH following 'A, overcome, reductive elimination through the bent 'Af state could occur. There are no other excited states of CH3CuH expected to exhibit visible or near-UV absorptions. The only other electronic transition that may yield a bent excited state involves a population of the nonbonding orbitals composed mainly of Cu 4p, and 4p, atomic orbital contributions, 'E ,A,, giving an excited-state electronic configuration of dgas2apirp'. As the px and pv orbitals of Cu are degenerate in C3, symmetry, Jahn-Teller coupling to the C-Cu-H bending mode should lead to a bent excited state, A', and a nearly linear state, A" (CJ. Both states are expected to be either weakly bound or unbound due to the lowering in the number of bonding a-orbital electrons to three. Excitation to these states should therefore lead rapidly to fragmentation and possibly reductive elimination. In the absence of accurate calculations on the energies of these excited states, it is not possible to determine whether this transition is energetically feasible at 300-700 nm. It seems likely that the excitation is quite high in energy, probably in excess of the 2P 'S excitation of Cu atoms. This, in conjunction with the anticipated shift in the absorption to higher energy due to the repulsive upper-state configuration, makes population of this state seem unlikely in the near-UV and especially in the visible. The experimentally observed photodissociation behavior of CH3CuH may now be considered with the above discussion of the excited states in mind. The gradual shift in product yield from mainly Cu atom production at >700 nm to mainly CH, + CuH production at 300 nm can be interpreted in two ways. A single excited state may be populated, from which the observed final product distribution is dependent upon the excess translational, rotational, and vibrational energy of the fragments following initial dissociation. Alternatively, two excited states, from which partial fragmentation or reductive elimination occur, could overlap in the region 300-700 nm. The observed decrease in the rate of photodecomposition of CH3CuH at longer wavelengths could reflect a less favored excitation beyond 500 nm. although it more likely simply reflects the decrease in extinction coefficient at, these longer wavelengths. If two excited states were to be invoked to account for the observed photochemistry of CH3CuH, it is expected that both would have to involve excitation of 0-bonding electrons and +
-
+
(23) Chen, C . .I.; Osgoo.1.. R. M. J . C'hem. Ph),s. 1984. 81. 3 2 7 (24) Blomberg. hl. R. A,; Siegbahn. P. E . hl. J. C'hent. Phys. 1983, 78. 5682. ( 2 5 ) Garcia-Prietu. J . . Kui7. M .I . , Pouliiin, E.: Ozin. G . A . ; Uov.iro. 0. J . Chrm. Phi,s. 1984. 8 2 . 5920.
4028 The Journal of Physical Chemistry, Vol. 93, .No. IO, 1989 SCHEME I1
-
CH3CuH ( 2 A i )
hv
-
CH,CuH* ( * A l ) CH, E-V 1 cage CH,CuH** (,A’, bent) SCHEME 111
hu
CH3CuH (*AI) CH3CuH* (’A1)
-
+ CuH, CuCH3 + H
-
CH4 + Cu
-
CH3* + CuH* CH, intracage 1 recombination CH4 + C U
+ CuH
not a d-d transition. Therefore, one would have to postulate reductive elimination from a lower lying state than one leading to partial fragmentation. As is noted above, reductive elimination ,Al and partial fragmentation could occur from both the ,E ,Al transitions (C,”). If the latter transition led to and ,Al a significant production of the bent ,A’ state of CH3CuH, discussed above, and if the former led mainly to partial fragmentation, an overlap of these two transitions could be consistent with the observed photochemistry (Scheme I). However, as noted above, ,A, transition is expected to occur in the UV and involve the ,E significant reductive elimination. This, along with the absence of structure on the optical absorption of CH3CuH3suggests that overlapping absorptions from these two states are unlikely. We feel that it is much more likely that the observed photochemistry arises from a single excited state, ,Al, whose photodissociation is mediated through interactions between the excited state and the surrounding matrix cage. Access to the bent ,A‘ state of CH,CwH followed by reductive elimination to regenerate a Cu atom could result from impulsive interactions between the upper state and the cage. These would serve to transfer translational and vibrational energy into the skeletal bending mode such that the barrier to formation of the bent state is overcome and reductive elimination occurs (Scheme 11). The probability of such a transformation would have to decrease with increased excitation energy, leading to increased yields of partial fragmentation products at shorter wavelengths. The wavelength dependence of the product yields from CH3C u H photolysis could also be due to secondary interactions between fragments formed from the dissociation of a single excited state. It is well-known that the matrix cage can heavily influence the outcome of normally highly exothermic photodissociations. For example, photolysis of CH,I in rare-gas matrices between 200 and 300 nm yields only small fragment yields, in spite of the unbound nature of the upper state.26 The quantum yield for this process in rare-gas solids is estimated at 6 = 2 X lo-* (Ne) and 2 X IOw3 (Kr) for 262.5-nm excitation, while the same photolysis in the gas phase leads to highly efficient photodissociation. It therefore seems reasonable to consider the possibility of secondary reactions, particularly between CH, and CuH, the major partial fragmentation products, but also between CH3Cu and H (Scheme I l l ) . As the thermal reaction between H and CuH is known to give rise exclusively to Cu H2.,’ analogous production of Cu atoms through secondary reactions of CH,CuH photolysis fragments seems quite plausible. This would be expected to occur despite the ground-state barrier to reductive elimination implied by the thermal annealing studies,, since the fragments formed in the initial photolysis of CH,CuH would possess far more energy ( 1 5-70 kcal/mol) than those involved in the thermal annealing studies ( < I kcal/mol). The observed trend toward Cu production at longer wavelengths would then be simply a manifestation of the decreased ability of the initially formed fragments to escape the cage in which they are formed. I n light of the precedent existing in the C u H + H thermal reaction, we favor this “secondary encounter” mechanism. However, both these “one state’’ explanations appear to be consistent with all the observations
-
-
-
+
( 2 6 ) Brus. L E . Bondbbey, V E J Chem Phis 1976. 6 5 ~71
Parnis and Ozin in CH,CuH photochemistry (excluding the absence of H atom growth following low-energy photolysis) and are therefore preferred to the “two excited state” approach. Assignment of the broad absorption between 275 and >700 nm would therefore be ,Al electronic transition of CH3CuH that is made to the ,A, ,E, transition of CuH,. analogous to the ,E, Comparison with the Copper1Hydrogen System. The experimental details of the reaction between photoexcited copper atoms and molecular hydrogen in rare-gas matrices suggest several fundamental differences between processes leading to the formation of CuH2 and CH3CuH. Matrix isolation studiesz8 have ,S absorption in shown that excitation of Cu atoms in the ,P the presence of H2 (1:lO Hz/rare gas) leads to formation of CuH and H atoms with no evidence for a CuH, intermediate. Attempts to form CuH, by thermal annealing of matrices containing large amounts of CuH and H led to highly efficient recovery of copper atoms:’ either through a H atom abstraction or through formation of CuH,. The latter requires that CuH, be highly unstable with respect to reductive elimination. The experimental results did not allow a distinction to be made between these two possibilities. Pseudopotential calculations on the C u / H 2 system indicate that the formation of CuH, from C u H and H can proceed with no energy barrier and predict a small barrier for the abstraction route of 6.9 kcal/m01.~~ The calculations do not, however, allow elimination of the abstraction mechanism as a viable route to Cu atom production in rare-gas matrices, since the predicted barrier is small. The Cu/H2 system therefore differs fundamentally from the Cu/CH, system in its apparent inability to generate significant quantities of the suspected initial insertion product either through copper atom excitation or thermal annealing of CuH + H. This behavior may be indicative of several differences between the properties of CuH, and CH,CuH. The recombinations of C u H H or CH, CuH are simple processes involving the formation of similar electronic and structural configurations in the presence of very little excess energy except that released during the bond formation. The fact that CH,CuH can be formed and stabilized in a methane matrix suggests that if CuH, were stable at 12 K, it would be formed to some extent during CuH + H recombination. Since the C-H bonds of CHI and the H-H bond of H2 are almost equal in strength, as are the CuH and CuCH, bonds, there is little reason to suppose that CuH, is less stable in a thermodynamic sense with respect to Cu + H 2 than CH3CuH is to Cu + CH4. Thus, it appears that CuH2 is kinetically less stable than CH,CuH. This may be interpreted as being indicative of a greater barrier to reductive elimination of CH4 from CH3CuH than H, from CuH2. Such a difference in the relative instability of metal dihydrides with respect to methylmetal hydrides has been predicted for thermally induced reductive elimination from PdH,, Pd(CH3)H, and Pd(CH,),, where barriers of 1.5, 10.4, and 22.0 kcal/mol, respectively, have been c a l c ~ l a t e d . The ~ ~ increase in the barrier to reductive elimination when a hydride is replaced by a methyl group has been attributed to the directional nature of the CH, sp3 orbital with respect to the spherical H 1s orbital. It seems likely that such an effect is responsible for the apparent instability of CuH, with respect to reductive elimination. The proposed kinetic instability of CuH, cannot alone explain the absence of CuH, during the primary photoexcitation of Cu atoms in the presence of H, in which CuH + H atoms are formed. This follows since the decomposition of CuH,, if formed through annealing, leads only to Cu atoms and H,. Therefore, one might expect the Cu H, reaction to be nonproductive, that is, formation of CuH, could be immediately followed by reductive elimination of H 2 to regenerate a Cu atom. Observation of partial-fragmentation products clearly indicates that, if CuH, is formed, it
--
-
+
+
+
(27) Ozin, G . A,; Gracie, C. I . J . Phys. Chem. 1984, 88, 643. (28) Ozin, G. A.; Garcia-Prieto, J.; Mitchell, S. A. Angew. Chem. Int. Ed. Engl. Suppl. 1984, 785. (29) Ruiz, M. E.; Garcia-Prieto, J.; Poulain, E.; Ozin, G . A,; Poirier, R. A.; Mattar, S. M.;Csizmadia, I. G.; Gracie, C.: Novaro. 0. J . Phys. Chem. 1986. 90. 279. (30) Low. J . J.; Goddard, W . A . J . A m . Chem. SOC.1984, 106, 8321.
4029
J . Phys. Chem. 1989, 93, 4029-4033 must decompose via a different mechanism than that involved during thermal annealing studies. There are two possible routes by which fragmentation might occur. The first is that CuH2 may be photochemically active a t the Cu ,P 2S excitation wavelength, as is CH3CuH. As both C H 3 and H as ligands do not possess any low-lying orbitals, it is likely that the electronic spectrum of CuH, exhibits an absorption similar to that of 2Stransition. If the CH3CuH in the region of the Cu atom ,P efficiency of fragmentation of CuHz due to secondary photolysis were greater than that of CH3CuH, it is possible that CuH2 would never be formed in appreciable amounts and would therefore escape detection. Alternatively, the vibrationally relaxed ground state of CuH2 may not be formed during the Cu/H2 photochemical reaction in rare-gas matrices. This is highly likely since the reaction of ,P Cu with H2 is quite exothermic (about 80 kcal/mol; 90 kcal/mol excitation energy, 104 kcal/mol H-H bond strength, 65 + 30 kcal/mol CuH2 bond strengths, discussed above). This large amount of excess energy must be disposed of through energy transfer to the phonon modes of the matrix if CuH2 is to be stabilized. It is anticipated that CuH2 would be formed with a great deal of vibrational energy and this could lead to a partial fragmentation of CuH, to yield C u H + H in a manner that is similar to the fragmentation of gas-phase dihydrides such as MgH2 and HgH2.31,32These species are presumed to exist as short-lived intermediates in metal atom excited-state quenching reactions but are not observed. Cleavage of the HCu-H bond should be a relatively facile process, since the bond is expected to be quite weak as a result of the high promotion energy required to achieve a d9aszupzCu( 11) configuration, as discussed above. Therefore, the question of CuH, photochemical activity at 300-330 nm may not be relevant, since CuHz may only be formed in the matrix Cu/H2 reaction as a fleeting, highly vibrationally excited molecule. With respect to CH3CuH, one might anticipate a fragmentation similar to that observed for the C u / H 2 reaction to occur, since its formation from ,P Cu CHI is a highly exothermic process (about 70 kcal/mol). The major difference seems to be the presence of a larger number of vibrational degrees of freedom in CH3CuH (1 2 vs 4 for CuH,), including high-frequency vibrations of C-H bonds. These modes are expected to share the excess energy through an equilibration of vibrational energy and in this +
+
+
(31) Breckenridge, W. H.; Umemoto, H.J . Chem. Phys. 1984,80,4168. (32) Callear, A. B.; McGurk, J. C. J . Chem. Soc., Faraday Trans. 2 1973, 69, 97.
way reduce the extent of excitation of any one mode. At the same time, the neat CHI matrix material possesses a much greater capacity for assimilation of vibrational energy through the internal vibrational modes of methane not available in rare-gas supports, the latter being necessarily employed in the studies of Cu/H2 at 12 K. These two points, as well as the apparent stability disparity between CH3CuH and CuH,, probably account for the principle difference between the Cu/CH4 and C u / H 2 systems. Summary
The photochemistry of CH3CuH has been examined over the wavelength range 270-700 nm. Production of CH3 C u H is found to be the dominant decomposition route between 300 and 500 nm, while production of Cu atoms is dominant between 500 and 700 nm. Hydrogen atom yields diminish between 300 and 400 nm such that no H atom growth is observed a t wavelengths longer than 450 nm. The photochemical reaction of C u atoms with methane is considered from the point of view of the known reactant and product bond dissociation energies as well as quantum mechanical calculations on the reaction of ground- and excited-state Cu atoms with H,. The structure and bonding of CH,CuH is considered with regard to theoretical and experimental data for related molecules. CH,CuH is predicted to have a d9as2up2ground-state electronic configuration. Bond dissociation energies of CH3Cu-H of 1 3 0 kcal/mol and HCu-CH, of 5 2 5 kcal/mol are predicted on the basis of the promotion energy for Cu(1) to Cu(I1). The photochemistry of CH3CuH is discussed with respect to the calculated electronic structures of CuHz and CH,CuH. Wavelength-dependent matrix-cage effects involving a single state of CH,CuH are proposed to account for the variation in CH, and Cu atom yields during secondary photolysis of CH,CuH in solid methane. These are thought to involve either a bent excited state exhibiting s-d hybrid bonding or secondary interactions between fragmentation products, either process leading to Cu atom regeneration. A comparison is made between the Cu/CH, and Cu/H2 reactions, where differences between the two are discussed with respect to the kinetic instability of the CuH2 insertion product.
+
Acknowledgment. The generous financial assistance of the Natural Sciences and Engineering Research Council of Canada's Operating and Strategic Grants Programmes is deeply appreciated. J.M.P. thanks the NSERC for a postgraduate scholarship. Registry No. CH3CuH, 88778-41-0;CH3, 2229-07-4; CuH, 1351700-5; CU,7440-50-8.
Evidence for Homolytic Decomposition of Ammonium Nitrate at High Temperature K. R. Brower,* Jimmie C. Oxley, and Mohan Tewari Department of Chemistry, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801 (Received: August I , 1988; In Final Form: January 9, 1989)
Rates of decomposition of ammonium nitrate in the liquid and vapor state have been measured at temperatures up to 400 O C . The evidence indicates that an ionic mechanism operating at temperatures below 290 OC is overtaken by a homolytic mechanism at higher temperatures. The activation energy increases to 193 kJ/mol, which is nearly equal to the N - 0 bond energy in HNO,. Water and NH, strongly inhibit the ionic reaction at low temperature, but the effect fades away at high temperature. There is no primary H / D kinetic isotope effect. The reaction rates of liquid and vapor are nearly the same at high temperature. The rate at high temperature is given by (kT/h)e4.06e-233"lT.
Introduction Because of its technical importance the thermal decomposition of fused ammonium nitrate to nitrous oxide and water has been the subject of many kinetic investigations.'-s The decomposition ( 1 ) Hainer, R. M. 5th Combust. Symp. 1954, 224. (2) Shah, M . S.; Oza, T. M. J . Chem. SOC.1932, 725.
0022-3654/89/2093-4029$01 .50/0
at temperatures near 200 OC has often seemed to show variable induction periods and capricious variations in rate. It has been reported, for example, that completely dry a - r " n i u m nitrate does (3) Saunders, H . L. J . Chem. SOC.1922, 121, 698. (4) Delsemme, A. H. Compt. Rend. 1950, 230, 1858. (5) Friedman. L.; Bigeleisen, J . J . Chem. Phys. 1950, 18, 1325.
0 1989 American Chemical Society
