J. Phys. Chem. 1985,89, 2019-2021 feature; and (v) a consistency of disagreement between laserschlieren and ARAS measurements of dissociation rate. Laserschlieren rates on propaneI6 are about a factor three higher than the ARAS values,z8 and some recent measurements on toluene pyrolysisz9are similarly distant from ARAS ratesM There would seem to be some problem in extracting rate constants from the ARAS measurements. The RRKM model used here, although crude, has at least no prior prejudice in favor of any particular channel for dissociation. The parameters required to fit the measurements seem quite reasonable, although an average energy transfer of 70 cm-' may be a bit small. This could reflect the high temperatures of this study, since there are indications a slight negative temperature dependence for (-AE)," would improve the fit. The rate constant (28) C. Chiang and G. B. Skinner, Symp. (Inr.) Combust., [Proc.],18rh, 1981,915 (1982). (291 H.-C. Wei and J. H. Kiefer, unpublished laser-schlieren measure-
ments o n toluene pyrolysis. (30) V. S. Rao and G. B. Skinner, J. Phys. Chem., 88, 4362 (1984).
2019
for reaction 2 is only well established for the lower temperatures, so the assigned activation energy is rather uncertain. However, extrapolation of this expression to 800 K gives 1 X 1Olocm3/(mol s), close to the upper limit of 9 X lo9 cm3/(mol s) suggested by Nicovich and R a ~ i s h a n k a r a . ~ ~ The experiments and analysis presented here define the major pathways and their kinetic parameters in high-temperature benzene pyrolysis. The remaining problems in this pyrolysis concern the paths to minor products, about which laser-schlieren measurements provide no information.
Acknowledgment. We thank Profs. R. D. Kern and G. B. Skinner for helpful discussion and transmission of their results prior to publication. This research was supported by the Department of Energy under Contract No. DE AC02-78ER,4759. Registry NO. C6H6,71-43-2; C6H5,2396-01-2. (31) J. M. Nicovich and A. R. Ravishankara, J. Phys. Chem., 88, 2534 (1984).
Gallium Dicarbonyi: Matrix Isolation ESR Study Paul H. Kasai* and Paul M. Jones IBM Instruments, Inc.,-Orchard Park, Danbury, Connecticut 06810 (Received: December 3, 1984)
ESR spectra of a gallium carbonyl generated in argon matrices by co-condenstation of gallium atoms and carbon monoxide were observed and analyzed. It is shown that the carbonyl consists of one gallium atom and two CO molecules. It has a bent planar structure OC-Ga-CO, and a semifilled orbital representing the back-donation from the Ga pr orbital into the antibonding x orbitals of the CO moiety.
Introduction Many mononuclear transition-metal atom carbonyls M(CO), have been prepared by co-condensation of metal atoms and carbon monoxide molecules in inert-gas matrices.' All of these carbonyls have been identified and examined by their vibrational spectra (IR and Raman). For CO(CO)~, CU(CO)~, and Ag(C0)3, ESR (electron spin resonance) spectra have also been Ogden and his co-workers5 reported that co-condensation of aluminum atoms and carbon monoxide in a krypton matrix led to formation of an aluminum carbonyl. Based on the effect of CI8O upon the I R spectrum, they demonstrated that the species had the formula Al,(CO)z, but refrained from asserting the number of aluminum atoms involved. Ozin et ale4suggested that it might be A12(C0)z. Detection by I R of similarly generated Ga,(CO)z has also been reported.6 Recently we reported on our ESR study of an aluminum carbonyl generated in argon matrices.' The study showed that the carbonyl involved one aluminum atom and two carbon monoxide molecules. It also showed that the carbonyl had a bent, planar structure and its semifilled orbital represented the backdonation from the A1 pI orbital into the antibonding x orbitals of the CO molecules. (1) See,for example, Moskovitz, M.; Ozin,G. A. 'Cryochemistry"; Wiley: New York, 1976; Chapters 7 and 8. (2) Hanlan, L. A.; Huber, H.; Kiindig, E. P.; McGarvey, B. R.; Ozin, G. A. J. Am. Chem. Soc. 1975, 97,1054. (3) Ozin, G. A. Appl. Specrrosc. 1976, 30, 573. (4) McIntosh, D.; Ozin, G. A. J. Am. Chem. Soc. 1976, 98, 3167. (5) Hinchcliffe, A. G.; Ogden, J. S.; Oswald, D. D. J. Chem. Soc., Chem. Commun. 1972, 338. (6) Ogden, J. S.,ref 1, p 247. (7) Kasai, P. H.; Jones, P. M. J. Am. Chem. Soc. 1984, 106, 8018.
0022-3654/85/2089-2019$OlSO/O
We report in this paper ESR spectra of a gallium carbonyl generated in argon matrices. The spectra unequivocally demonstrated the presence of one gallium atom and two carbon monoxide molecules in the complex. They also revealed that the structure and the bonding scheme of Ga(CO)z were essentially identical with those of Al(CO)z.
Experimental Section A liquid helium cryostat that would enable trapping of vaporized species in an inert-gas matrix and examination of the resulting matrix by ESR has been described earlier.8 In the present series of experiments, gallium atoms were generated from a resistively heated (- 1400 "C) tantalum cell and were trapped in argon matrices containing a controlled amount of carbon monoxide (-20%). The ESR spectrometer used was an IBM Model ER2OOD system. A low-frequency (375 Hz) modulation was employed for the signal detection. All the spectra reported here were obtained while the matrix was maintained at -4 K, and the spectrometer frequency locked to the sample cavity was 9.4275 GHz. Research grade argon and CP grade carbon monoxide were obtained from Matheson, while 13C-enriched(enrichment > 90") carbon monoxide was obtained from MSD Isotopes. Gallium metal (99.999%) was obtained from Ventron Corp. Results The ground-state electronic configuration of Ga atoms is 4s2 4p'. Thus, owing to the degeneracy of the p orbitals, the ESR signal of the Ga atoms situated at sites with a cubic symmetry would be broadened beyond detection. However, it has been shown (8) Kasai, P. H. Acc. Chem. Res. 1971, 41, 329.
Q 1985 American Chemical Society
2020
The Journal of Physical Chemistry, Vol. 89, No. 10, 1985
Kasai and Jones
50 G
H 50
0
(a) ESR spectrum of gallium carbonyl observed from the Ga/I3CO (20%)/Ar system. (b) Computer-simulatedspectrum based upon the g tensor and the Ga hf tensors determined from the non-13Clabeled spectrum and the hf interaction with two equivalent ”C nuclei as given in the text. Figure 2.
Figure 1. (a) ESR spectrum of gallium carbonyl observed from the Ga/CO (20%)/Ar system. Weak signals due to formyl radicals are indicated. The carbonyl signals can be accounted for by an orthorhombic g tensor, and an orthorhombic hf tensor of the Ga nucleus as shown. The powder pattern of the 69Gaspecies is shown at the bottom. (b) Computer-simulated spectrum based upon the parameters given in the text. The spectra due to the 69Gaand 71Gaspecies were superimposed.
that rare-gas matrices containing a high concentration (a few tenths of a percent) of group 13 metal atoms often exhibit strong ESR signals. These signals were designated as the AI-X signals, for example, and were attributed to isolated metal atoms occupying axially distorted substitutional sites of the host m a t r i ~ . ~ For J~ the Al/CO/Ar system, it was shown that only the AI-X signals were observed when the CO concentration was less than 2% and only the aluminum carbonyl signals were observed when the CO concentration was 10% or higher.7 There are two major gallium isotopes, 69Ga ( I = 3/2, natural abundance = 60%, p = 2.0108 @”),and 71Ga ( I = 3/2, natural abundance = 40%, p = 2.5549 @,). Figure l a shows the ESR spectrum observed from the Ga/CO (20%)/Ar system. The spectrum is completely different from the Ga-X spectrumlo reported earlier. Weak signals due to inadvertently formed formyl radicals, HCO, are indicated.” The remaining signals are attributed to a gallium carbdnyl; they can be accounted for by a radical having an orthorhombic g tensor and an orthorhombic hf (hyperfine) coupling tensor with one gallium nucleus.12 The Ga hf tensor of the carbonyl is further characterized by an extremely large coupling constant along the principal axis 1 and relatively small coupling constants along the other principal axes. Thus the (9)
Knight, L. B.;Weltner, W., Jr. J . Chem. Phys. 1971, 55, 5066.
(10) Ammeter, J. H.; Schlosnagle, D. C. J . Chem. Phys. 1973, 59, 4784. (11) Adrian, F.J.; Cochran, E. L.; Bowers, V. A. J . Chem. Phys. 1962,
36, 1661. (12) For analyses of
respective quartet splittings due to the 69Gaand ’lGa nuclei are resolved along axis 1, but not along axes 2 and 3. The “house figures” at the bottom of Figure l a indicate the powder pattern of the 69Gaspecies. The g tensor and the 69Ga hf tensor of the carbonyl were thus assessed as follows. axis g
L4(69Ga)I,G
1 2.0008 (3)
2 2.0090 (3)
103.0 ( 5 )
15.0 (5)
1.9809 (3) 17.0 ( 5 )
Figure 1b is a computer-simulated spectrum based upon these values; the spectra due to both the 69Ga and 71Gaspecies were considered and s~perimposed.’~The agreement between the observed and simulated spectra is excellent except the disparity in the line width of the outermost gl hf components. The observed extra width of these components is attributed to a scatter of A,(Ga) due to matrix effects. Figure 2a shows the ESR spectrum observed from the Ga/I3CO (20%)/Ar system. Though surprisingly small, effects of the hf interaction with the ”C nucleus/nuclei are conspicuous. They are manifested as additional broadening of the glpeaks and loss of detailed features in the central region due to overlaps of increased number of hfcomponents. Thus the magnitude of A,(13C) may be assessed from the line width increase of the inner gl hf components. The magnitudes of A2(I3C)and A3(13C)may then be assessed through a trial and error process for the best fit between the observed and simulated spectra in the central region. The result obtained assuming the hf interaction with two equivalent I3C nuclei is as follows: IAl(”C)l = 3.0 ( 5 ) G lA2(l3C)1 = 7.5 ( 5 ) G
ESR powder patterns,
see, for example, Ayscough, P. B. ‘Electron Spin Resonance in Chemistry”; Methuen: London, 1967; pp 323-332.
3
(13) Kasai, P. H. J . Am. Chem. SOC.1972, 94, 5950.
Matrix Isolation ESR Study of Ga(C0)2
The Journal of Physical Chemistry, Vol. 89, No. 10, 1985 2021 where
IA3(13C)(= 8.0 (5) G Figure 2b shows the simulated spectrum based upon the g tensor and the Ga hf tensor determined from the non-13C-labeled spectrum, and the I3C hf tensor given above. No reasonable fit was obtained when the interaction with one 13C nucleus was assumed. The number of carbon monoxide involved in the complex was thus established as two. The observed spectrum was hence assigned to Ga(C0)2.
Discussion The Ga hf tensor of Ga(CO), observed here is small but extremely anisotropic. It signifies a large spin density in the Ga 4p orbital and no “direct occupation” of its 4s orbital. It is strongly suggested that Ga(C0)2 has structural and bonding features similar to those of A1(C0)2.7 The Ga atom is thus sp2 hybridized, and the two vacant hybrid orbitals are utilized for the a-type dative interaction with two CO molecules depicted as follows:
R0
.C A‘
A11(69Ga) = +lo3 G
+ JA31)/2 = f 1 6 G A,(13C) = -(IAzl + IA31)/2 = -7.8
A,(69Ga) = f(1A21
Y
t
AII(13C)= f3.0 G
Back-donation is then possible from the semifilled Ga 4p, orbital into the vacant a,* orbitals of the C O moiety. The semifilled orbital of Ga(CO), is thus given by
CP = ~ + ~ , ( 4 p ,+ ) b(a,*
Here 1+(0)12 represents the spin density at the nucleus, r the separation between the unpaired electron and the nucleus, and a the angle between r a n d the symmetry axis. Only spin density in an s orbital contributes to Ai,, and that in a non-s orbital contributes to &p. The second expression for Adip applies when a unit spin density is located in a p orbital of the magnetic nucleus. Equation 3 states that, for a p orbital case, Adi is positive, and hence A,, > A,. From the observed 69Ga and ‘IJ, hf constants the hf tensors of axial symmetry (with signs) are thus deduced as follows:
+ a,*’)
(1)
where u,* and a,*‘ represent the antibonding azorbitals of the two carbonyls. It has been shown that, for a radical having a nondegenerate ground state IO), deviation of the g tensor from the spin only value g, (= 2.0023) is given byI4
I
G
We shall assume A,(69Ga) = -16 G and AIl(l3C)= +3.0 G since they lead to a more reasonable overall unpaired electron distribution (vide infra). Analyses of the tensors in terms of eq 3 then yield the following: Ai,(69Ga) = +24 G
Adip(69Ga)= +40 G
Aiso(13C)= -4.2 G
Adip(13C) = +3.6 G
The atomic values Aoi, and A’dip for a unit spin density in the valence s and p orbitals of atoms have been computed theoretically.16 The results for Ga and C are A0iso(69Ga)= 4361 G, Aodip(69Ga)= 73 G, Aod(l3C) = 1350 G, and A0dip(13C)= 38 G. The spin density distribution in Ga(C0)2 was thus determined as follows: P(4S)Ga = 0.006 p(4p,)Ga = 0.55 ~ ( 2 s =) ~-0.003
Here i (= x, y, z,) represents a principal axis of the g tensor, L, the orbital angular momentum operator, and the one electron spin-orbit coupling constant. The summation is performed over all the excited states. In evaluating eq 2 in terms of LCAO-MOs, only on-centered integrals need to be retained, and for each atomic integral the spin-orbit coupling constant of the particular atom is used. Thus for the ground state 10) given by eq 1 , we can immediately sate Ag, = 0.0. The highest doubly occupied M O of the system would be the orbital of the lone pair electrons of Ga. A large positive g shift is hence expected along t h e y axis. Thus axis 1 of Ga(C0)2 showing the smallest deviation from the spin only value is identified as the z axis and axis 2 showing a large positive g shift is identified as the y axis. Anisotropy of the hf tensor to a magnetic nucleus should reflect the symmetry of the distribution of the unpaired electron in the vicinity of the nucleus. Thus the hf tensors of the Ga and 13C nuclei in Ga(C0)2 should be approximately axially symmetric about the z axis. The tensors determined above for the 69Ga and I3Care indeed nearly axially symmetric about the z axis. A small deviation from axial symmetry is expected since the molecule itself lacks axial symmetry. It has been shown that the principal elements, Ail and A,, of an axially symmetric hf tensor are related to the isotropic term Ai, and the anisotropic term Adip as follows:’5
p(2p,)c = 0.09 The balance of spin density must be at oxygens. Thus ~ ( 2 p , ) ~ = 0.14. The extremely small but nonzero spin densities computed above for the Ga 4s and C 2s orbitals should not be taken too literally. The small Ah’s determined here are the sum attributes of the spin polarization in all the filled s orbitals of the respective atoms.” However, Ai, of the 13C nucleus induced by spin polarization by an unpaired electron in the 2p orbital of the same atom is usually positive.’* A negative spin density determined here can be best accounted for by polarization of electrons in the u dative bond by the large spin density in the Ga 4p, orbital. For Al(CO)z, the spin density distribution was determined as follows:’ ~ ( 3 s =) 0.02, ~ ~ ~ ( 3 p , ) ~=]0.42, ~ ( 2 s =) -0.004, ~ and p(2p2)c .= 0.09. The similarity between the spin density distributions in Al(CO)z and Ga(CO), is remarkable. According to Ogden’s IR w ~ r k ,the ~ , symmetric ~ and antisymmetric stretches of the carbonyls occur at 1988 and 1890 cm-I, respectively, for Al(C0)2, and at 2006 and 1912 cm-’ for Ga(CO)z. Stronger CO bonds in the latter complex indicate less back-donation from the metal pz orbital, hence a larger spin density in the metal p, orbital. The spin density in the metal p, orbital presently determined for Ga(C0)2 is indeed larger than that determined for Al(CO),. Registry No. Ga(C0)2, 95646-95-0. (15) Smith, W. V.; Sorokin, P. P.; Gelles, I.L.; Lasher, G . L. Phys. Rev. 1959, 115, 1546.
(14) Pryce, M. H. L. Proc. Phys. Soc., London, Sect. A . 1950, 63, 25.
(16) Morton, J. R.; Preston, K. F. J . Mag. Reson. 1978, 30, 577. (17) See, for example, Abragam, A.; Bleaney, B. “Electron Paramagnetic Resonance of Transition Ions”; Oxford University Press: London, 1970; Chapter 17. (18) See review article by Morton, J. R. Chem. Rea 1964, 64, 453.
