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J. Phys. Chem. 1981, 85,10-1 1
Theoretical Studies on the Reaction of Atomic Oxygen (3P) with Acetylene Lawrence B. Harding Theoretlcal Chemistry Group, Chemistry Division, Argonne National Laboratory, Argonne, Iillnois 60439 (Received: October 3, 1960; I n Final Form: November 19, 1980)
Ab initio, configurationinteraction calculationswith a polarization, double-lbasis set are reported on the barriers to decomposition of triplet formylmethylene. It is found that the barrier to hydrogen atom elimination is 37 kcal/mol while that to 1,2 hydrogen migration is 53 kcal/mol. On the basis of these results it is concluded that the dominant products resulting from the reaction of O(3P) with acetylene should be atomic hydrogen and the HCCO radical.
Acetylene is an important intermediate in most hydrocarbon flames.' A major pathway for destruction of acetylene in these flames is through reaction with atomic oxygena2 Experimental studies of this reaction have led to the identification of two important product channels, one being CH2(3B1)+ CO and the second being H HCC0.3i4 Attempts to determine the ratios of these two products, however, have led to widely varying results. Early work by Williamson indicated HCCO to be a major product (42 f More recently, low-pressure reactor studies have been interpreted to show that production of HCCO is negligible (go%) products.6 The mechanism for this reaction is postulated to involve an initial addition of atomic oxygen to a T bond of acetylene, forming triplet formylmethylene, OHCCH.' This adduct then rapidly decomposes either by the direct elimination of atomic hydrogen, forming HCCO, or via a 1,2 hydrogen migration leading to a triplet state of ketene. Triplet ketene can then undergo carbon-carbon bond cleavage forming CH2(3Bl)+ C 0 . 8 This mechanism is summarized in Figure 1. The question of the products of the 0 + CzHzreaction then centers on the relative ease of hydrogen atom dissociation vs. 1,2 hydrogen migration in the OHCCH adduct. We report here ab initio, configuration interaction calculations aimed at determining the relative heights of the barriers to hydrogen elimination and migration in the 3A" state of formylmethylene. The basis sets employed were the Dunningg (3s,2p) contractions of Huzinaga's (9s,5p) primitive Gaussian sets for carbon and oxygen. For the active hydrogen, Ha, a (3s) contraction of Huzinaga's (5s) primitive set was used while for the remaining hydrogens a (2s) contraction of the (4s) primitive set (scale factor = 1.2) was used. These functions were augmented by sets of d polarization functionsg on the carbons (a = 0.75) and the oxygen (a = 0.85) and a set of p polarization functions (a = 1.1236)'O on the active hydrogen.
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(1) A. Williams and D. B. Smith, Chem. Reus., 70, 267 (1970). (2) K. H. Eberius, K. Hoyermann, and H. Gg. Wagner, Symp. (Znt.) Combust., [Proc.],I4th, 147 (1973). (3) (a) I. T. N. Jones and K. D. Bayes, J. Am. Chem. SOC.,94,6869 (1972); (b) Symp. (Znt.)Combust., [Proc.],14th, 277 (1973). (4) J. R. Kanofsky, D. Lucas, F. Pruss, and D. Gutman, J. Phys. Chem., 78, 311 (1974). (5) (a) D. G. Williamson and K. D. Bayes, J. Phys. Chem., 73, 1232 (1969); (b) D. G. Williamson, ibid., 75, 4053 (1971). (6) B. Blumenberg, K. Hoyermann, and R. Sievert, Symp. (Int.) Combust., [Proc.], 16th, 841 (1977). (7) C. P. Fenimore and G. W. Jones, J. Chern. Phys., 39,1514 (1963). (8) J. W. Simons and R. Curry, Chem. Phys. Lett., 38, 171 (1976). (9) (a) T. H. Dunning, Jr., and P. J. Hay, "Modern Theoretical Chemistry", Vol. 2, H. F. Schaefer, 111, Ed., Plenum Press, New York, 1976. (b) S. Huzinaga, J. Chem. Phys., 58, 4452 (1973). 0022-3654/81/2085-0010$01.00/0
TABLE I: Relative Energies of Intermediates and Transition Structures in the O(3P)+ HCCH Reaction structure GVB(4/SO) POL-CI OHCCH(3A") 0.oa 0.oa OHCCH*H+ HCCO 58.5 31.3 OHCCH OCCH, 68.0 52.9 H + HCCO(zA') 51.4 31.8 OCCHz(3A") -8.2 -19.6 a The total GVB(4/SO) and POL-CI energies for OHCCH OHCCH(3A")are -151.729589 and -151.869263 hartree, respectively.
*
The transition structures for the two reactions were located with a GVB(2/S0)l1JZwave function of the form $GVB(2/SO)
--
4(core)[4,2 - WUt2- +,+14u1)14a'4b'~) where 4gand describe the C-Ha bond pair, 4, and & are the two singly occupied, carbene-center orbitals, core refers to all other (doubly occupied) orbitals, and x is a general, optimized spin function. With this wave function, the saddle points were located by minimizing the energy with respect to n - 1 coordinates and maximizing the energy with respect to the remaining coordinate (the reaction coordinate). The success of this procedure, of course, depends on the choice of the reaction coordinate. Calculations on hydrogen migration and elimination in the vinyl radical13 have shown that to a very good approximation the reaction coordinate for elimination can be taken to be the C-Ha bond distance and the migration reaction coordinate can be taken to be the CMH, angle where M is the midpoint of the C-C bond. For the equilibrium structures of OHCCH and OCCHz (3A") restricted Hartree-Fock calculations with a double-{ basis were used to determine the optimum geometries. For these two species the CH bond lengths were assumed to be 1.09 A and the bond angles about the trisubstituted carbons were assumed to be 120O. For HCCO, it was found that the combined effects of correlation and d-polarization functions lead to a substantial change in the geometry and so this structure was reoptimized with the polarized double-{ basis and the GVB wave function. The results of the geometry optimizations are summarized in Figure 2. At these optimized geometries, POL-CI ~ a l c u l a t i o n s ~ ~ were carried out based on GVB(4/SO) wave functions in (10) S. P. Walch and T. H. Dunning, Jr., J. Chem. Phys., 72, 1303 (1980). (11) F. W. Bobrowicz, Ph.D. Thesis, California Institute of Technology, 1979. (12) B. J. Moss, F. W. Bobrowicz, and W. A. Goddard, 111, J.Chem. Phys., 63, 4632 (1975). (13) L. B. Harding, to be published. (14) P. J. Hay and T. H. Dunning, Jr., J. Chem. Phys., 64,5077 (1976).
0 1981 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 85, No. 1, 198 1
O(3P) + C2H2-C, -
H + HCCO (2A”l
,H
0 \
/
Figure 1. Mechanism of the reaction of O(3P)
+ C2H2.
i
0 \.zoo
\
O\
/H
I I68
9
I
i
\ H
i 2.065
\ H Ho Flgure 2. Calculated geometries of intermediates and transition structures in the O(3P) HCCH reaction. Bond lengths are in A. Unspecified bond angles were assumed to be 120’ and unspecified C-H bond lengths were assumed to be 1.09 A.
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which both the C-C u and the C-0 a bonds were correlated (in addition to the C-Ha bond). The POL-CI calculations include all single and double excitations relative to each of the three GVB(2ISO) configurations with the restriction that at most one electron is allowed into the virtual orbitals. This leads to a total of 46393 spin eigen-functions. These were selected relative to the GVB(2/SO) configurations by using the AK selectionl5J6scheme with a cummulative energy lowering criterion of 1mhmtree on the discarded configurations. Analogous calculations on barriers to hydrogen atom elimination from vinyl and ethyl radicals have led to barriers within 3 kcal/mol of the observed activation energies.13 We know of only one measurement of an activation energy for 1,2 hydrogen migration in a free radical, this being on the ethyl radical. The observed barrier is 41 f 4 kcal/moP7 while the calculationalmethod used here predicts a barrier of 45.4 kcal/mol. Thus we conclude that these methods are capable of predicting barrier heights for hydrogen elimination and migration reliable to approximately *5 kcal/mol. The results on OHCCH are summarized in Table I. The calculated barrier for elimination of hydrogen is 37 kcal/mol while that for 1,2 hydrogen migration is 53 kcal/mol. In addition, we note that the transition structure for elimination is very loose, involving a CH, distance of 2.06 A while the migration transition structure is much tighter (CIH, = CzH, = 1.4 A). Qualitatively then we (15)Z. Gershgorn and I. Shavitt, Int. J. Quantum Chem., 2, 751 (1968). (16)T. H. Dunning, Jr., Chem. Phys., 42, 249 (1979). (17)A.S.Gordon, D. C. Tardy, and R. Ireton, J. Phys. Chem., 80,1400 (1976).
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expect the A factors to also favor the HCCO product. In support of this we note that both trajectory calculations18 and experimental measurements” of the relative A factors for hydrogen migration and dissociation in the ethyl radical led to the conclusion that the A factors favor dissociation by a factor of 10. Together then the calculated 16 kcal/mol difference in activation energies and the expected factor of 10 difference in A factors make it very unlikely that significant amounts of CHz(3B1)+ CO result from this decomposition.lg There are several complicating features of this reaction that deserve consideration. First, the calculations reported here have considered only the 3A” state of the OHCCH adduct. There is a second triplet state, 3A’, of this species that can result from addition of atomic oxygen to acetylene. Preliminary GVB-CI calculations on this state indicate that it lies 37 kcal/mol above the 3A” state placing it slightly aboue H + HCCO. Hydrogen migration from the 3A’ state leads most directly to the second triplet state of ketene. A preliminary search in the region of this transition structure indicates that it lies at .an energy well above that of the reactants and thus the 3A’ state is also expected to lead to HCCO formation. In addition to these two triplet states of OHCCH there also exists a relatively low-lying singlet state. Calculations by Bargon et aLZohave shown that this state has at most a small barrier to hydrogen migration and thus if formed it would result in formation of CHz(’A1) + CO. However, GVB-CI calculations predict that the 3A’’ state of OHCCH is formed with -55 kcal/mol of excess energy, while the barrier to hydrogen elimination is only 37 kcal/mol. At low pressure then it appears unlikely that intersystem crossing to the singlet state would compete with hydrogen elimination. The singlet state may, however, be of importance either at high pressures or in the oxidation of highly substituted alkynesz1due to the expected longer lifetimes of the initial adducts. In summary, the calculations reported here predict that the dominant product of the gas-phase reaction of atomic oxygen (3P)with acetylene are atomic hydrogen and the HCCO radical. This conclusion is supported by a recent single-collision molecular beam study of this reaction in which it was concluded that HCCO is a major product.zz In addition, analysis of the translational energy distribution of the products was interpreted to indicate a barrier of several kcal/mol in the exit channel. This is also in accord with the results reported here which predict a barrier of 5.4 kcal/mol occurring quite far out in the exit channel leading to HCCO. Acknowledgment. I thank T. H. Dunning, Jr., D. Gutman, R. J. Buss, and Y. T. Lee for many stimulating discussions on this topic. This work was performed under the auspicies of the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Energy. (18)(a) W. L. Hase, R. J. Wolf, and Christine S. Sloane, J. Chem. Phys., 71, 2911 (1979). (b) W. L. Hase, private communication. (19)These calculations do not include the effects of zero point energies. It is expected, however, that the zero point energy will be larger for the tight, migration, transition structure than for the loose, dissociation structure. Thus zero point energies would also favor the formation of HCCO. (20)J. Bargon, K. Tanaka, and M. Yoshimine, “Computational Methods in Chemistry”, J. Bargon, Ed., Plenum Press, New York, 1980. (21)J. J. Have1 and K. H. Chan, J. Org. Chem., 42, 569 (1977). (22)R.J. Buss, R. J. Baseman, P. Casavecchia,and Y. T. Lee, private communication.
