Calculated barrier heights for OH + C2H2 and OH + C2H4 using unrestricted Moeller-Plesset perturbation theory with spin annihilation

Abstract

The reactions of OH radical with C2H2 and CzH4 have been studied with ab initio molecular orbital techniques. Reactants, loose clusters, transition structures, and products were optimized at UHF/3-21G and UHF/6-3 1G*. The barrier heights have been computed by using unrestricted Hartree-Fock and Mdler-Plesset perturbation theory up to fourth order, including single, double, and quadruple excitations. Spin contamination in the UHF wave function has been corrected by annihilating the largest spin contaminant. The vibrational frequencies were computed by using analytical derivative methods at the UHF/3-21G level. The barrier heights for both reactions are overestimated by 7-15 kcal/mol at the UMPZ, UMP3, and UMP4 levels. Annihilation of the largest spin contaminant lowers the barrier heights by 7-15 kcal/mol. Calculations at the PMP4/6-3 lG* level are in good agreement with the estimated experiment barrier heights. The reactions of OH with acetylene and ethylene are known to be important in hydrocarbon combustion as well as atmospheric chemistry.'-5 Experimental studies on OH + C2H21-12 and OH + c2H4'-43'1-25 have shown that near room temperature the predominant mechanism involves the electrophilic addition of the OH radical to the 7r bond, forming an activated complex which can be collisionally stabilized. OH + C2H2, - C2H2,0H* (n = 1, 2) (1) (2) C2H2,0H* + M --* C2H2,OH + M In early experimental work on OH + C2H2, no pressure de- pendence of the rate constant was found at low temperature^.^^^ However, recent studies covering a wider range of temperature and pressure, and using flash photolysis/resonance fluorescence and laser pyrolysis/laser fluorescence techniques,8-10 indicate that the rate constant does depend on the pressure, consistent with eq 1 and 2. Analysis of the temperature and pressure dependence yields an activation energy of 1.3 =! 0.1 kcal/m~l,~.'~ based on an estimated heat of reaction of -36 f 6 kcal/mol for OH + C2H2 - C2H20H. At temperatures higher than 1000 K, the addition channel becomes less important because of competition from hydrogen abstraction (estimated activation energy 6-8 kcal/ The OH + C2H4 system has also received considerable atten- ti~n.-~~ In accord with eq 1 and 2, the rate for OH + C2H4 is found to be dependent on the total pressure. However, unlike addition to acetylene, a small negative activation energy has been found (-0.9 * 0.2,'3*14 -0.7 f 0.3,15 -0.6 f 0.316 kcal/mol). Several explanations have been given for this, including the for- mation of a weakly bound complex.26 Estimates of the heat of reaction for the addition process range from -29 to -32 kcal/ m0I.~3'~-'* Mass spectral methods have been used to observe directly the primary adduct, C2H40H, and to study its decom- position into CH, + CH20 and H + CH3CH0.18,'9 At higher temperatures, hydrogen abstraction becomes the dominant pathway (estimated activation energy ca. 3 kcal/m~l).',~~~~ ~ In an earlier theoretical study, Melius, Binkley, and Koszy- kowskiZ7 examined the reactions of OH with C2H2, C2H4, and HCN. Geometries were optimized at the HF/6-31G* level and energies were calculated by using fourth order unrestricted Mdler-Plesset perturbation theory (UMP4/6-3 lG**). The heats of reaction and barrier heights were estimated by applying bond additivity corrections to the UMP4 calculations (BAC-MP4). These corrections were determined by a least-squares fit to ca. 50 molecules with well-established heats of formation. For m01).~J~