5010 J. Phys. Chem. 1994, 98, zyxwvu 5010-5018 The Last Chapter on Chlorofluorocarbon Photooxidation Processes: Formation and Dissociation of FC(0)ONO T. S. Dibble and J. S. Francisco. Department of Chemistry, Wayne State University, Detroit, Michigan 48202 Received: September 3, 1993; In Final Form: March 9, 1994" The key features of the potential energy surface of FC(0)ONO and its FC(O)N02 isomer have been determined using ab initio molecular orbital theory. This species, which may be regarded as the N O adduct of the FC(0)O radical, is postulated to be an intermediate in the atmospheric degradation of chlorofluorocarbons. Hartree- Fock (HF) and second-order Merller-Plesset perturbation theory (MP2) are used to characterize the structure and vibrational frequencies of two FC(0)ONO chain conformers and two higher energy isomers, including a stable cyclic species whose structure can be represented as FN zyxwv (g ) C=O. Relative energies are determined using MP4 and QCISD(T) (quadratic configuration interaction with single, double, and triple excitations) methods. Transition states are found connecting the two FC(0)ONO chain conformers to each other, to FC(O)N02, and to stable degradation products. The exothermicity of formation of FC(0)ONO by any pathway is more than sufficient to overcome the barrier to formation of FNO + C02, the most exothermic species on the entire potential energy surface. Thus, the reaction of FC(O)O, radicals with NO, radicals terminates chlorofluorocarbon photooxidation by converting FC(O)O, radicals to C02 and FNO, which is suggested as a hitherto undetected fluorine reservoir species. Introduction Radicals of the type FC(O)O, are believed to be formed in the atmospheric photooxidation of FzCO and FC1CO,l4 which are reservoir species in the degradation of CX3 fragments produced by the photodissociation of chlor~fluorocarbons,~~~ FXCO + zyxwvut hv - FCO + X X = (F,Cl) (1) FCO + 0, + M - FC(O)O, + M (2) FC(O)O, + NO - FC(0)O + NO, (3) It has been proposed that these radicals could couple to the NO, cycle to create a catalytic cycle which could regenerate ozone in the upper atmosphere. The cycle begins with reactions 2 and 3 above and is continued via FC(0)O + NO - FC(0)ONO (4) FC(0)ONO - FCO + NO, (5) (2[NO, + 0, - zyx 0, + NO]) (6) net: 30, - 20, (7) However, reactions 4 and 5 are not the only chemistry of FC- (0)ONO. This adduct could isomerizeor undergo conformational change through FC(0)ONO - FC(O)NO, (8) syn-FC(0)ONO - anti-FC(0)ONO (9) Two other reactions produce a pair of closed-shell products FC(0)ONO - FNO + CO, (10) a Abstract published in Aduance ACS Abstracts, April 15, 1994. 0022-3654/94/2098-50lOSO4.50/0 FC(O)NO, - FNO, + CO (11) FC(0)ONO and its isomers have not been studied previously, either computationally or experimentally. In this work we use ab initio molecular orbital theory to calculate geometries and vibrational frequencies for two FC(0)ONO conformers and two isomers. We report absolute intensities of the vibrational modes as an aid to experimentalists wishing to search for these compounds, and we determine the energetics of reactions 4 and 5. To investigate the fate of these adducts in the atmosphere, and thus the ultimate fate of fluorinated methyl radicals produced in the photodissociation of chlorofluorocarbons, it is necessary to determine the activation barriers to reactions 8-1 1. We have calculated the structures of the transition states for reactions 8-1 1 and the energetics of thesereactions. This report is intended to describe the most significant features of the potential energy surface of this system. As the first comprehensive examination of this system, this paper should provide a firm basis for further study, both experimental and computational. Computational Methodology Ab initio molecular orbital calculations were performed with the GAUSSIAN 907 and GAUSSIAN 92* series of programs on a Kubota-Pacific (formerly Stardent) GS 3000 minisupercom- puter. Geometry optimizations and frequency calculations were carried out using the Hartree-Fock method and second-order Maller-Plesset perturbation theory. An unrestricted Hartree- Fock wave function was used for radical species. Due to the number of species and transition states whose structures needed to be determined, and the number of heavy atoms in the adducts and transition states, we only used the 6-31G* basis set.9 Geometry optimizations were carried out using the gradient method of Schlegel.10 Geometries were optimized to better than 0.001 A for bond lengths and 0.lo for angles. With a SCF convergence of at least 10-9 on the density matrix, the rms force was less than 10-4 atomic units. Harmonic frequencies and zero- point energies were computed using analytical second derivatives at the Hartree-Fock and MP2 levels. Single-point calculations were carried out at the MP4SDTQ level of theory. Because the energetics of some species (FN02, FNO) which might be produced 0 1994 American Chemical Society