The reaction OH -+ CH 2 F 2 f products has been investigated by both selected ion flow tube (SIFT) experiments and ab initio molecular calculations. The SIFT experiments showed that a bimolecular process, leading to two major anionic products, CHF 2 -(86%) and F -(11%), and one minor anionic product, HF 2 -(3%), is in competition with a three-body association leading to OH -‚CH 2 F 2 (where values in parentheses are the relative values of the detected anionic products at 300 K). From a pressure dependence study, an upper limit of the bimolecular reaction rate coefficient at 300 K is determined to be (2.4 ( 1.4) × 10 -12 cm 3 molecule -1 s -1 . This shows a small negative temperature dependence, suggesting that the reaction proceeds via an ion-complex intermediate. These experimental results were rationalized using ab initio molecular orbital calculations. Stationary points on the reaction paths of the two main reaction channels were located at both the HF/6-31++G ** and MP2/6-31++G ** levels. The relative energies of the located stationary points were calculated at up to the CCSD(T)/6-311++G 3df,2p //MP2/6-31++G ** level. The CHF 2 -+ H 2 O channel was found to be endothermic by 7.5 kcal mol -1 and the F -+ CH 2 (OH)F channel was found to be exothermic by 20.4 kcal mol -1 . It was found that both reaction channels proceed via the reactant-like ion-molecule complex intermediate, OH -‚CH 2 F 2 , in agreement with the conclusion drawn from the experimental negative temperature dependence of the overall rate coefficient. The fact that the product anion yields show that [CHF 2 -] > [F -], despite the fact that the CHF 2 -+ H 2 O channel is endothermic whereas the F -+ CH 2 (OH)F channel is exothermic, has been rationalized using transition-state theory.

The title reactions have been investigated in the gas phase using UV photoelectron spectroscopy. By recording spectra at different reaction times, it was possible to detect reaction intermediates and hence identify pathways by which each reaction proceeds. The F2 + CH3SCH3 reaction appears to proceed via the intermediate CH3-SFCH3 whereas the F2 + CH3SSCH3 reaction gives H2CS and HF as the first observed products. For both reactions at longer reaction times, a new spectrum associated with a reaction product, consisting of three photoelectron bands with adiabatic ionization energies of 10.15 f 0.01, 11.49 f 0.02, and 13.71 0.01 eV, has been observed. This spectrum is assigned to the HFCS molecule. The first two adiabatic ionization energies of HFCS have been calculated using the Gaussian-2 (G2) theoretical procedure as 10.09 and 1 1.5 1 eV, respectively, supporting this assignment. This work represents the first observation and spectroscopic study of this molecule. A broad band with a vertical ionization energy of 10.58 f 0.06 eV, assigned to the first ionization energy of HCF, has also been observed as a secondary product from both reactions under conditions of excess F2 partial pressure. This assignment is supported by first adiabatic and vertical ionization energies of HCF computed at the G2 level of theory. For each reaction, a reaction mechanism is proposed on the basis of the results obtained.

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