Theoretical Investigations on Photodissociation Dynamics of Deuterated Alkyl Halides CD₃CH₂F

The product branching ratio between different products in multichannel reactions is as important as the overall rate of reaction, both in terms of practical applications (\emphe.g. models of combustion or atmosphere chemistry) in understanding the fundamental mechanisms of such chemical reactions. A global ground state potential energy surface for the dissociation reaction of deuterated alkyl halide CD _3 CH _2 F was computed at the CCSD(T)/CBS//B3LYP/aug-cc-pVDZ level of theory for all species. The decomposition of CD _3 CH _2 F is controversial concerning C - F bond dissociation reaction and molecular (HF, DF, H _2 , D _2 , HD) elimination reaction. Rice-Ramsperger-Kassel-Marcus (RRKM) calculations were applied to compute the rate constants for individual reaction steps and the relative product branching ratios for the dissociation products were calculated using the steady-state approach. At the different energies studied, the RRKM method predicts that the main channel for DF or HF elimination from 1, 2-elimination of CD _3 CH _2 F is through a four-center transition state, whereas D _2 or H _2 elimination from 1, 1-elimination of CD _3 CH _2 F occurs through a direct three-center elimination. At 266, 248, and 193 nm photodissociation, the main product CD _2 CH _2 +DF branching ratios are computed to be 96.57%, 91.47%, and 48.52%, respectively; however, at 157 nm photodissociation, the product branching ratio is computed to be 16.11%. Based on these transition state structures and energies, the following photodissociation mechanisms are suggested: at 266, 248, 193 nm, CD _3 CH _2 F \rightarrow absorption of a photon \rightarrow TS5 \rightarrow the formation of the major product CD _2 CH _2 +DF; at 157 nm, CD _3 CH _2 F \rightarrow absorption of a photon \rightarrow D/F interchange of TS1 \rightarrow CDH _2 CDF \rightarrow H/F interchange of TS2 \rightarrow CHD _2 CHDF \rightarrow the formation of the major product CHD _2 +CHDF.