Unimolecular Rate Constants for the HF and HCl Elimination Reactions from Chemically Activated CF₂ClSH

Abstract

Chemically activated CF 3 SH, CFCl 2 SH, and CF 2 ClSH were formed through combination of SH and CF 3 , CFCl 2 , and CF 2 Cl radicals, respectively. The SH radical was prepared by abstraction of an H-atom from H 2 S by the halocarbon radical produced during photolysis of (CF 3 ) 2 C=O, (CFCl 2 ) 2 C=O, or (CF 2 Cl) 2 C=O. 1,2-HX (X = F, Cl) elimination reactions were observed from CF 3 SH, CFCl 2 SH, and CF 2 ClSH with products detected by GC-MS. The combination reaction of CF 2 Cl radicals with SH radicals prepared CF 2 ClSH molecules with approximately 318 kJ/mol of internal energy. The experimental rate constants for elimination of HCl and HF from CF 2 ClSH were 3 ± 3 × 10 10 and 2 ± 1 × 10 9 s −1 , respectively. Comparison to Rice-Ramsperger-Kassel-Marcus (RRKM) calculated rate constants assigned the threshold energies as 171 ± 12 and 205 ± 12 kJ/mol for the unimolecular elimination of HCl and HF, respectively. Theoretical calculations using the B3PW91, MP2, and M062X methods with the 6311+G(2d,p) and 6-31G(d',p') basis sets established that for a specific method the threshold energies differ by only 4 kJ/mol between the two different basis sets. There was wide variation among the three methods, but the M062X approach appeared to give threshold energies closest to the experimental values. Chemically activated CF 3 SH and CFCl 2 SH were also prepared with about 318 kcal mol −1 of internal energy, and the HX (X = F, Cl) elimination reactions were observed. Only HCl loss was detected from CFCl 2 SH, but the rate was too fast to measure with our kinetic method; however, based on our detection limit the HF elimination channel is at least 50 times slower. C