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

Abstract

ABSTRACTChemically activated CF3SH, CFCl2SH, and CF2ClSH were formed through combination of SH and CF3, CFCl2, and CF2Cl radicals, respectively. The SH radical was prepared by abstraction of an H‐atom from H2S by the halocarbon radical produced during photolysis of (CF3)2C=O, (CFCl2)2C=O, or (CF2Cl)2C=O. 1,2‐HX (X = F, Cl) elimination reactions were observed from CF3SH, CFCl2SH, and CF2ClSH with products detected by GC‐MS. The combination reaction of CF2Cl radicals with SH radicals prepared CF2ClSH molecules with approximately 318 kJ/mol of internal energy. The experimental rate constants for elimination of HCl and HF from CF2ClSH were 3 ± 3 × 1010 and 2 ± 1 × 109 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 CF3SH and CFCl2SH 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 CFCl2SH, 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.