Photoionization mass spectrometric studies of the isomeric transient species CH2SH and CH3S

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Abstract
The products of the reaction of F atoms with CH3SH are studied by photoionization mass spectrometry. Two thresholds, at 7.536±0.003 eV and 9.262±0.005 eV, are observed at mass 47, and identified with the adiabatic ionization potentials of CH2SH and CH3S, respectively. These conclusions are confirmed by additional experiments with CD3SH, where an adiabatic ionization potential of 7.522±0.003 eV is obtained at mass 49 (CD2SH+) and a value of 9.268±0.005 eV is found at mass 50 (CD3S+). From an earlier appearance potential for CH2SH+ (CH3SH) and the present ionization potential of CH2SH, an upper limit for Dd0d (H–CH2SH ) of ≤93.97±0.13 kcal/mol is deduced. By giving weight to the measured proton affinity of CH2S, one obtains D0(H–CH2SH )=92.4±2.0 kcal/mol. The corresponding S–H bond energy in CH3SH derived from a recent kinetics-based value for ΔH0f0(CH3S) is 86.1±0.6 kcal/mol. A discrepancy in the difference of ΔH0f0(CH3S+) and ΔH0f0(CH2SH+) arising from recent theoretical and experimental determinations is resolved in favor of the ab initio values.

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