PHOTOIONIZATION MASS-SPECTROMETRY OF CH2S AND HCS

The transient species CH2S and HCS were studied by photoionization mass spectrometry. They were prepared in situ from CH3SH by sequential hydrogen abstraction with fluorine atoms. CH2S was also prepared by pyrolysis of CH3SCl and CH3SSCH3. The photoion yield curve of CH2S displays an abrupt threshold, and is similar in overall shape to that of the homologue CH2O. The adiabatic ionization potential of CH2S is found to be 9.376 +/- 0.003 eV. Evidence has been found for nd and/or ns and np Rydberg states converging to the first excited state of CH2S+. In addition, the HCS+ fragment from CH2S has been determined to appear at less-than-or-equal-to 11.533 +/-0.021 eV at 0 K. In contrast to CH2S, the photoion yield curve of HCS+ from HCS displays a very broad Franck-Condon envelope, consistent with a transition from bent HCS to linear HCS+. A Poisson fit to the experimental Franck-Condon factors indicates that the adiabatic ionization potential of HCS is less-than-or-equal-to 7.499 +/- 0.005 eV, and perhaps as low as 7.412 +/- 0.007 eV. The fragment curves at m/e = 46, 47, 48, and 49 from CH3SSCH3 have also been examined, and their relative shifts in energy determined. Together with measurements on CH2S and HCS, and the previously reported DELTAH(f0)-degrees (CH2SH+) = 211.5 +/- 2. 0 kcal/mol (less-than-or-equal-to 213.1 +/- 0.2 kcal/mol), this is sufficient to establish DELTAH(f0)-degrees (CH2S) = 28.3 +/- 2.0 kcal/mol (less-than-or-equal-to 29.9 +/- 0.9 kcal/mol) and DELTAH(f0)-degrees (HCS) = 71.7 +/- 2.0 kcal/mol (less-than-or-equal-to 73.3 +/- 1.0 kcal/mol), greater-than-or-equal-to 69.7 +/- 2.0 kcal/mol). These values are in very good agreement with recent ab initio calculations. The implications for various bond energies within the CH(n)S system are also discussed.