The pyrolytic reaction of ketonic hydrazones from S-methyl dithiocarbazate: a combined online GC-MS pyrolysis and DFT study.

The gas-phase pyrolysis of ketonic hydrazones from S-methyl dithiocarbazate R(1)R(2)C=N-NHC(=S)SCH(3) (R(1), R(2) = alkyl or aryl) was investigated by online GC-MS pyrolysis and theoretical calculation. Both of these pyrolytic products, ascribed to methanethiol and the corresponding N-isothiocyanate imines, were detected in the total ion chromatography (TIC) of GC-MS. Calculation results exhibit two stable configurational structures for reactants (Re), which can interconvert with relatively low barriers (<78 kJ/mol). DFT calculations showed that the two unimolecular pyrolytic processes, a direct 1,2-elimination of CH(3)SH for syn-Re and a two-step reaction pathway for trans-Re involving tautomer interconversion followed by decomposition of CH(3)SH, are competitive in the reaction. Both syn-Re and trans-Re exhibit lower critical energies in the propagation step of the radical pyrolysis than that in the unimolecular pyrolysis process (187.76 kJ/mol via 131.91 kJ/mol for syn-Re, and 159.15 kJ/mol via 98.92 kJ/mol for trans-Re). However, much more energy is needed to excite the compound to produce the methylthio radical, with 262.03 and 253.60 kJ/mol for syn-Re and trans-Re, respectively. Therefore, the unimolecular pyrolysis rather than the radical one occurs in the condition of this study.