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 R1R2C═N—NHC(═S)SCH3 (R1, R2 = 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 CH3SH for syn-Re and a two-step reaction pathway for trans-Re involving tautomer interconversion followed by decomposition of CH3SH, 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 tran...