An insight into methimazole phototautomerism: Central role of the thiyl radical and effect of benzo substitution

The thione/thiol tautomerism in methimazole (1-methyl-2-thioimidazole; MTI) and 1-methyl-2-thiobenzimidazole (MTBI) has been investigated by matrix isolation infrared spectroscopy and quantum chemical calculations performed at the DFT(B3LYP)/6-311++G(d,p) level. The two compounds were shown to pass to the gas phase from the crystalline state, where they exist in the thione form, upon sublimation, without tautomerizing. The thione form of the compounds could then be isolated in the cryogenic matrices and vibrationally characterized. Narrowband UV-irradiation of the thione forms at 261 or 307 nm, for MTI and MTBI, respectively, allowed their conversion into the corresponding thiol forms, which were observed experimentally for the first time and could then be also characterized vibrationally in details. MTBI thiol could subsequently be converted back to the thione form upon irradiation at 246 nm, while the thiol→thione tautomerization could not be induced for MTI. The experimental results were rationalized in terms of the PhotoInduced Detachment Association (PIDA) mechanism and taking into account both the structures of the thiyl radicals resulting from the photodetachment process and the effects due to the benzo-substituent. In the case of MTI, besides the photoinduced thione→thiol tautomerization, photofragmentation was observed, through imidazole ring disassembling, yielding N-vinylidenemethanamine (NVM). On the whole, this study aims to further understanding on thione/thiol tautomerism and photochemistry of thiones and thiols.