General rights This document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www. Abstract The ultraviolet (UV) photodissociation of gas phase methimazole has been investigated by H Rydberg atom photofragment translational spectroscopy methods at many wavelengths in the range 222.5 -275 nm and by complementary electronic structure calculations. Methimazole is shown to exist

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... antly as the thione tautomer, 1-methyl-2(3H)-imidazolinethione, rather than the commonly given thiol form, 2-mercapto-1-methylimidazole. The UV absorption spectrum of methimazole is dominated by the S 4 S 0 transition of the thione tautomer, which involves electron promotion from an a′ (p y ) orbital localized on the sulfur atom to a σ* orbital localized around the N-H bond. Two H atom formation pathways are identified following UV photoexcitation. One, involving prompt, excited state N-H bond fission, yields vibrationally cold but rotationally excited methimazolyl (Myl) radicals in their first excited ( ) electronic state. The second yields H atoms with an isotropic recoil velocity distribution peaking at low kinetic energies but extending to the energetic limit allowed by energy conservation given a ground state dissociation energy D 0 (Myl-H) ~24 000 cm -1 . These latter H atoms are attributed to the unimolecular decay of highly vibrationally excited S 0 parent molecules. The companion electronic structure calculations provide rationales for both fragmentation pathways and the accompanying product energy disposals, and highlight similarities and differences between the UV photochemistry of methimazole and that of other azoles (e.g. imidazole) and with molecules like thiourea and thiouracil that contain similar N-C=S motifs.