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Publication - Professor Mike Ashfold

    Photofragment Translational Spectroscopy Studies of H Atom Loss Following Ultraviolet Photoexcitation of Methimazole in the Gas Phase

    Citation

    Cooper, GA, Hansen, CS, Karsili, TNV & Ashfold, M, 2018, ‘Photofragment Translational Spectroscopy Studies of H Atom Loss Following Ultraviolet Photoexcitation of Methimazole in the Gas Phase’. Journal of Physical Chemistry A, vol 122., pp. 9869-9878

    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 of 222.5–275 nm and by
    complementary electronic structure calculations. Methimazole is shown to
    exist predominantly 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 S4 ← S0 transition of the thione tautomer, which involves electron promotion from an a′ (py)
    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 (Ã) 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 D0(Myl–H) ∼24 000 cm–1. These latter H atoms are attributed to the unimolecular decay of highly vibrationally excited S0
    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.

    Full details in the University publications repository