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

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 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 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.

Effects of Ring Fluorination on the Ultraviolet Photodissociation Dynamics of Phenol

“…All the spectra show a broad, smooth feature rising at low kinetic energies. Such signals have been observed in many previous experiments involving a range of polyatomic hydrides and are generally attributed to processes where the electronically excited parent internally converts to vibrationally hot S 0 levels and then loses an H atom, or to processes involving multiple photons. ,,,,,,,− Of greater current interest, the spectra also show a number of well-resolved peaks at higher TKER values, the positions and relative intensities of which vary with the photolysis wavelength. For completeness, we note that the TKER spectra recorded at the longest excitation wavelengths (i.e., when exciting low vibronic levels in the S 1 potential well) also show a weak tail extending to much higher kinetic energies, consistent with unintended resonance enhanced two photon excitation via these S 1 levels and subsequent dissociation, as observed previously in other phenols with relatively long-lived S 1 states. ,, …”

“…Such signals have been observed in many previous experiments involving a range of polyatomic hydrides and are generally attributed to processes where the electronically excited parent internally converts to vibrationally hot S0 levels and then loses an H atom, or to processes involving multiple photons. 7,11,21,34,40,47,48,[62][63][64] Of greater current interest, the spectra also show a number of well-resolved peaks at higher TKER values, the positions and relative intensities of which vary with the photolysis wavelength. For completeness, we note that the TKER spectra recorded at the longest excitation wavelengths (i.e.…”