Vibronic spectroscopy of methyl anthranilate and its water complex: hydrogen atom dislocation in the excited state?

Physical Chemistry Chemical Physics Pub Date: 2019-09-11 DOI: 10.1039/C9CP04556B

Abstract

Laser-induced fluorescence (LIF) excitation, dispersed fluorescence (DFL), UV–UV-hole burning, and UV-depletion spectra have been collected on methyl anthranilate (MA, methyl 2-aminobenzoate) and its water-containing complex (MA–H2O), under jet-cooled conditions in the gas phase. As a close structural analog of a sunscreen agent, MA has a strong absorption due to the S0–S1 transition that begins in the UV-A region, with the electronic origin at 28?852 cm?1 (346.6 nm). Unlike most sunscreens that have fast non-radiative pathways back to the ground state, MA fluoresces efficiently, with an excited state lifetime of 27 ns. Relative to methyl benzoate, inter-system crossing to the triplet manifold is shut off in MA by the strong intramolecular NH?O[double bond, length as m-dash]C H-bond, which shifts the 3nπ* state well above the 1ππ* S1 state. Single vibronic level DFL spectra are used to obtain a near-complete assignment of the vibronic structure in the excited state. Much of the vibrational structure in the excitation spectrum is Franck–Condon activity due to three in-plane vibrations that modulate the distance between the NH2 and CO2Me groups, ν33 (421 cm?1), ν34 (366 cm?1), and ν36 (179 cm?1). Based on the close correspondence between experiment and theory at the TD-DFT B3LYP-D3BJ/def2TZVP level of theory, the major structural changes associated with electronic excitation are evaluated, leading to the conclusion that the major motion is a reorientation and constriction of the 6-membered H-bonded ring closed by the intramolecular NH?O[double bond, length as m-dash]C H-bond. This leads to a shortening of the NH?O[double bond, length as m-dash]C H-bond distance from 1.926 ? to 1.723 ?, equivalent to about a 25% reduction in the H?O distance compared to full H-atom transfer. As a result, the excited state process near the S1 origin is a hydrogen atom dislocation that is brought about primarily by heavy atom motion, since the shortened H-bond distance results from extensive heavy-atom motion, with only a 0.03 ? increase in the NH bond length relative to its ground state value.

Graphical abstract: Vibronic spectroscopy of methyl anthranilate and its water complex: hydrogen atom dislocation in the excited state
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