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

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 S-0-S-1 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 center dot center dot center dot O=C H-bond, which shifts the (3)n pi* state well above the (1)pi pi* S-1 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, nu(33) (421 cm(-1)), nu(34) (366 cm(-1)), and nu(36) (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 center dot center dot center dot O=C H-bond. This leads to a shortening of the NH center dot center dot center dot O=C H-bond distance from 1.926 angstrom to 1.723 angstrom, equivalent to about a 25% reduction in the HMIDLINE HORIZONTAL ELLIPSISO distance compared to full H-atom transfer. As a result, the excited state process near the S-1 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 angstrom increase in the NH bond length relative to its ground state value.