Franck–Condon analysis of laser-induced fluorescence excitation spectrum of anthranilic acid: Evaluation of geometry change upon S→S1 excitation

Abstract

Geometry and vibrational modes of the anthranilic acid molecule in the S(0) and S(1) states were computed using ab initio methods: Hartree-Fock (HF) and configuration interaction of singly excited configurations (CIS) as well as the density functional theory with time-dependent perturbation (TD-DFT). The intensity distribution in the laser-induced fluorescence excitation spectra was modeled in two ways: using displacement parameters for independent modes and using multidimensional Franck-Condon integrals. The change in the molecular geometry upon excitation was calculated from the band intensities within the above two models. Displacement parameters of eight in-plane modes active in the excitation spectrum were optimized to reproduce the experimental intensities of about 40 most intensive and well-separated vibrational bands, while displacement parameters of other in-plane modes were kept frozen at the values resulting from the quantum chemical calculations. The intramolecular hydrogen bond is significantly stronger in the S(1) state than in the ground state. Additionally, bond lengths and angles in the aromatic ring, within the substituents and between the ring and the substituents undergo significant changes and they induce the presence of strong fundamentals in the excitation spectrum.

LIF excitation spectra for S0→S1 transition of deuterated anthranilic acid COOD, ND2 in supersonic-jet expansion

“…In spite of the uncertainty caused by the presence of two closely lying bands and significant anharmonicity, it can be concluded that the 6S mode possesses a relatively large isotopic shift, consistent with the shift of the computed vibrational frequency. This supports the assignment of the band at 594:6 cm À1 (AA) to the 6S mode [13,14]. Moreover, the assignment of this band to the combination of overtones of out-of-plane modes 2 Â 2A + 2 Â 3A [4] is not supported by relative band intensities.…”

“…The spectrum of the deuterated anthranilic acid is shown on The modes are ordered according to increasing frequencies, so that consistency with the previously used notation [13,14] is maintained.…”

“…Nevertheless, those effects are relatively weak in the considered case. For the investigated modes the frequency changes upon excitation are smaller than 5% and Dushinsky effect is negligible [13,14].…”

“…S 1 transition of anthranilic acid molecules deuterated in the substituent groups (COOD, ND 2 ) was investigated. The examination of the alternative assignments proposed in earlier studies [1,4,13,14] based on the of deuteration shifts as well as the PED's and overlaps of the modes active in the LIF spectra and and the experimental anharmonicity pattern are presented.…”

“…Based on this spectrum, and with the aid of the ab initio (HF, CIS) as well as TD-DFT calculations, an estimation of geometry change upon the electronic excitation using multidimensional Franck-Condon factors were done by Leśniewski et al [14]. The 00 transition for the monomeric species appears at 28591 cm À1 (vacuum-corrected value).…”