Consistent Franck–Condon modeling of geometry changes for the S0→S1(ππ*) excitation in anthranilic acid: LIF spectroscopy aided by CC2 or TDDFT vibrations

Przemysław Kolek , Marcin Andrzejak , Tomasz Uchacz , Paweł Szlachcic

Abstract

Experimentally-based evaluation of the changes of equilibrium geometry for the S0→S1(ππ*) excitation in anthranilic acid via fitting of Franck–Condon factors is tested for reliability. The modeling is based on the experimental band intensities in the LIF spectrum that are transformed into geometry changes with the aid of the vibrational modes computed using both the CC2 method (the perturbative approximation to CCSD coupled-cluster) and TDDFT(B3LYP). This modeling is a challenging task not only for quantitative spectroscopy, but also for the quantum chemistry calculations, since the adequate reproduction of vibrational modes for the excited state requires a balanced treatment of the π-electronic conjugation and dynamic electron correlation. The most reliable approach (LIF+CC2 Franck–Condon fitting) predicts a dramatic shortening of the O···H distance by 31 ± 1.5 pm, from 1.91 Å in the ground state to 1.60 Å in the excited state. The changes in bond alternation in the H-chelate ring (around 5 pm according to the LIF+CC2 fit) and the shortening of the O···H bond are typically 1 pm larger than modeled with the LIF+TDDFT fit. Changes of valence angles of magnitude around 5° are larger by 0.5°-1.0° according to LIF+CC2 FC fit than the LIF+DFT fit. Consistency of the obtained results shows that the Franck-Condon fitting provides a reliable, experimentally-based approach to evaluation of the excitation-related geometry changes. It has been achieved owing to the quantitatively reliable spectroscopic results and vibrational analysis computed using the quantum chemistry methods that are able to provide correct description of the excited-state vibrational modes involving the intramolecular hydrogen bond.