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

Laser induced fluorescence (LIF) excitation spectrum for the S0 → S1 transition of anthranilic acid molecules deuterated in the substituent groups (COOD, ND2) was investigated. Analysis of the LIF spectrum allowed for the assignment of the six most prominent fundamental in-plane modes of frequencies up to ca. 850cm-1. The experimental results show good correlation with the frequency changes upon deuteration computed with CIS (CI-Singles) and TD-DFT for the S1 state. Deuteration induced red-shifts of the identified fundamental bands are used for examination of the alternative assignments proposed in earlier studies. Potential energy distributions (PED) and overlaps of the in-plane normal modes with frequencies below 850cm-1 indicate that the correspondence of the respective vibrations of the deuterated and non-deuterated molecule is very good. A blue-shift of the 00 transition due to the isotopic substitution, is equal to 47cm-1. This relatively large value is caused primarily by a significant decrease of the N-H stretching frequency associated with the increase of strength of the intramolecular hydrogen bond upon the electronic excitation. The deuteration shift of the 00 band was interpreted in terms of the differences of the zero point energy (ZPE) between the S0 and S1 electronic states, computed with DFT and TD-DFT methods, respectively.