Quantitatively Adequate Calculations of the H-Chelate Ring Distortion upon the S0 → S1(ππ*) Excitation in Internally H-Bonded o-Anthranilic Acid: CC2 Coupled-Cluster versus TDDFT

The S-0 -> S-1(pi -> pi*) excitation in o-aminobenzoic acid causes strengthening of the N-H center dot center dot center dot-O intramolecular hydrogen bond. The interplay of the hydrogen bond shortening, the hydrogen atom dislocation along the hydrogen bond, and the skeletal relaxation is investigated. These effects often cause the appearance of dual fluorescence from the pi-conjugated internally H-bonded molecules, which is traditionally interpreted as the evidence of the excited-state intramolecular proton transfer process: ESPIT. Hence, their quantitative modeling is an important but demanding task for computational photochemistry. Extensive calculations using CC2 method (the perturbative approximation to CCSD coupled-cluster) and TDDFT(B3LYP) were performed with the series of (aug)-cc-pVXZ(X = D,T,Q) basis sets. CC2 predicts remarkable shortening of the O center dot center dot center dot H distance by 0.273 angstrom accompanied by the skeleton relaxation that involves considerable distortions of valence angles of the amino group (up to 7.3 degrees) and within the benzene ring (up to 5 degrees). Additionally, moderate changes (<0.046 angstrom) of the bond alternation in the pi-electronic system and the hydrogen atom dislocation along the hydrogen bond (0.043 angstrom) are predicted. The CC2 method yields 90% of the magnitude of the experimentally based geometry changes, estimated in the earlier studies via Franck Condon fit to the LIF spectra, while the TDDFT results approach only 65% of the experimental values.