Intramolecular hydrogen bonding in the S1(ππ*) excited state of anthranilic acid and salicylic acid:: TDDFT calculation of excited-state geometries and infrared spectra

The equilibrium geometries and vibrational spectra of anthranilic acid (AA) and salicylic acid (SA) in the S-1(pipi*) excited state have been determined with time-dependent density functional theory (TDDFT). A single minimum of the excited-state potential-energy surface, rather than separate amino/imino (enol/keto) minima, is found for AA and SA. A significant strengthening of the intramolecular hydrogen bond upon electronic excitation is predicted for both systems. The excited-state minimum corresponds to a shortening of the intramolecular hydrogen bond by 0.226 Angstrom (0.322 Angstrom) in AA (SA). The calculated 0-0 excitation energies are within 0.1 eV of the experimental values. An exceptionally large red shift of 1400 cm(-1) and a strong mixing with H-chelate ring bending and stretching motions is predicted for the phenolic OH stretching vibration in the S, state of salicylic acid. In anthranilic acid, a red shift of 500 cm-1 is obtained for the H-bonded amino NH stretching vibration, in excellent agreement with a recent UV-IR double-resonance experiment [Southern, D. A.; Levy, D. H.; Florio, G. M.; Longarte, A.; Zwier, T. S. J. Phys. Chem. A 2003, 107, 4032]. The results suggest that the TDDFT method with the B3LYP functional is a viable method for the calculation of excited-state potential-energy surfaces of intramolecularly hydrogen-bonded aromatic systems.