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 (ππ*) 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 Å (0.322 Å) 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 1 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.