Electronic and infrared spectroscopy of anthranilic acid in a supersonic jet

The electronic and infrared spectra of anthranilic acid in a supersonic jet were measured. The fluorescence excitation spectrum is extremely congested. IR-UV hole-burning measurements indicate that only a single ground-state rotamer contributes to the observed spectrum. Vibrational progressions in 252 and 418 cm(-1) modes were observed. Density functional theory calculations indicate that these modes involve the in-plane bending motions of the amino and carboxyl groups of anthranilic acid. The presence of vibrational progressions in these modes suggests that the relative positions of the amino and carboxyl groups are different in the ground and excited electronic states of anthranilic acid. This observation is supported by the fluorescence-dip infrared spectra acquired, which show a shift in the lower frequency NH stretch fundamental from 3394 to 2900 cm(-1) upon electronic excitation, suggesting a dramatic strengthening of the intramolecular hydrogen bond in the excited state. The change in the hydrogen-bond strength does not lead to full excited-state intramolecular hydrogen-atom transfer, as the strongly red-shifted emission feature associated with this process is not observed. Instead, the excited-state behavior of anthranilic acid is best described as intramolecular hydrogen-atom dislocation, as has been postulated for the related molecule salicylic acid.