INTRAMOLECULAR HYDROGEN-BOND IN THE GROUND AND EXCITED ELECTRONIC STATES OF 2-HYDROXYQUINOLINE - A STUDY USING HIGH-RESOLUTION LASER SPECTROSCOPY

2-Hydroxyquinoline (2HQ) is known to exist in two tautomeric forms, the lactim ("enol") and lactam ("keto"), which exhibit different electronic spectra. Reported herein are spectra of the isolated molecule at full rotational resolution which show (1) that the higher frequency electronic origin is that of the lactim form, (2) that in the ground electronic state (S0) the more stable rotamer of the lactim has an O-H bond that is cis with respect to the naphthalene frame, (3) that this rotamer exhibits an out-of-plane intramolecular hydrogen bond involving the OH group and the ring nitrogen, and (4) that on electronic excitation of this rotamer to the S1 state there is only a small shift of the hydroxy hydrogen toward the nitrogen atom. Additionally, it is found (5) that the S1 <-- S0 transition is mainly long-axis polarized, demonstrating that the S1 state is principally pi-pi* in character. The interpretations of these results and their implications for the acid-base properties of 2HQ in both electronic states are discussed.