Excitation of the 7-hydroxyquinoline.(NH3)(3) [7HQ.(NH3)(3)] cluster to the S-1 (1)pipi* state results in an O-H-->NH3 hydrogen atom transfer (HAT) reaction. In order to investigate the entrance channel, the vibronic S-1<---->S-0 spectra of the 7HQ.(NH3)(3) and the d(2)-7DQ.(ND3)(3) clusters have been studied by resonant two-photon ionization, UV-UV depletion and fluorescence techniques, and by ab initio calculations for the ground and excited states. For both isotopomers, the low-frequency part of the S-1<--S-0 spectra is dominated by ammonia-wire deformation and stretching vibrations. Excitation of overtones or combinations of these modes above a threshold of 200-250 cm-1 for 7HQ.(NH3)(3) accelerates the HAT reaction by an order of magnitude or more. The d(2)-7DQ.(ND3)(3) cluster exhibits a more gradual threshold from 300 to 650 cm-1. For both isotopomers, intermolecular vibrational states above the threshold exhibit faster HAT rates than the intramolecular vibrations. The reactivity, isotope effects, and mode selectivity are interpreted in terms of H atom tunneling through a barrier along the O-H-->NH3 coordinate. The barrier results from a conical intersection of the optically excited (1)pipi* state with an optically dark (1)pisigma* state. Excitation of the ammonia-wire stretching modes decreases both the quinoline-O-H...NH3 distance and the energetic separation between the (1)pipi* and (1)pisigma* states, thereby increasing the H atom tunneling rate. The intramolecular vibrations change the H bond distance and modulate the (1)pipi*<---->(1)pisigma* interaction to a much smaller extent. (C) 2004 American Institute of Physics.