H atom transfer along an ammonia chain: Tunneling and mode selectivity in 7-hydroxyquinoline·(NH3)3.

Excitation of the 7-hydroxyquinoline·(NH₃)₃ [7HQ·(NH₃)₃] cluster to the S₁ ¹ππ^* state results in an O-H→NH₃ hydrogen atom transfer (HAT) reaction. In order to investigate the entrance channel, the vibronic S₁↔S₀ spectra of the 7HQ·(NH₃)₃ and the d₂-7DQ·(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 S₁←S₀ 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 d₂-7DQ·(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 ¹ππ^* state with an optically dark ¹πσ^* state. Excitation of the ammonia-wire stretching modes decreases both the quinoline-O-H...NH₃ distance and the energetic separation between the ¹ππ^* and ¹πσ^* states, thereby increasing the H atom tunneling rate. The intramolecular vibrations change the H bond distance and modulate the ¹ππ^*↔¹πσ^* interaction to a much smaller extent. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]