7-Hydroxyquinoline·(NH3)3: A model for excited state H-atom transfer along an ammonia wire

Abstract

The 7-hydroxyquinoline·(NH3)3 cluster has been used as a model to study excited state H-atom transfer along a hydrogen-bonded NH3⋯NH3⋯NH3 wire. Excitation of the supersonically cooled cluster to its vibrationless S1 state produces no reaction, whereas excitation of ammonia-wire vibrations induces fluorescence of the excited 7-ketoquinoline·(NH3)3 cluster, revealing the excited state enol → keto tautomerization. This reaction is assumed to occur via successive H transfers along the ammonia wire in a Grotthus-like process. Ab initio calculations of the reaction path show that proton and electron movement along the wire are closely coupled. The rate-controlling S1 state barriers arise from crossing of a ππ* and a Rydberg-type πσ* electronic excited state.