Probing the Threshold to H Atom Transfer Along a Hydrogen-Bonded Ammonia Wire

Abstract

We characterized the entrance channel, reaction threshold, and mechanism of an excited-state H atom transfer reaction along a unidirectionally hydrogen-bonded “wire” –O–H···NH₃···NH₃···NH₃···N. Excitation of supersonically cooled 7-hydroxyquinoline·(NH₃)₃ to its vibrationless S₁ state produces no reaction, whereas excitation of ammonia-wire vibrations induces H atom transfer with a reaction threshold ≈ 200 wave numbers. Further translocation steps along the wire produce the S₁ state 7-ketoquinoline·(NH₃)₃ tautomer. Ab initio calculations show that proton and electron movement along the wire are closely coupled. The rate-controlling S₁ state barriers arise from crossings of a ππ* with a Rydberg-type πσ* state.