Probing the threshold to H atom transfer along a hydrogen-bonded ammonia wire

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 S-1 state produces no reaction, whereas excitation of ammonia-wire vibrations induces H atom transfer with a reaction threshold approximate to 200 wave numbers. Further translocation steps along the wire produce the S-1 state 7-ketoquinoline.(NH3)(3) tautomer. Ab initio calculations show that proton and electron movement along the wire are closely coupled. The rate-controlling S-1 state barriers arise from crossings of a pipi* with a Rydberg-type pisigma* state.