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 hydrogenbonded "wire"$-O-H \cdot \cdot \cdot NH_3 \cdot \cdot \cdot NH_3 \cdot \cdot \cdot NH_3 \cdot \cdot \cdot N$. Excitation of supersonically cooled$7-hydroxyquinoline\cdot (NH_3)_3$to its vibrationless S1state 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 S1state$7-ketoquinoline\cdot (NH_3)_3$tautomer. Ab initio calculations show that proton and electron movement along the wire are closely coupled. The rate-controlling S1state barriers arise from crossings of a ππ*with a Rydberg-type$\pi\sigma^\ast$state.