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

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Science  05 Dec 2003:
Vol. 302, Issue 5651, pp. 1736-1739
DOI: 10.1126/science.1091708

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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.

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