Excited state hydrogen atom transfer in ammonia-wire and water-wire clusters

We review experimental and theoretical investigations of excited-state hydrogen atom transfer (ESHAT) reactions along unidirectionally hydrogen bonded solvent 'wire' clusters. The solvent wire is attached to the aromatic 'scaffold' molecule 7-hydroxyquinoline (7HQ), which offers an O - H and an N hydrogen bonding site, spaced far enough apart to form two- to four-membered wires. S-1 <-- S-0 photoexcitation renders the O - H group more acidic and the quinolinic N more basic. This provides a driving force for the enol --> keto tautomerization, probed by the characteristic fluorescence of the 7-ketoquinoline in the molecular beam experiments. For 7-hydroxyquinoline center dot (NH3)(3), excitation of ammonia-wire vibrations induces the tautomerization at similar to 200 cm(-1). Different reaction pathways have been explored by excited-state ab initio calculations. These show that the reaction proceeds by H-atom transfer along the wire as a series of Grotthus-type translocation steps. There is no competition with a mechanism involving successive proton translocations. The rate-controlling S-1 state barriers arise from crossings of a pi pi* with a Rydberg- type pi sigma* state and the proton and electron movements along the wire are closely coupled. The excited state reactant, H-transferred intermediates and product structures are characterized. The reaction proceeds by tunnelling, as shown by deuteration of the solvent molecules (ND3) in the wire. The first step of the reaction exhibits intra/intermolecular vibrational mode selectivity. Substitution of NH3 by one, two or three H2O molecules in the wire leads to increasing threshold with each additional H2O molecule, up to > 2000 cm(-1) for the 7-hydroxyquinoline center dot (H2O)(3) water-wire cluster. No 7-ketoquinoline fluorescence is observed upon insertion of even a single H2O molecule. The calculations show that insertion of each H2O molecule into the solvent wire introduces a high barrier, which blocks any further H-atom transfer.