title: Excited-state hydrogen-atom transfer along solvent wires: water molecules stop the transfer.
abstract: Excited-state hydrogen-atom transfer (ESHAT) along a hydrogen-bonded solvent wire occurs for the supersonically cooled n = 3 ammonia-wire cluster attached to the scaffold molecule 7-hydroxyquinoline (7HQ) [Tanner, C.; et al. Science 2003, 302, 1736]. Here, we study the analogous three-membered solvent-wire clusters 7HQ.(NH3)n.(H2O)m, n + m = 3, using resonant two-photon ionization (R2PI) and UV-UV hole-burning spectroscopies. Substitution of H2O for NH3 has a dramatic effect on the excited-state H-atom transfer: The threshold for the ESHAT reaction is approximately 200 cm(-1) for 7HQ.(NH3)3, approximately 350 cm(-1) for both isomers of the 7HQ.(NH3)2.H2O cluster, and approximately 600 cm(-1) for 7HQ.NH3.(H2O)2 but increases to approximately 2000 cm(-1) for the pure 7HQ.(H2O)3 water-wire cluster. To understand the effect of the chemical composition of the solvent wire on the H-atom transfer, the reaction profiles of the low-lying electronic excited states of the n = 3 pure and mixed solvent-wire clusters are calculated with the configuration interaction singles (CIS) method. For those solvent wires with an NH3 molecule at the first position, injection of the H atom into the wire can occur by tunneling. However, further H-atom transfer is blocked by a high barrier at the first (and second) H2O molecule along the solvent wire. H-atom transfer along the entire length of the solvent wire, leading to formation of the 7-ketoquinoline (7KQ) tautomer, cannot occur for any of the H2O-containing clusters, in agreement with experimentally observed absence of 7KQ fluorescence.
authors.affiliation: Departement für Chemie und Biochemie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
authors.forename: Christian
authors.initials: C
authors.lastname: Tanner
authors.forename: Markus
authors.initials: M
authors.lastname: Thut
authors.forename: Andreas
authors.initials: A
authors.lastname: Steinlin
authors.forename: Carine
authors.initials: C
authors.lastname: Manca
authors.forename: Samuel
authors.initials: S
authors.lastname: Leutwyler
mesh_terms: D006859:Hydrogen / Q000737:chemistry*; D006860:Hydrogen Bonding; D012997:Solvents / Q000737:chemistry; D014466:Ultraviolet Rays; D014867:Water / Q000737:chemistry
chemical_list: D012997:Solvents; D014867:Water; D006859:Hydrogen