Mass-and isomer-selected S 1 ←S 0 resonant two-photon ionization and S 1 →S 0 fluorescence spectra were measured for the 7-hydroxyquinoline•͑NH 3 ) 2 ͓7HQ•͑NH 3 ) 2 ] and d 2 -7-hydroxyquinoline •͑ND 3 ) 2 clusters cooled in supersonic expansions. UV/UV hole burning measurements prove that a single cluster isomer is formed. Ab initio self-consistent field and density functional calculations predict that the most stable cluster form has an "ammonia wire" hydrogen bonded to the -OH and N groups

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... the cis-7HQ rotamer. The experimental S 0 and S 1 frequencies are in very good agreement with the calculated normal mode frequencies for both the normal and deuterated ammonia-wire clusters. S 1 ←S 0 excitation leads to contractions of the -O-H¯N and NH 3¯N H 3 hydrogen bonds, as well as smaller displacements for the NH 3¯N ͑quinoline) stretch and the in plane rotation ͑or bend͒ of the ammonia dimer relative to 7HQ. The coupling of these modes to the S 1 ←S 0 electronic excitation indicates that hydrogen bond contractions in the excited state are important and may be prerequisite for the S 1 state proton transfer processes that occur in the larger 7HQ•͑NH 3 ) n (nу4) clusters. The calculated electron density differences upon S 1 ←S 0 excitation show large -electron flows on the 7HQ moiety. However, the -electronic rearrangements that directly drive the hydrogen bond rearrangements are one to two orders of magnitude smaller.