PROTON-TRANSFER REACTION-RATES AS A PROBE OF SIZE-DEPENDENT PROPERTIES OF LARGE WATER CLUSTERS

Solvent-controlled excited state intermolecular proton transfer kinetics in 1-naphthol . (H2O)n (n less-than-or-equal-to 800), clusters were observed by picosecond time-resolved fluorescence in a molecular beam. Fast ( < 60 ps) and slow (0.5-1.5 ns) components of the naphtholate product fluorescence appearance were observed. The slower component decreased in rate with increasing cluster size; the time scale is much slower than for relaxation in bulk liquid water. The occurrence of the proton transfer reaction indicates liquid-like fluxionality of the water clusters in spite of their low temperature,