TUNNELING MECHANISM FOR EXCITED-STATE PROTON-TRANSFER IN PHENOL AMMONIA CLUSTERS

Reaction rate measurements and modeling calculations are presented that support a tunneling mechanism for excited-state proton transfer (ESPT) in phenol-ammonia clusters. Rates of ESPT were measured by picosecond pump-probe resonance-enhanced multiphoton ionization in a molecular beam mass spectrometer. In earlier work on C6H5OH.(NH3)n, we measured size-specific excited-state lifetimes and observed a decrease from 5 ns (n = 4) to 65 ps (n = 5). Here we report excited-state lifetimes for partially and fully deuterated clusters, C6H5OD.(ND3)n and C6D5OD.(ND3)n, that are significantly longer (> 1 ns for n > 4) than for the protiated clusters. The barrier to reaction is due to a crossing between a covalent state and a solvent-stabilized ion-pair state of phenol. A barrier model and tunneling calculations are presented in support of the experimental results.