PREDICTIONS OF RATE CONSTANTS AND ESTIMATES FOR TUNNELING SPLITTINGS OF CONCERTED PROTON-TRANSFER IN SMALL CYCLIC WATER CLUSTERS

We present transfer rates for the concerted hydrogen exchange in cycli c water clusters (H2O)(n) (n=3,4) based on ab initio hypersurfaces. Th e studied hydrogen exchange involves bond breaking and forming and is in contrast to flipping motions of ''free'' hydrogen atoms in a ''chem ical'' reaction. The rates are calculated for gas-phase systems using canonical, variational transition state theory. Multidimensional tunne ling corrections are included assuming both a small and a large reacti on path curvature. Hybrid density functional theory [B3LYP/6-31+G(d)] was used to evaluate the potential energy hypersurface with interpolat ed corrections of second order perturbation theory [MP2/6-311+ +G(3pd, 3df)] at the three stationary points for both systems. Large curvature tunneling corrections are included in dual-level direct ab initio dyn amics for the cyclic tri- and tetramer of water. The ridge of the reac tion swath serves as an estimate for the tunneling probability of vari ous straight-line corner cutting paths. Our results suggest that the i nvestigated species interconvert on a time scale of seconds. The groun d-state tunneling splitting is proportional to the square root of the transition probability at the energy of the minima, which is available from the calculation of tunneling corrections. The associated tunneli ng splittings are estimated to be between 10(-4) and 10(-5) cm(-1), wh ich is close to the experimental resolution limit. (C) 1998 American I nstitute of Physics.