NEW INSIGHTS INTO THE DYNAMICS OF CONCERTED PROTON TUNNELING IN CYCLIC WATER AND HYDROGEN-FLUORIDE CLUSTERS

The concerted proton transfer hypersurface of cyclic water and hydroge n fluoride clusters has been described by high-level molecular quantum mechanical calculations. For the cyclic water clusters the concerted proton transfer transition states have been investigated for the first time with methods including treatment of dynamic electron correlation . The crucial importance of dynamic electron correlation for the barri er heights is demonstrated, A detailed analysis of the minimum energy path has been performed. The reaction swath of the concerted proton tr ansfer was examined indicating a reasonable description by harmonic ap proximation of the energy hypersurface. Transfer rates have been calcu lated by means of variational transition state theory with interpolate d corrections (VTST-IC) and dual-level direct dynamics (DLDD), both wi th semiclassical tunneling corrections. Tunneling is very efficient in the concerted proton exchange reaction of the cyclic hydrogen-bonded clusters under investigation. Rate constants for the concerted exchang e of hydrogens in important hydrogen fluoride vapor phase species are reported for the first time. In the hydrogen fluoride tetramer and pen tamer the concerted proton exchange of four and five protons, respecti vely, takes place with reaction rates that an comparable with the conc erted exchange rates in carboxylic acid dimers and is not hindered by the large number of simultanously moving protons. The concerted proton exchange rates in the studied water clusters are comparably low becau se of higher exchange barriers. It is shown that hydrogen fluoride clu sters can be used to a large extent as ''simplified'' experimental and theoretical models for water clusters.