Variational transition state theory calculations of tunneling effects on concerted hydrogen motion in water clusters and formaldehyde/water clusters.

摘要

The direct participation of water molecules in aqueous phase reaction processes has been postulated to occur via both single-step mechanisms as well as concerted hydrogen atom or proton shifts. In the present work, simple prototypes of concerted hydrogen atom transfer processes are examined for small hydrogen-bonded water clusters -- cyclic trimers and tetramers -- and hydrogen-bonded clusters of formaldehyde with one and two water molecules. Rate constants for the rearrangement processes are computed using variational transition state theory, accounting for quantum mechanical tunneling effects by semiclassical ground-state adiabatic transmission coefficients. The variational transition state theory calculations directly utilize selected information about the potential energy surface along the minimum energy path as parameters of the reaction path Hamiltonian. The potential energy information is obtained from ab ignite electronic structure calculations with an empirical bond additivity correction (the BAC-MP4 method). Tunneling is found to be very important for these concerted rearrangement processes -- the semiclassical ground-state adiabatic transmission coefficients are estimated to be as high as four order of magnitude at room temperature. Effects of the size of the cluster (number of water molecules in the cyclic complex) are also dramatic -- addition of a water molecule is seen to change the calculated rates by orders of magnitude. 36 refs., 10 figs.