Reaction rates for proton transfer over small barriers and connection to transition state theory

Reaction rates of chemical reactions can be generally well described by cla ssical transition state theory (TST) when considering zero-point energy and tunneling effects. TST is, however, not applicable for small energy barrie rs comparable in size to the zero-point energy or for cases where even no e nergy barrier is present. These situations are common for proton transfer i n bulk water. Here, energy profiles for proton transfer between water and s mall organic molecules were computed quantum mechanically and were used as input for solving the time-dependent Schrodinger equation in one dimension. Proton transfer over small barriers occurs very fast and is completed afte r 10-40 fs. Transition probabilities can reach values as high as 100%. They can easily be fitted by an analytical expression. An interpolation for pro ton transfer rates is then derived for connecting the low-barrier-regime th at should be treated by solving the time-dependent Schrodinger equation wit h the high-barrier-regime where TST applies. (C) 2001 American Institute of Physics.