TUNNELING PATHS IN INTRAMOLECULAR PROTON-TRANSFER

We recently introduced a procedure for determining effective tunneling paths in nonthermal symmetric proton-transfer reactions. The procedure involves a "maximum probability path" (MPP) based on the ground-state vibrational wave function, which can be determined within the reaction surface Hamiltonian framework by ab initio electronic structure methods. In this paper we further define the MPP concept and explore this path using the malonaldehyde molecule and the formic acid dimer as examples, as well as calculations on simpler model systems. The results verify our qualitative expectations for tunneling in heavy-light-heavy systems. Furthermore, a comparison of the MPP to other more conventional paths, such as the IRC, helps elucidate certain physical aspects of multidimensional tunneling. Finally, simple extensions of the MPP procedure suggest possible directions for new methods for calculating tunneling splitting in complex molecules.