A method for calculating tunneling splitting and isotope effects in hydrogen-bonded molecules

1. V. K. Babamov, Chem, Phys. Lett. 217, 254(1994) 2. V.K. Babamov and R. A. Marcus, J. Chem. Phys. 74, 1790 (1981).Author Institution: Chemical Abstracts ServiceA method for calculating hydrogen tunneling splitting in a molecule with an intramolecular hydrogen bond is presented and applied to selected systems. The method utilizes analytical formula for tunneling splitting for a particle in a double-well potential linearly coupled to a low-frequency $bath^{1}$ and another previously developed treatment suitable for quadratic $couplings.^{2}$ The method can be viewed as a simple extension of the usual normal-mode treatment molecular vibrations to incorporate the large-amplitude proton motion. It highlights the surprisingly large role of the heavy-atom motion in the hydrogen-atom tunneling, Excellent results are obtained in the tests against multidimensional numerical calculations on model systems. The method is used to calculate tunneling splittings and isotope effects in malonaldehyde and tropolone and to estimate the barrier sizes. The relation of the approach to the macroscopic electron-transfer and proton-transfer theories is discussed