QUASI-ADIABATIC CHANNELS AND EFFECTIVE TRANSITION-STATE BARRIERS FOR THE DISROTATORY INPLANE HYDROGEN-BOND EXCHANGE MOTION IN (HF)2

The clamped-coordinate diffusion quantum Monte Carlo technique for the calculation of quasiadiabatic channels and transition-state energies is presented. Calculations are carried out for the hydrogen-bond exchange isomerization reaction in (HF)2 using an analytical, complete six-dimensional potential hypersurface, which agrees with available experiments. The anharmonic zero-point energy contribution amounts to about 30% (100 cm-1) of the isomerization barrier in a very tight transition state and shows an inverse deuterium isotope effect, contrary to a standard harmonic calculation. The effects of promoting and inhibiting modes are discussed and found to agree with experiment.