Large Curvature Tunneling Effects Reveal Concerted Hydrogen Exchange Rates in Cyclic Hydrogen Fluoride Clusters Comparable to Carboxylic Acid Dimers

Abstract

Reaction rate constants for the concerted proton transfer in cyclic hydrogen fluoride clusters obtained by using a variational transition state theory approach with high-level data at three stationary points and with interpolated corrections based on semiempirical Hamiltonians specially parametrized for these systemsMP2/6-311++G(3df,3pd)///PM3-SRPare reported. For the first time multidimensional tunneling is included in the large curvature limit. The corresponding small curvature limit is benchmarked against higher level calculations. The ridge of the reaction swath is defined in a general way and serves together with an estimate of the barrier width as a validation of the potential energy hypersurfaces. Recent experimental and theoretical results concerning carboxylic acids, whose hypersurfaces are found to be very similar to those of the HF clusters, underline the relevancy of the investigated concerted proton exchange motion. A reaction rate constant k(300 K) = 2.86 × 109 s-1 is found for (HF)5, which is similar to carboxylic acid dimers, whose experimental k(300 K) lie near 1010 s-1.