LARGE CURVATURE TUNNELING EFFECTS REVEAL CONCERTED HYDROGEN-EXCHANGE RATES IN CYCLIC HYDROGEN-FLUORIDE CLUSTERS COMPARABLE TO CARBOXYLIC-ACID DIMERS

Reaction rate constants for the concerted proton transfer in cyclic hy drogen fluoride clusters obtained by using a variational transition st ate theory approach with high-level data at three stationary paints an d with interpolated corrections based on semiempirical Hamiltonians sp ecially parametrized for these systems-MP2/ 6-311++C(3df,3pd)///PPM3-S RP-are reported. For the first time multidimensional tunneling is incl uded in the large curvature limit. The corresponding small curvature l imit is benchmarked against higher level calculations. The ridge of th e reaction swath is defined in a general way and serves together with an estimate of the barrier width as a validation of the potential ener gy hypersurfaces. Recent experimental and theoretical results concerni ng carboxylic acids, whose hypersurfaces are found to be very similar to those of the HF clusters, underline the relevancy of the investigat ed concerted proton exchange motion. A reaction rate constant k(300 K) = 2.86 x 10(9) s(-1) is found for (HF)(5), which is similar to carbox ylic acid dimers, whose experimental k(300 K) lie near 10(10) s(-1).