Double proton transfer in the complex of acetic acid with methanol: Theoryversus experiment

To test the approximate instanton approach to intermolecular proton-transfe r dynamics, we report multidimensional ab initio bimolecular rate constants of HH, HD, and DD exchange in the complex of acetic acid with methanol in tetrahydrofuran-d(8), and compare them with the NMR (nuclear magnetic reson ance) experiments of Gerritzen and Limbach. The bimolecular rate constants are evaluated as products of the exchange rates and the equilibrium rate co nstants of complex formation in solution. The two molecules form hydrogen-b ond bridges and the exchange occurs via concerted transfer of two protons. The dynamics of this transfer is evaluated in the complete space of 36 vibr ational degrees of freedom. The geometries of the two isolated molecules, t he complex, and the transition states corresponding to double proton transf er are fully optimized at QCISD (quadratic configuration interaction includ ing single and double substitutions) level of theory, and the normal-mode f requencies are calculated at MP2 (Moller-Plesset perturbation theory of sec ond order) level with the 6-31G (d,p) basis set. The presence of the solven t is taken into account via single-point calculations over the gas phase ge ometries with the PCM (polarized continuum model). The proton exchange rate constants, calculated with the instanton method, show the effect of the st ructure and strength of the hydrogen bonds, reflected in the coupling betwe en the tunneling motion and the other vibrations of the complex. Comparison with experiment, which shows substantial kinetic isotopic effects (KIE), i ndicates that tunneling prevails over classic exchange for the whole temper ature range of observation. The unusual behavior of the experimental KIE up on single and double deuterium substitution is well reproduced and is relat ed to the synchronicity of two-atom tunneling. (C) 2001 American Institute of Physics.