Direct ab initio dynamics studies of proton transfer in hydrogen-bond systems

We present systematic direct ab initio dynamics studies of proton transfer in hydrogen-bond systems using the tautomerization in gas phase formamidine and its monohydrated complex as model reactions. The thermal rate constants were calculated using a canonical variational transition state theory (CVT) with multidimensional semiclassical tunneling corrections within a small-curvature ground-state approximation. The reaction valleys were calculated at the second-order MdllerPlesset (MP2) perturbation theory, Hartree-Fock (HF) and nonlocal Becke’s half-and-half exchange and Lee-Yang-Parr correlation (BH&H-LYP) density functional theory (DFT) levels of theory using the 6-3 1 G(d,p) basis set. For accurate rate constants, the potential energy along the minimum energy path was scaled to match the single-point coupled cluster calculations including single and double excitations plus correction for triple excitation [CCSD(T)] at the MP2/6-3 1 G(d,p) classical barrier for each reaction. In the HF rate calculations, the HF frequencies were also scaled by a factor of 0.9. We found that adding a water to assist the proton transfer significantly enhances the tautomerization rate. Tunneling contributions in both systems are quite substantial and cannot be corrected by the Wigner approximation. We found that vibrational excitation of the solvent symmetriclike stretching mode would significantly enhance the rate of proton transfer in the formamidine-water complex. We also found that nonlocal DFT methods, particular the BH&HLYP functionals studied here, can provide accurate potential energy information for dynamical calculations. Due to the computational advantage of DFT methods, prospects for dynamical studies of large polyatomic chemical reactions are quite encouraging.