A DIRECT AB-INITIO DYNAMICS STUDY OF THE WATER-ASSISTED TAUTOMERIZATION OF FORMAMIDE

Direct ab initio dynamics calculations based on a canonical variationa l transition-state theory with several multidimensional semiclassical tunneling approximations were carried out to obtain rate constants for the water-assisted tautomerization of formamide. The accuracy of the density functionals, namely, B-LYP, B3-LYP, and BH&H-LYP, were examine d. We found that the BH&H-LYP method yields the most accurate transiti on-state properties when comparing it to ab initio MP2 and QCISD resul ts, whereas B-LYP and BS-LYP methods predict barrier heights too low. Reaction path information was calculated at both the MP2 and nonlocal hybrid BH&H-LYP levels using the 6-31G(d,p) basis set. At the BH&H-LYP level, we found that the zero-point energy motion lowers the barrier to tautomerization in the formamide-water complex by 3.6 kcal/mol. Whe n tunneling is considered, the activation energy at the BH&H-LYP level at 300 K is 17.1 kcal/mol. This is 3.4 kcal/mol below the zero-point- corrected barrier and 7.0 kcal/mol below the classical barrier. Excell ent agreement between BH&H-LYP and MP2 rate constants further supports the use of BH&H-LYP for rate calculations of large systems. (C) 1997 John Wiley & Sons, Inc.