PRIMARY AND SOLVENT KINETIC ISOTOPE EFFECTS IN THE WATER-ASSISTED TAUTOMERIZATION OF FORMAMIDINE - AN AB-INITIO DIRECT DYNAMICS STUDY

We present an ab initio direct dynamics study of the primary and solve nt kinetic isotopes effects (KIEs) for the water-assisted tautomerizat ion in the formamidine-water complex. These calculations are based on a variational transition state theory plus multidimensional semiclassi cal tunneling corrections with potential energy information calculated at the MP2 level of theory using the 6-31G(d,p) basis set. We found t har both the primary and solvent KIEs are large at low temperatures an d are due not only to tunneling but also to quantum effects in vibrati onal motions. The primary KIEs were larger than the solvent KIEs. This results from differences due to the effect of deuteration on vibratio nal modes. Bending modes show significant inverse KIEs, while those of the reactive OH and NH vibrations show both inverse and normal effect s. Such effects can be explained by examining changes in the zero-poin t energies of these modes. The adherence of the HH/HD/DD rates to the rule of the geometric mean is also examined.