Dynamics and kinetic isotope effects for the intramolecular double proton transfer in oxalamidine using direct semiempirical dynamics calculation

We have carried out direct semiempirical dynamics calculations for the doub le proton transfer in oxalamidine using variational transition state theory with multidimensional semiclassical tunneling approximations. This double proton transfer occurs stepwise, with an intermediate. The energy of the,in termediate relative to the reactant and the barrier height have been calcul ated at the G2* level of theory, which yields 20.8 and 25.1 kcal mol(-1), r espectively. A quantum mechanical potential energy surface has been constru cted using the AM1 Hamiltonian with specific reaction parameters (AM1-SRP) which are obtained by adjusting the standard AM1 parameters to reproduce th e energetics given by the G2* level of theory. The minimum energy path has been calculated on the AMI-SRP potential energy surface, and other characte ristics of the surface were calculated as needed. The hydrogenic motion is separated from the heavy atom motion along the reaction coordinate. The pro ton hops about 0.32 Angstrom by tunneling, but heavy atoms do not move much while tunneling occurs. Tunneling reduces the adiabatic energy barrier by 0.67 kcal mol(-1). Rate constants and kinetic isotope effects (KIEs) have b een determined experimentally in methylcyclohexane and acetonitrile :soluti ons for a bicyclic oxalamidine. The calculated KIEs agree very well with th e experimental values. The calculated activation energy is about 35% higher than the measured value. The equilibrium isotope effects and the quasiclas sical secondary KIEs reveal that proton transfer and the change in the forc e constants are asynchronous. Although the geometric parameters for the tra nsition state (TS) are closer to those for the intermediate than those for the reactant (TS is late geometrically), the force constants are more simil ar to those of the reactant (TS is early in terms of force constants). The change in force constants is a nonlinear function of the geometric paramete rs, and depends on the position.