We have carried out direct semiempirical dynamics calculations for the double 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,intermediate relative to the reactant and the barrier height have been calculated at the G2* level of theory, which yields 20.8 and 25.1 kcal mol(-1), respectively. A quantum mechanical potential energy surface has been constructed using the AM1 Hamiltonian with specific reaction parameters (AM1-SRP) which are obtained by adjusting the standard AM1 parameters to reproduce the energetics given by the G2* level of theory. The minimum energy path has been calculated on the AMI-SRP potential energy surface, and other characteristics of the surface were calculated as needed. The hydrogenic motion is separated from the heavy atom motion along the reaction coordinate. The proton 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 been determined experimentally in methylcyclohexane and acetonitrile :solutions for a bicyclic oxalamidine. The calculated KIEs agree very well with the experimental values. The calculated activation energy is about 35% higher than the measured value. The equilibrium isotope effects and the quasiclassical secondary KIEs reveal that proton transfer and the change in the force constants are asynchronous. Although the geometric parameters for the transition state (TS) are closer to those for the intermediate than those for the reactant (TS is late geometrically), the force constants are more similar 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 parameters, and depends on the position.