Dynamics and Kinetic Isotope Effects for the Intramolecular Double Proton Transfer in Oxalamidine Using Direct Semiempirical Dynamics Calculation

Abstract

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 AM1-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 Å 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.

Towards the Experimental Decomposition Rate of Carbonic Acid (H2CO3) in Aqueous Solution

“…Systems showing an asynchronous proton transition behavior are, for example, the hydration equilibrium of SO 3 ,56–59 which plays a very important role in atmospherical chemistry, as well as the competing hydration equilibrium of SO 2 ,56, 60 and the hydrolysis of N 2 O 5 61. Other examples of asynchronous transition behavior are the water‐assisted tautomerizations of formamide62 and 7‐azaindole,63, 64 which play a fundamental role in biological systems. The formamidine dimer has a concerted, synchronous proton transfer in the gas phase but a stepwise mechanism is followed in aqueous solution 65…”

Theoretical Studies for the Rates and Kinetic Isotope Effects of the Excited-State Double Proton Transfer in the 1:1 7-Azaindole:H2O Complex Using Variational Transition State Theory Including Multidimensional Tunneling

“…Variational transition state theory including multidimensional tunneling (VTST/MT) has been successfully used to study multiple proton transfer reactions. ,− To apply transition state theory in excited-state reactions, the excited species should be thermally equilibrated with the environment during the reactive processes. In solution, this equilibrium is reached by solvent relaxation.…”

Kinetic Isotope Effects as a Probe for the Protonolysis Mechanism of Alkylmetal Complexes: VTST/MT Calculations Based on DFT Potential Energy Surfaces

“…Because of the large activation energy, the predicted rate constants of this route are very small compared to those of the concerted pathway. The tunneling coefficients and KIEs were not very large compared to those from the concerted protonolysis, however they were in the normal range observed in many hydrogen transfer reactions of organic molecules, , where both tunneling and quasi-classical effects play some role. It is interesting that the S E (ox) mechanism does not necessarily produce an inverse or very small KIE value.…”