AM1 Specific Reaction Parameters for Reactions of Hydroxide Ion with Halomethanes in Complex Environments:...

A specific reaction parameters (SRP) approach is used to reparameterize the semiempirical AM1 method to represent two SN2 reactions of OH(-) with halomethanes (CH3F and CH3Cl). The objective is to develop SRP models that are able to accurately reproduce ab initio potential energy surfaces but with a reduction of several orders of magnitude in computational cost. The developed models, labeled AM1-SRP, have been tested in gas and aqueous phases. The results show that the AM1-SRP models are able to predict a minimum reaction path in close agreement with high level ab initio calculations in gas phase. A QM/MM simulation method with the solutes represented by the AM1-SRP models is used to compute the free energy profiles of the reactions in aqueous phase. The AM1-SRP models predict reaction free energies and free energy barriers in good agreement with experimental values. The current study indicates that semiempirical models may be improved to accurately reproduce an ab initio minimum reaction path at a considerably lower cost. The SRP approach is expected to be useful in long time scale molecular dynamics simulations where it is very expensive to use ab initio methods, specially in condensed phases.