Empirical valence bond models for reactive potential energy surfaces. II. Intramolecular proton transfer in pyridone and the Claisen reaction of allyl vinyl ether

Empirical valence bond (EVB) surfaces have been constructed for 2-pyridone-2hydroxypyridine proton transfer and for the Claisen rearrangement of allyl vinyl ether at the MP2/6-311þG(d,p) level of theory. A distributed Gaussian approach is used to approximate the interaction matrix elements. Parameters for the distributed Gaussians are determined by fitting to energy, gradient and Hessian data obtained from ab initio electronic structure calculations at one to nine points along the reaction path. An efficient DIIS (direct inversion of iterative subspace) method is used to solve the fitting equations. Criteria for choosing internal coordinates for the representation of the potential energy surfaces and for the interaction matrix element are discussed. Practical techniques for determining the placement and exponents of the Gaussians are described. With one set of s-, pand d-type Gaussians at the transition state, the error in the energy along the reaction path is less than 10 kJmol"1 for pyridone tautomerization. Five sets of Gaussians reduces the error to less than 5 kJmol"1 and seven Gaussians drops the error below 1 kJmol"1. The Claisen rearrangement is more challenging and requires seven Gaussians to achieve an error of less than 4 kJmol"1 for energies along the reaction path.