The application of density functional theory to the optimization of transition state structures. I. Organic migration reactions

Abstract

We present here the first determination of transition state structures based on Hartree–Fock–Slater (HFS) density functional calculations. The optimization of the transition state structures utilized the analytical energy gradients evaluated by a recently developed method. The transition state structures obtained by the HFS method are similar to those determined by ab initio methods in which electron correlation is partly taken into account. The energy barrier for the CH₃NC→CH₃CN isomerization process, corrected for the zero‐point energy, is 40.1 kcal mol⁻¹ which is in good agreement with the experimental value of 38.4 kcal mol⁻¹. Experimental barriers are not available for the HNC→HCN and N₂H₂(trans)→N₂H₂(cis) isomerization processes. However, the barriers calculated by the HFS method are in good accordance with the barriers obtained from electron‐correlation calculations. Furthermore, the vibrational frequencies calculated by the HFS method are in good agreement either with experiments in the case of stable molecules or with electron‐correlation calculations in the case of transition states. The HFS method seems in general to provide better estimates of vibrational frequencies and activation barriers than the Hartree–Fock method in which electron correlation is lacking. The later method generally afford too high values for both frequencies and activation barriers.