An assessment of density functional theory on evaluating activation barriers for small organic gas-phase rearrangement reactions

Abstract

Several gas-phase isomerization reactions have been studied with density functional theory (DFT) using both the local density approximation (LDA) and non-local (NL) gradient-corrected methods. Besides the validity of each of these pure density functional procedures, the relative accuracy of the hybrid B3LYP model (Becke's 3 parameter-functional with the NL correlation of Lee, Yang and Parr) has also been tested. The predicted geometries and relative energetics for the reactions have been examined and compared with those of Hartree-Fock (HF), MP2 and QCISD (quadratic configuration interaction with single and double excitations) calculations. Our results indicate that the NL level of DFT is reliable at estimating reaction barriers and transition state structures, especially when using the B3LYP functional, which outperforms or at least equals MP2 calculations. This theoretical investigation demonstrates that DFT methods can be used to obtain thermochemical information as accurate as that provided by means of some ab initio post-HF methods.