Systematic study of the potential energy surface for the base-induced elimination reaction of fluoride ion with ethyl fluoride using density functional theory

The potential energy surface for the reaction between fluoride ion and ethyl fluoride was explored using density functional theory and a high-level ab initio procedure (G2+). A wide variety of functional and basis sets were systematically examined. Four reactions with known thermochemistry were used as benchmarks, and while the experimental results could be reproduced, no single functional or class of functionals consistently outperformed the others. Transition structures and an ion-molecule complex, F^-··CH₃CH₂F, are discussed. The DFT barriers are consistently smaller and looser than those obtained with high-level ab initio calculations. Moreover, the anti elimination transition state is unusually sensitive to the choice of functional and basis set; in many cases the transition structure does not appear to exist on the potential energy surface. The origin of this problem may reside in the large charge inhomogeneities characteristic of transition states and/or an inadequate description of the exchange interaction. This shortcoming, clearly, demands a great deal more attention.