SYSTEMATIC STUDY OF THE POTENTIAL-ENERGY SURFACE FOR THE BASE-INDUCEDELIMINATION-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 hig h-level ab initio procedure (G2+). A wide variety of functionals and b asis sets were systematically examined. Four reactions with known ther mochemistry were used as benchmarks, and while the experimental result s could be reproduced, no single functional or class of functionals co nsistently outperformed the others. Transition structures and an ion-m olecule complex, F-.. CH3CH2F, are discussed. The DFT barriers are con sistently smaller and looser than those obtained with high-level nb in itio 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 potent ial 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.