An MNDO SCF-MO study of proton transfer from fluoroethanols

Abstract

Proton exchange between β-fluorinated ethanols and ethoxide ions has been studied using the MNDO SCF-MO method. Calculations were performed on reactions of ethoxide ion with ethanols substituted in the β-position with 0, 1, 2, and 3 fluorine atoms as well as on reactions where both the ethanol and the ethoxide ion were substituted with the same number (1, 2, 3) of fluorine atoms in the β-position. The energies obtained for the ion–molecule reactant complexes and the transition states from these reactions have been analyzed using the Marcus equation. Through the calculated force-constant matrices of reactants and transition states we also calculated the kinetic isotope effects for the proton-transfer reactions. The semiclassical isotopic rate constant ratios (kH/kD)s were found to be of rather normal magnitude and showed a variation with the energy of reaction. The calculated ratios of tunnel correction factors, QtH/QtD, proved to be unrealistically high. These factors were also calculated with the frequencies scaled down by 10% and this was found to reduce the QtH/QtD ratios to more realistic values.