CHEMICAL CONSEQUENCES OF ORBITAL INTERACTIONS .14. ABINITIO MOLECULAR-ORBITAL STUDY OF GEOMETRIES, PROPERTIES, AND PROTONATION OF SIMPLE ORGANOFLUORIDES

SCF calculations with minimal, STO-3G, and extended, 4-31G, basis sets have been used to study the geometries and properties of the simple alkyl fluorides, HF to t-BuF, and vinyl fluoride. Particular emphasis is placed on examining the corresponding protonated fluorides in order to better understand the gas-phase behavior of these species as well as to provide insight into the solvation of carbonium ions in a superacid medium such as liquid HF. In contrast to an earlier study of alkyl chlorides, computed properties for the protonated fluorides are poorly described with the minimal basis set calculations. This necessitates limited geometry optimization at the 4-31G level to obtain reasonable agreement with experimental proton affinities and carbonium ion-FH dissociation energies (Δ Es). The small Δ Es values (<8 kcal/mol) for protonated isopropyl and tert-butyl fluorides account for the inability to observe these ions in ICR experiments and provide evidence for the weak solvation of secondary and tertiary carbonium ions in superacid solutions. It is also found that an HF binds to bisected ethyl cation more strongly than the bridged ethyl cation by 4 kcal/mol. Although this is consistent with previous work, the conclusion could be modified by a more sophisticated theoretical treatment. As an exception, vinyl fluoride is predicted to protonate on carbon rather than fluorine; the reason is illuminated by a simple frontier orbital analysis. Various correlations involving the properties of the fluorides are noted, while differences in the behavior of chlorides and fluorides can usually be ascribed to the greater basicity and weaker electronegativity of chlorine. © 1978, American Chemical Society. All rights reserved.