Negative Hyperconjugation in Acidity of Polyfluorinated Alkanes. A Natural Bond Orbital Analysis

Abstract

Abstract Natural bond orbital (NBO) analysis has been applied to various polyfluorinated carbanions. The E(2)[LP(Cα)→σ*(Cβ-F) or σ*(Cβ-Cγ)] values that are interaction energies between a lone pair NBO at the anionic center carbon and the σ*(Cβ-F) or σ*(Cβ-Cγ) NBO increased with the corresponding Cβ-F or Cβ-Cγ bond distances, respectively, being consistent with the molecular orbital theory on negative hyperconjugation. The total E(2) values for the interactions of a lone pair orbital with all σ*(Cβ-F) and σ*(Cβ-Cγ) orbitals were linearly correlated with gas-phase acidities of the corresponding alkanes, giving two lines for the carbanions having no β-fluorine atom and for the primary and secondary carbanions having β-fluorine atoms (slopes of 0.57 and 1.56, respectively). The major E(2)[LP(Cα)→σ*(Cβ-F)] values in the respective anions were found to be linearly correlated with the ΔG°β-F values as an empirical measure of β-fluorine negative hyperconjugation obtained from an increased acidity of the molecule owing to the presence of β-fluorine. However, the magnitude of ΔG°β-F was much smaller than the E(2)[LP(Cα)→σ*(Cβ-F)] value, indicating that the absolute values of the β-fluorine negative hyperconjugation are smaller than the E(2) interaction energies.