Validation of valence bond and molecular orbital models in analyzing the anionic hyperconjugation effect on the stability of alkyl and fluorinated carbanions in gaseous phase

In this paper, the anionic hyperconjugation occurring in alkyl and fluorinated carbanions in the gaseous phase is examined at density functional theory (DFT) at the level of theory B3LYP/6‐31++G (d,p) with empirical dispersion‐GD3 theory. First, a contribution of this study is to confirm that the C‐H bond in (CH3)3C‐H (isobutane) is more acidic than the C‐H bonds in CH3CH2‐H (ethane) and (CH3)2CH‐H (propane) based on a comparison of conjugate base stability. Also, the natural population analysis (NPA) and CHELPG methods are well‐performed for calculating charge distribution on carbon atoms, and in contrast, the Mulliken population is better for hydrogen atoms to describe the anionic hyperconjugation in the valence bond (VB) model. The molecular orbital model accounts for the stability order of alkyl and perfluorinated carbanions through the number, strength, and overlap geometry between the lone electron pair on carbon atom and the vicinal σ*C‐H/σ*C‐F orbitals. On the other hand, the utility of VB theory becomes invalid because of the strong inductive effect of the fluorine element and the steric repulsion among CF3 groups in perfluorinated carbanions. Also, the hybridization of the carbon atom holding the lone electron pair can become more s‐character or p‐character dependent on the size of surrounding groups. In this paper, anionic hyperconjugation (HCJ) occurring in alkyl and fluorinated carbanions in the gaseous phase is examined at density functional theory (DFT) at the level of theory B3LYP‐D3/6‐31++G (d,p). To describe the HCJ in the valence bond (VB), the NPA and CHELPG populations are well‐performed for calculating charge distribution on carbon atoms, whereas the Mulliken population is better for hydrogen atoms. Additionally, the strong inductive effect of the fluorine element and the steric repulsion among CF3 groups in perfluorinated carbanions make the VB model less efficient than the MO model to describe several properties of the carbanion. Finally, the hybridization of the carbon atom holding the lone electron pair can become more s‐character or p‐character dependent on the size of surrounding groups.