A natural bond orbital analysis of aryl-substituted polyfluorinated carbanions: negative hyperconjugation

Aryl-substituted polyfluorinated carbanions, ArCHR_f ⁻ where R_f = CF₃ (1), C₂F₅ (2), i-C₃F₇ (3), and t-C₄F₉ (4), were analyzed by means of the natural bond orbital (NBO) theory at the B3LYP/6-311+G(d,p) computational level. A lone pair NBO at the formal anionic center carbon (Cα) was not found in the Lewis structure. Instead, significant donor/acceptor NBO interactions between π(Cα-C1) and σ*(Cβ-F) or σ*(Cβ-Cγ) were observed for 1, 2, 3a (strong electron-withdrawing substituent, from p-CF₃ to p-NO₂), and 4. Their second-order donor/acceptor perturbation interaction energy, E(2), values decreased with the increase of the stability of carbanions. A larger E(2) value corresponds to longer Cβ-F and Cβ-Cγ bonds and a shorter Cα-Cβ bond, indicating that the E(2) values can be associated with the negative hyperconjugation of the Cβ-F and Cβ-Cγ bonds. In accordance with this, the E(2) values for π(Cα-C1) → σ*(Cβ-F) were linearly correlated with the ΔG^o _β-F values (an empirical measure of β-fluorine negative hyperconjugation obtained from an increased acidity). In 3b (weak electron-withdrawing substituents, from H to m-NO₂) very large E(2) values for LP(Fβ) → π*(Cα-Cβ) were obtained. This was attributed to the Cβ-F bond cleavage and the Cα-Cβ double bond formation in the Lewis structure that is caused by the extremely strong negative hyperconjugation of the Cβ-F bond.