Direct Evaluation of the Hyperconjugative Interactions in 1,1,1-Trihaloethane (CH3CX3, X = F, Cl, and Br)
Following the computational strategy proposed by Mulliken in 1939 (J. Chem. Phys. 1939, 7 (5), 339-352), when the concept of hyperconjugation was coined, we evaluated the hyperconjugative stabilization energy in 1,1,1-trihaloethane using the block-localized wave function (BLW) method. The BLW method is the simplest and most efficient variant of ab initio valence bond (VB) theory and can derive the strictly electron-localized state wave function self-consistently. The latter serves as a reference for the quantification of the electron delocalization effect in terms of the resonance theory. Computations show that the overall hyperconjugative interactions in 1,1,1-trihaloethane, dominated by sigma(CH) -> sigma(CX) with minor contribution from sigma(CX) -> sigma(CX) ranges from 9.59 to 7.25 kcal/mol in the staggered structures and decreases in the order Br > Cl > F. This is in accord with the H-1 NMR spectra of CH3CX3. Notably, the hyperconjugation effect accounts for 35 40% of the rotation barriers in these molecules, which are dominated by the conventional steric repulsion. This is consistent with the recent findings with 1,2-difluoroethane (Freitas; Buhl; O'Hagan. Chem. Comm. 2012, 48, 2433-2435) that the variation of (1)J(CF) with the FCCF torsional angle cannot be well explained by the hyperconjugation model.