Direct Evaluation of the Hyperconjugative Interactions

Following the computational strategy proposed by Mulliken in 1939 (J. Chem. Phys. 7 (5), 339-352 (1939)) when the concept of hyperconjugation was coined, we evaluated the hyperconjugative stabilization energy in 1,1,1-trihaloethane using the block-localized wavefunction (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 wavefunction 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 σ_"CH" →σ_"CX" ^* with minor contribution from σ_"CX" →σ_"CH" ^* , 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 1H 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, Bühl and O’Hagan, Chem. Comm. 48, 2433-2435 (2012)) that the variation of 1JCF with the FCCF torsional angle cannot be well explained by the hyperconjugation model.