Diffuse functions in natural bond orbital analysis.

We show that diffuse function augmentation of Pople basis sets at the 6-311G RHF and B3LYP levels strongly impact conclusions drawn from natural bond orbital (NBO) analysis. The large spatial extent of high quantum number Rydberg orbitals introduced by augmentation contribute importantly to the valence space of neighboring atoms due to the likely inadequacy of the 311 valence functions. In contrast, lesser anomalies are found for augmentation of double zeta type 6-31G.

Gauche effect in 1,2-difluoroethane. Hyperconjugation, bent bonds, steric repulsion.

Natural bond orbital deletion calculations show that whereas the gauche preference arises from vicinal hyperconjugative interaction between anti C-H bonds and C-F* antibonds, the cis C-H/C-F* interactions are substantial (approximately 25% of the anti interaction). The established significantly >60 degrees FCCF dihedral angle for the equilibrium conformer can then be rationalized in terms of the hyperconjugation model alone by taking into account both anti interactions that maximize near 60 degrees and the smaller cis interactions that maximize at a much larger dihedral angle. This explanation does not invoke repulsive forces to rationalize the 72 degrees equilibrium conformer angle.

1-Fluoropropane. Torsional Potential Surface.

The systematic deletion of orbital interactions, using natural bond orbital (NBO) theory at the B3LYP/ 6-311++G(3df,2p) level, provides validation for the anti-C-H/C-F* hyperconjugative interaction providing the backbone for the gauche preference of 1-fluoropropane (FP). The FCCC torsional coordinate taking trans FP to gauche FP is predicted to be strongly contaminated by CCC bending with the result that a large part of the trans → gauche stabilization energy stems from mode coupling. The anti-C-H/C-F* hyperconjugative interaction is also found to play a major, if not determining, role in the coupling.