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. The results of Rydberg deletion calculations suggest that Rydberg interactions play a role in NBO analysis, contrary to the usual assumption that interactions involving Rydberg orbitals can be ignored.