BOND ORBITALS AND INTRA-MOLECULAR INTERACTIONS .1. BARRIERS TO ROTATION AND NONBONDED INTERACTIONS

Hypotheses of Brunck and Weinhold (BW) concerning the origin of barriers to internal rotation are examined in the light of BW's bond-orbital approach. Predictions by BW, based on an INDO study with an implicitly orthogonal AO set, are tested using nonorthogonal AOs in the framework of extended Hiickel theory. In accordance with these predictions, we find that the stability of staggered relative to eclipsed conformations of ethane and other simple compounds is lost when σ/σ* mixing is prevented by deletion of antibond basis orbitals, provided that basis orbitals are LÖwdin orthogonalized. As forecast by BW, however, the barrier is little affected when σ* orbitals are deleted if nonorthogonalized basis σ and σ* orbitals are adopted. Our results are, then, substantially in accord with the BW interpretation of the σ/σ* interaction origin of the barrier except as follows. Their original suggestion that the effect reduces to an intrinsic difference, mediated by σ/σ* interactions, between cis and trans stabilities is not, by itself, a complete rationale for threefold barriers. The BW scheme of employing orthogonalized bond orbitals as basis orbitals is shown to have several interpretational advantages. It is found, among other things, that intramolecular nonbonded atom-atom interactions reduce to the form popularly associated with nonbonded repulsions only when through-bond effects are shut off by deleting σ* basis orbitals. © 1979, American Chemical Society. All rights reserved.