QUANTUM-MECHANICAL STUDIES ON THE ORIGIN OF BARRIERS TO INTERNAL-ROTATION ABOUT SINGLE BONDS

An analysis of the forces responsible for internal rotation barriers in ethane-like molecules is carried out in terms of a linear combination of bond orbitals (LCBO) representation of approximate self-consistent field molecular orbitals. The primary contributions to the barrier are found to arise from vicinal interactions between orbitals of bond and antibond type. The preference for staggered conformations in these molecules is traced to the differential stabilizing effects of cis and trans bondantibond interactions, which in turn can be rationalized from the general shapes of these localized orbitals. Various chemical trends and “effects” of conformational analysis are analyzed from this point of view, and relationships to previous work on the barrier origin are discussed. © 1979, American Chemical Society. All rights reserved.