Ode to overlap, patroness of perpetual perplexity


Frank Weinhold, weinhold@chem.wisc.edu, Theoretical Chemistry Institute and Department of Chemistry, University of Wisconsin, Madison, WI 53706
"Overlap" plays a dominant role in the conceptual domain of physical organic chemistry. Overlap-based concepts are sometimes useful, as, e.g., in Mulliken-type approximation of Hamiltonian matrix elements or Weisskopf-type description of steric repulsions. However, overlap is intrinsically an enemy of clear discourse when employed as a verbal substitute for the underlying quantum mechanics of a perturbed physical system. The verbal and conceptual perplexity arising from overlap-based PMO or Morokuma-type analysis of rotation barrier or steric phenomena promotes two unhealthy consequences: (1) silly arguments, because different workers inevitably attach different verbal labels to what are inherently overlap-contaminated mixtures; (2) misguided chemistry, because the chosen verbal labels inspire unproductive theoretical or experimental approaches to "simulating" or controlling the phenomenon of interest. In contrast, we show how overlap-free analysis in terms of natural atomic and bond orbitals (NAO/NBO) leads to a simpler and chemically fruitful hyperconjugative view of ethane-type rotation barriers and related donor-acceptor phenomena.