Total pi-Electron Energy
Total pi-electron energy (Epi) is one of the most important pieces of information about a conjugated molecule which can be obtained from simple Hückel molecular orbital (HMO) calculations. Moreover, HMO Epi ’s are often of the same degree of quantitative accuracy as those calculated by much more sophisticated SCF MO models. An intriguing result along these lines was obtained by Schaad and Hess,139 who showed that in many cases Epi follows linearly the total (thermodynamically measurable) energy of a conjugated compound. The physical reasons of how it is possible that such a simple model like HMO can give not only qualitative but sometimes also fair quantitative agreement with experimental findings are at present not well understood. However, in this chapter we shall not be involved in clarification of this complicated question, but we proceed with an “empirical” assumption that HMO theory works and that it is expecially reliable in reproducing total pi-electron energies. Our considerations will be always on a qualitative level.
KeywordsBipartite Graph Molecular Graph Resonance Theory Fundamental Identity Graph Spectrum
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