Molecular orbital topological effect

Keywords Agostic interaction Weak interaction Bond activation Computational studies DFT QM/MM calculations Molecular orbitals Topological analysis Steric effects... 

The applicability of Eq. (45) to a broad range of biological (i.e., toxic, geno-toxic) structure-activity relationships has been demonstrated convincingly by Hansch and associates and many others in the years since 1964 [60-62, 80, 120-122, 160, 161, 195, 204-208, 281-285, 289, 296-298]. The success of this model led to its generalization to include additional parameters in attempts to minimize residual variance in such correlations, a wide variety of physicochemical parameters and properties, structural and topological features, molecular orbital indices, and for constant but for theoretically unaccountable features, indicator or dummy variables (1 or 0) have been employed. A widespread use of Eq. (45) has provided an important stimulus for the review and extension of established scales of substituent effects, and even for the development of new ones. It should be cautioned here, however, that the general validity or indeed the need for these latter scales has not been established. 

Abstract The agostic bond defines an intramolecular interaction where a a bond is geometrically close to an electron deficient centre (often a transition metal). The computational studies on this energetically weak interaction are reviewed and discussed. Various types of a bonds have been considered (C-H, C-C, Si-H, Si-C, B-H). It is suggested that a C-X bond in which X carries a lone pair should preferably not be viewed as agostic. The factors that contribute to his occurrence are discussed. In particular, the agostic interaction is very sensitive to steric effects. Explanations based on molecular orbital analysis, electron delocalization and topological analysis of the electron density are presented. 

The most frequently used single-determinant descriptions of the magnetic systems incorporate spin polarization effects through spin-symmetry breaking of the molecular orbitals. Since the next section is devoted to comparisons between the predictions of the topological approaches and the results of non-empirical calculations, which are performed along the Unrestricted Density Functional Theory (UDFT) approximation, it is worth recalling briefly the main features of the spin polarization phenomenon. 

KSC = Kekule structure count QSAR = quantitative structure-activity relationships QSPR = quantitative structure-property relationships SRWAC = self-returning walk atomic code TEMO = topological effect on molecular orbitals TRE = topological resonance energy TREPE = TRE per r-electron. 

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