Sigma, pi

The unusual form of the C=C NBOs in Fig. 3.50 is clearly related to hypercon-jugative interactions with the nN lone pair. In (he 4> = 80° geometry, the nN has comparable delocalizations into both banana antibonds (stabilization energies of 8.41 and 7.06 kcal mol-1). The C=C NBOs revert rather abruptly to their usual sigma/pi form as nN is twisted to somewhat smaller

[Pg.220]

Valence indices, (denoted with a v superscript) which encode sigma, pi, and lone electrons and thus include more information about the specific elements included in the PAH compound. 

Much of the interest in the polysilanes, polygermanes, and polystannanes involves their sigma delocalization and their sigma-pi delocalization when coupled with arenes or acetylenes. This is not unexpected since silicon exists as a covalent network similar to diamond. In exhibiting electrical conductivity, germanium and tin show more typical metallic bonding. Some polystannanes have been referred to as molecular metals. ... 

The basis of the VSEPR theory is that the shape of a molecule (or the geometry around any particular atom connected to at least two other atoms) is assumed to be dependent upon the minimization of the repulsive forces operating between the pairs of sigma (a) valence electrons. This is an important restriction. Any pi (7t) or delta (8) pairs are discounted in arriving at a decision about the molecular shape. The terms sigma , pi and delta refer to the type of overlap undertaken by the contributory atomic orbitals in producing the molecular orbitals, and are referred to by their Greek-letter symbols in the remainder of the book. 

The view that theoretical considerations and experimental evidence are in much better agreement with an (equivalent) bent-bond description of a multiple bond [0, 24, 25] than with the widely accepted sigma-pi-description is also supported by... 

Substituent Parameters. A significant advance was made, in the structure-activity studies with the Hill reaction, when Hansch and Deutsch (12) evaluated some of the published Hill inhibition data with a multiple regression analysis, an extra-thermodynamic approach, or the so-called sigma, pi (a, T ) regression analysis. The principle of the approach rests on the assumption that changes in biological activity can be correlated with measurable molecular or substituent parameters. This analysis involved equations of the following type ... 

Iida T, Ohshita J, Ohta N, Komaguchi K, Itagaki Y, Shiotani M, Kunai A (2003) Spin-spin interaction between phenoxyl radicals through sigma-pi system. J Organomet Chem 688 192-199... 

All of these excited states are subject to differential relaxation and correlation effects. Some ligand excited states, such as the (mr ) Soret bands of porphine, are subject to actual and artifactual Rydberg-valence mixing. Even in the absence of the metal, these can only be unraveled by calculations which include extensive sigma-pi correlation. 

Sigma-pi rearrangements of 11 organo-transition metal com- (108) pounds... 

Stufkens, D. J. Aarnts, M. P. Rossenaar, B. D. Vlcek, A. A new series of Re and Ru complexes having a lowest sigma pi excited state that varies from reactive to stable and long lived. Pure Appl. Chem. 1997, 69, 831-835. 

---