P-type lone pair and adjacent

The Interaction between the Ring-Oxygen p-Type Lone Pair and Adjacent cr-Bonds in Pyranose Derivatives... 

Interaction between the Oxygen p-type Lone Pair and Adjacent q-bonds The interaction between the oxygen p-type lone pair and the adjacent antibonding orbital of a carbon-halogen bond was first considered in 1959 by Lucken as a possible explanation of the abnormally low NQR frequencies of Ot-haloethers (33). Later on, Altona showed that the same phenomenon could explain the peculiarities of some bond lengths in similar compounds (2). The spectroscopic properties of pyranose sugars which we have described in the first two sections of this article seem to imply that the existence of such an interaction lies beyond all possible doubt in these molecules. It does not follow that they can explain the Anomeric Effect (34) (35). For this, we need a quantitative estimation of the stabilization introduced in a molecule by such interactions. 

NBOs 3-5 are the three fluorine lone pairs ( LP ). As shown by the occupancies and hybrid composition, these lone pairs are inequivalent. LP(1) is the s-rich sigma-type sp0 26 lone pair (nF(cr) 79% s character), directed along the bond axis. LP(2) andLP(3) are the p-rich pi-type lone pairs (nF(7t) and nF(rf) 99.97% p-character), perpendicular to the bond axis. The lone pairs have occupancies slightly less than 2.000 00 (due to weak delocalization into Rydberg orbitals of the adjacent H), but overall, the correspondence with the elementary Lewis-structure description is excellent. 

For anions or molecules with many adjacent lone pairs, the basis set must be augmented with diffuse functions to allow the electron density to expand into a larger volume. For split-valence basis sets, this is designated by as in 6-31-l-G(d). The diffuse functions added are a full set of additional functions of the same type as are present in the valence space. So, for carbon, the diffuse functions would be an added s basis function and a set of p basis functions. The composition of the... 

Special substrates Allylic, H2C=CH-CH2-L, benzylic, PhCH2-L, and Z-CH2-L react even more quickly than methyl. The overlap of the p orbital (of the double bond, aromatic, or lone pair) with the adjacent p orbital of the Sn2 transition state forms a pi-type bond that lowers the energy of the transition state, speeding up the reaction. 

In ground-state acetaldehyde (2), the principal delocalizations involve vicinal hyperconjugation from the p-type oxygen lone pair Uq" into the adjacent ch and cc antibonds... 

In the classical picture each dicoordinate chalcogen atoms bears two lone pair orbitals. It has been shown that a marked s-p separation is observed and that the nature of these two lone pair orbitals is mainly s and p ( = 3-5). The s -type orbital is usually not involved in the stereochemistry and it is clear that the structures of the species described in this chapter are determined by the repulsion between the adjacent occupied p lone pair orbitals. Thus, usually the systems try to minimize coulombic repulsion by adopting E-E-E-E torsion angles of about 90 20° (Figure 1). 

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