P-type lone pairs

This discrepancy may be ascribed to the differing electronic situation of both groups in that the chalcogens bear an additional occupied p-type lone pair orbital, which can be used to delocalize the positive charge over all atoms. A similar stabilization is impossible for the group 15 cations and may account for their rare occurrence as compounds in the bottle. 

Structure C is usually written with a pentavalent phosphorus this description, however, should merely denote that the phosphorus lone pair is delocalized and no allenic bond with two perpendicular jr-systems has been formed. H2C=P(H)=CH2, a typical representative of structure C phosphorus, can be described by an allyl-like r-system, ° with the p -type lone pair of a planar tricoordinate phosphorus participating in the 4-electron-3-center jT-bonding, without invoking d-orbital participation. " Thus, the C -type bonding of phosphorus involves tricoordination, with a planar bonding environment, whereby the lone pair is available for jT-bonding. 

To gain a quantitative view of the delocalization of the p-type lone pair (LP) localized at the central C atom, the partial charges, occupancies of the p-type LP (occ(LP(C)p2)) and the charge transfer upon substitution have been listed in Table 9. The overall charge transfer + o)) was divided into a n ( ct(7t)) and a contribution ( cr(o ). Table 9). The... 

By definition, a generalized anomeric effect is observed at carbon of an XCY system when a molecule preferentially adopts a conformation that optimizes a secondary, stabilizing electronic interaction involving overlap between the lone pair on one heteroatom with the a orbital of the bond between the central carbon atom and the second heteroatom . Figure 5a illustrates that in XNY systems, as with anomeric carbon centres, two anomeric interactions are possible and involve either an ny-CT x ° nx-o NY overlap where nx and ny represent the p-type lone pairs on X and Y and NX and NY represent the N—X and N—Y a orbitals. In either case, the result is a net stabilization of the lone pair of electrons (Figure 5b). Except where the nitrogen is symmetrically substituted, one of these interactions will be strongest. 

Anomeric effects in ONCl systems are Uo-Oj a even though oxygen is more electronegative than chlorine N and O orbitals are similar in size and chlorine is a 3p element, thus favouring overlap between the p-type lone pair on O with the low-energy N-Cl <7 orbital. In XNY systems, occupation by Uy leads to transfer of electron density to the X substituent and the substantially higher electron affinity of chlorine will also favour this anomeric interaction rather than an Uci-cTno overlap. 

Geometries of all four amides in the solid state indicate a preference for a conformation in which the alkoxyl oxygen p-type lone pair, noR, is largely collinear with the bond. Dihedral angles C(2) - 0(2) - 0(3) in Table 4 are all close to 90°. This anomeric... 

The symbol n denotes the p-type lone-pair orbital in the case of furan, pyrrole, and thiophene and the ir-orbital in the case of benzene the symbols n and v denote the usual tr and it orbitals of ethylene. 

On the frontier orbital picture of the anomeric effect the overlap of the p-type lone pair on oxygen with the a -orbital of a carbon-quaternary nitrogen bond should occur in exactly the same way as with a carbon-oxygen or carbon-halogen bond. The reverse anomeric effect (the original, experimentally-based one) should not therefore exist on the frontier orbital picture. This represents its major failure. 

Interaction of the C(l)—X pyranosyl derivative with (a) the p-type lone pair on oxygen when the C(l)-—X bond is axial and (b) the sp-type lone pair on oxygen when the C(I)—X bond is equatorial... 

Thus the assumption about the absence of Ip-lp interaction in dimethyl(phenyl-thio)arsine seems to be justified. One of the possible equilibrium conformations where the interaction is at minimum is shown in Figure 13. The plane of the paper contains the CphSAs moiety and bisects the CAsC angle the phenyl—S—As—methyl dihedral angle (O) is 50.2°, and the p-type lone pair of sulphur and the arsenic lone pair are perpendicular to each other. The dipole moment of the molecule is not consistent with this geometry, but the contradiction is resolved by the suggestion that the compound is an equilibrium mixture of two rotamers with O = 50.2° and [Pg.291]

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

This mesomeric picture gives an intuitive explanation of the higher s character of the C-H equatorial bond, as it indicates a measure of sp2 hybridization in this case. One should then be tempted to draw a mesomeric picture showing similar delocalization in axial C 1—H and 05—H bonds, and moreover to extend such representations to ordinary ethers. However calculations for methanol and dimethyl ether in the staggered conformation predict that the effect should be much smaller for these compounds. In this respect, we may mention that the practical significance of the interaction of the oxygen p-type lone pair with the antiperiplanar OH bond in methanol has been recently questioned (20). So the effect discussed in this section may be restricted to the pyranose-like, acetal or mixed acetal type of compounds. 

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. 

Figure 3. (a) Bisected and eclipsed conformations of an a-chloroanion (b) corresponding conformations of 2-chlo-rotetrahydropyran (c) the overlap between carbon-chlorine antibonding orbitals and the ring-oxygen lone pairs left, axial C-Cl bond and p-type lone pair right, equatorial C-Cl bond and a-type lone pair... 

What is true in the actual sugar model is that there are two lone pairs on oxygen, and that the p-type lone-pair axis is not parallel to the antibonding orbital, being in fact almost... 

The possibility of this second type of interaction had not been considered previously. Its effect is a stabilization of equatorial chlorine. We must also remember that in some respect axial hydrogen plays the same part as axial chlorine (Figure 2). Consideration of all relevant interactions led to the formula given in Figure 4 (37). One important novelty is the introduction of the energy difference between the o and p-type lone pairs of the ring oxygen, a consequence of the fact that one pair stabilizes the conformation with axial chlorine and the other the conformation with equatorial chlorine. 

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