Lone pair density

The atomic dipole moment can be attributed to the preferential population of specific nonspherical atomic orbitals. In particular, this is the case for atoms with doubly-filled nonbonding lone-pair orbitals, such as the oxygen atoms in C—O—H and H—O—H, or oxygen in a terminal position as it is in the carbonyl group. An early demonstration of the bias introduced in X-ray positions of non-hydrogen atoms was the combined X-ray and neutron study of oxalic acid dihydrate (Coppens et al. 1969), which showed the X-ray positions of the oxygen atoms to be systematically displaced by small amounts into the direction of the lone pair density. 

In contrast to the chemical properties of enolates, azaenolates of imines exhibit a marked thermodynamic preference for the syn configuration at the C-N partial double bond (syn effect)2 due to the repulsion between the lone pair density at nitrogen and the 7t-electron density at the carbon. Thus, lithiation of imines derived from ketones occurs with strong regioselectiv-ity at the least substituted carbon, followed by a regioselective reaction with electrophiles exclusively at the carbon of the ambident azaalkyl moiety. 

An amino substituent on the phosphenium phosphorus makes cationic phosphenium complexes stable, but the ji-donation from an amino substituent is only a minor contribution. So what is the role of an amino substituent The role may be, as is proposed for Arduengo carbene, to protect the approach of a nucleophile to phosphenium phosphorus by high p7i lone pair density flanking the phosphenium center.32... 

Lone-pair densities are less concentrated than bond densities in the same valence shell and occupy more space. 

Equivalent point charges to represent lone pairs were calculated as a/2, from the ionization radius of the central atom and their effective distance from the nucleus was related to the same parameter. These values are based on the assumption that lone-pair densities should relate to valence densities and that their distance from the nucleus should depend on atomic size... 

The hydrogen nuclei are distributed over a torus in a plane perpendicular to z and the resulting non-classical structure is indistinguishable from the recognized classical structure [196] with lone-pair density along the external H-O-H bisector. 

The lone-pair density of the Ooui centers is mainly directed toward the outside of the six-rings studied. Therefore, the centers Ooui can take part... 

Figure 2. Relief map of the negative Laplacian in the plane of the carboxyl group in glutamic acid. Sharp peaks in the close vicinity of the nuclei represent core charge concentrations. Double maxima in the C-N, C=0, and C-C covalent bonds are bonded VSCCs of the atoms participating in the bond. The two outer maxima at the keto oxygen atom are the non-bonded VSCCs associated with the lone-pair densities. Reprinted with permission from Ref. [77].

Cytldine. Near neutral pH, only the N(3) site on a cytosine base has lone pair density available for metal binding (Figure... 

An analysis of the effects of substituents on the basicity of phosphines has been presented,and the relationship between phosphine-proton affinities and lone pair density properties explored. A scale of a-donor strengths for phosphines and related systems has been developed. The model supports the view that as a-donor strength increases, so the 7C-acceptor power declines. Theoretical approaches to the electronic structures of phosphirane and 1,3-diphosphe-... 

O-H... O and N-H... O angles in intermolecular hydrogen-bonded systems vary from 140 to 180° and from 130 to 180°, respectively. Theoretical calculations show that large changes in angle correspond to only 1 or 2 kJ/mole for example + 30° for OH... 0=C costs ca. 3 kJ/mole. Incidentally, experimental and computational data do not support the hypothesis that the H bond lies along the maximum of the electron donor lone pair density. 

Fig. 1.18 Electron density difference maps for the (CH3)2TeCl2 molecule. On the left (a), in the equatorial plane defined by the atoms Te, S1, and C2, showing the Te-C bond and Te(I V) lone-pair densities. Contours are at 0.03e/A3 with negative contours broken. On the right (b), in the plane perpendicular to the plane of the left, showing the Te-Q (chlorine) bond and the Te(IV) lone-pair densities (Ref. [53], 1983 American Chemical Society)...

With respect to the 2D analysis, JP R clearly described for the first time the simultaneous hyperconjugative effects deriving from delocalization of lone-pair density on each oxygen atom into the (t co antibonding orbitals associated with their counterpart oxygens. While they did not refer to this by name, in essence this was the first demonstration of simultaneous anomeric and exo-anomeric effects. 

Fig. 2 Double bond-no-bond resonance depiction of hyperconjuga-tive delocalization of nitrogen lone pair density into C-X c ...

The lone pair density observed in the (Z)-lV-methyl-C-phenylnitrone example discussed in Section 7.1.2 (Figure 7.3) shows a similar nonspherical distribution of charge around the O atom of the N—0 group. MO calculations on the nitrone would give nonbonding... 

---