Lone pair, shielding

Most common single bonds (C — O, C — N) have shielding properties that parallel those of the C — C bond. There appears to be a sign reversal, however, for the C — S bond. In all these heteroatomic cases, the geometry is more complex than that for the C — C bond. In some instances, a lone electron pair can have a special effect. In N-methylpiperidine (3-7), the axial lone pair shields the vicinal by an n—o interaction without any effect on Hgq. As a result, increases to about 1.0 ppm or more. 

NMR data for 4-methyloxazole have been compared with those of 4-methylthiazole the data clearly show that the ring protons in each are shielded. In a comprehensive study of a range of oxazoles. Brown and Ghosh also reported NMR data but based a discussion of resonance stabilization on pK and UV spectral data (69JCS(B)270). The weak basicity of oxazole (pX a 0.8) relative to 1-methylimidazole (pK 7.44) and thiazole (pK 2.44) demonstrates that delocalization of the oxygen lone pair, which would have a base-strengthening effect on the nitrogen atom, is not extensive. It must be concluded that not only the experimental measurement but also the very definition of aromaticity in the azole series is as yet poorly quantified. Nevertheless, its importance in the interpretation of reactivity is enormous. 

The Lewis dot formalism shows any halogen in a molecule surrounded by three electron lone pairs. An unfortunate consequence of this perspective is that it is natural to assume that these electrons are equivalent and symmetrically distributed (i.e., that the iodine is sp3 hybridized). Even simple quantum mechanical calculations, however, show that this is not the case [148]. Consider the diiodine molecule in the gas phase (Fig. 3). There is a region directly opposite the I-I sigma bond where the nucleus is poorly shielded by the atoms electron cloud. Allen described this as polar flattening , where the effective atomic radius is shorter at this point than it is perpendicular to the I-I bond [149]. Politzer and coworkers simply call it a sigma hole [150,151]. This area of positive electrostatic potential also coincides with the LUMO of the molecule (Fig. 4). 

The localized quantities of the IGLO results allow separation of the influences of the different bonds, of the inner shells and of the lone pairs on the shielding of the resonance nuclei. It is evident from Table 1 that the SiX bonds with different substituents X do mainly contribute to the chemical shifts. On the other hand, the substituents X have distinct influences on the chemically unchanged parts of the molecules as in system II and on the inner L shell which, on their parts, influence the nuclear shielding, too. 

Summarizing the available bonding information, decamethylsilicocene (1) is regarded as an electron-rich silicon(II) compound containing a hypercoordinated silicon atom which is sandwiched between two rather weakly 7i-bonded pentamethylcyclopentadienyl ligands and thus is effectively shielded the lone-pair orbital at silicon is part of the frontier orbitals of the molecule. 

Conformational effects on 15N shifts in substituted cyclohexanes make an axial NH2 more shielded than an equatorial one. Also, 15N resonances are deshielded by ft substitution more extensively than are 13C resonances of cyclic hydrocarbons, but the magnitude of the effect depends on the degree of nitrogen substitution. Carbons in the y position shield the nitrogen in a manner analogous to 13C, but to a smaller extent in methanol than in cyclohexane solutions, and less for tertiary amines than for primary and secondary amines. These differences have been attributed in part to possible conformational influences on the stereoelectronic relationships between the lone pair and the C—C bonds. 

N-methylation of aniline results in shifts of the aniline nitrogen resonance to higher shielding (Table 6), and this has been discussed in terms of perturbation of the lone-pair jr-delocalization by the methyl group37. 

The picture is less clear when free electron pairs on other heteroatoms (O, S, etc.) are involved. There is only one clear-cut case providing evidence concerning endocyclic sulfoxides The unusually large shieldings of carbon atoms in gauche position relative to an axial oxygen atom in thiane oxides 79, 80, 82, and 83 (p. 252) (200,201) have been partially ascribed to an upfield-shifting effect of the equatorial antiperiplanar lone pair (cf. Section III-C). 

Analogously, shieldings of antiperiplanar carbons by nitrogen lone pairs have been reported. It was concluded that a considerable part of the 10 ppm difference of the C-2 chemical shifts in the isomers of 8-methyl-8-azabicyclo[3.2.1]octane must be due to lone-pair interaction, since a difference of only 6 ppm is expected from a y-ganche effect of the A-methyl group73. This is supported by the fact that the difference is reduced if the solvent is methanol in this case the lone pairs are involved in hydrogen bridging. 

The chemical shift of the angular proton in benzo[c]quinolizidines will differ from that in quinolizidine itself as a result of delocalization of the lone pair electrons over the aromatic ring in certain conformations. Examples are provided by 29 and 30. In trans-fused 29, the nitrogen lone pair is delocalized over the aromatic ring and thus the anti-coplanar shielding mechanism is lost and the angular proton absorbs to low field of that in the cis conformation 30.51... 

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