Nitrogen deshielding

Electron withdrawing substituents such as aldehyde or imonium functions at the other end of the alkene or the 1,3-diene transform enamines and 1,3-dienamines into push-pull systems characteristic of dyes. Electron release (push) of the donor group shields the / , S, e,... carbons and electron withdrawal (pull) of the acceptor group (carbonyl oxygen or imonium nitrogen) deshields in the a, y,... position [343],... 

The three carbons show a very wide range of chemical shifts. The lowest field resonance corresponds to the carbonyl carbon, which is highly deshielded by the double-bonded oxygen. The nitrogen deshields the CH carbon much less, and the CH3 carbon is the least deshielded of the three. 

Further aza substitution ortho or para in the same ring deshields the nitrogen the effect is moderate for a para-, and large for an ort/io-nitrogen. The latter is probably a special azo effect , since the nitrogens of a simple azo group absorb at still lower field (-130 p.p.m., in ether). 

Chemical shifts of pyridine and the diazines have been measured as a function of pH in aqueous solution and generally protonation at nitrogen results in deshielding of the carbon resonances by up to 10 p.p.m. (73T1145). The pH dependence follows classic titration curves whose inflexions yield pK values in good agreement with those obtained by other methods. 

Carbons next to amine nitrogens are slightly deshielded in the NMR spectrum and absorb about 20 ppm downfield from where they would absorb in an alkane of similar structure. In N-methylcyclohexylamine, for example, the... 

A trifluoromethyl group bound directly to phosphorous is deshielded slightly more than the respective nitrogen compound (Scheme 5.18). 

The equatorial fluorines of these nitrogen-bound SF3 groups are greatly deshielded as compared to those in their carbon-bound counterparts in Scheme 7.13. 

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. 

The amino nitrogens of N-nitroso compounds are deshielded substantially more than those of electronically analogous amides, and well separated resonances frequently can be detected65, where geometrical isomerism is possible. 

The apparent relationship between torsion angle and 170 chemical shift has been explored further by examination of the data for several ortho alkyl-substituted nitrobenzenes and dinitrobenzenes (Table 23). The deshielding values of the shifts reflect increasing nitrogen-oxygen double bond character, which is expected as the nitro group is rotated from the plane of the aromatic ring. 

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