Qualitative Discussion of Solvent-Induced Shifts

Tertiary amines undergo downfield shifts in polar or acid solvents, which have been explained on the basis of a decrease in electron density due to hydrogen bonding to the nitrogen lone pair or protonation (P 2). The situation is complicated when the nitrogen atom is bound to one or two protons and complex self-association or solvation processes occur (D 26). If basic solvents only are considered, a downfield shift may also be observed (P 2). 

On the other hand, the N = O nitrogen atom of nitrosamines moves towards low frequencies by about -5 ppm. 

The case of sulfonamides (R-S02NH2) is interesting to discuss in comparison with that of amides, since in the former, H-bond interactions occur mainly with the nitrogen atom, rather than with the oxygen. 

However, a paramagnetic shift occurs when the pH value of an aqueous solution of methanesulfonamide increases 

Small peptides have different kinds of nitrogen atoms which can be readily identified from their solvent - and protonation - shifts (G 13, K 17). Indeed the high-frequency shifts observed from DMSO- to water-or acid-solutions are not of the same magnitude for both nitrogen atoms 

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