Solvent and protonation effects on nitrogen NMR shieldings of isoamides (α-aminoethers)

High-precision solvent-induced 14N NMR shieldings are presented for some isoamides and their N-protonated forms. The results indicate that the major source of solute-solvent interactions, giving rise to nitrogen shielding variations, is solvent-to-solute hydrogen bonding where the lone pair electrons of the imino nitrogen act as the hydrogen-bond acceptor centre. This results in a remarkable shielding increase for the imino nitrogen atom. Another interaction which could be important, when NH moieties are present, is solute-to-solvent hydrogen bonding where the NH croup is engaged as a donor. In the present study this effect is found to be a modest one and produces nitrogen deshielding. The influence of non-specific interactions, arising from solvent polarity effects, is rather feeble compared with that for analogous imine systems. The direction and magnitude of this effect is correctly reproduced by molecular orbital calculations incorporating the Solvaton model for non-specific solute-solvent interactions. Protonation effects on the nitrogen shieldings of the isoamides are substantial, producing an increase in shielding of about 80 ppm. This is in accord with that observed for solvent-to-solute hydrogen bonding.