Effects of solvent, protonation, and N-alkylation on the nitrogen-15 chemical shifts of pyridine and related compounds

The ^(15)N chemical shift of pyridine has been measured in he gas phase and in a variety of solvents. The solvent-induced resonances are all upfield of the gas phase and correlate qualitatively with hydrogen-bonding capacity. The finding of a reasonable linear correlation with the Kosower 2 values indicates that the solvent shifts are dominated by the contribution of the n→^*π^* transition to the secondary paramagnetic shift term. No correlation was found with either the solvent dielectric constant, є, or the function (є - 1)/(2є + 2.5). Large solvent effects were found for the ^(15)N resonances of pyridine hydrochlorides, these require that interpretation of protonation shifts be made with care. In contrast to protonation shifts, methylation shifts are essentially solvent and counterion independent. The changes in ^(15)N chemical shifts of 4-substituted pyridines, on changing the solvent from benzene to methanol, correlate with the basicity of the solute, which also indicates the importance of hydrogen bonding in determining the solvent dependences of pyridine-nitrogen shifts.