Effects of Solvent, Protonation, and N-Alkylation on the ^(15)N 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, and 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

Nitrogen-15 Nuclear Magnetic Resonance Spectroscopy. Solvent Effects on the ^(15)N Chemical Shifts of Saturated Amines and their Hydrochlorides

Solvent effects were determined for the^(15)N NMR chemical shifts of some primary, secondary, and tertiary amines along with solvent, concentration, and counterion dependences of the corresponding amine salts. The solvent effects on tertiary-amine nitrogen resonances appear to be dominated by hydrogen bonding to the lone pair, which results in downfield shifts. For primary and secondary amines, the situation is much more complex. Hydrogen bonding to the lone pair is apparently important, as indicated by a linear correlation of shifts produced by going from nonpolar solvents to methanol as solvent and shifts produced by protonation. The very large solvent and counterion effects sometimes observed for the ^(15)N chemical shifts of the conjugate acids of saturated amines seem best explained by ion-pair aggregation in relatively nonpolar solvents like chloroform, and especially with “soft” polarizable anions like iodide. Solvation of the salts in polar solvents or changes of anions expected to cause less ion aggregation result in upfield shifts of the nitrogen resonances, thereby decreasing the magnitude of generally downfield protonation shifts in nonpolar solvents. For some amines, this solvent effect may be so large as to cause protonation of a saturated amine to result in an upfield rather than the usually expected downfield shift

Steric and Electronic Effects on ^(15)N Chemical Shifts of Piperidine and Decahydroquinoline Hydrochlorides

Natural-abundance ^(15)N NMR chemical shifts of a number of closely related C- and N-methyl-substituted piperidine and decahydroquinoline hydrochlorides have been measured in chloroform and methanol. For each solvent, the ^(15)N shifts of the salts of secondary and tertiary amines give different linear correlations with the ^(13)C shifts of the corresponding carbons of their hydrocarbon analogues. Additive shift parameters as well as protonation-shift parameters for carbon substitution near nitrogen have been determined. Three of the nine parameters have a pronounced solvent dependence. The substituent-shift parameters for hydrochlorides are in general closer to the analogous values for ^(13)C NMR than the corresponding parameters which correlate the I5N shifts of the free amines. The parameters for substitution on @carbons are an exception. The ^(13)C shifts of some of the compounds can be used to elucidate conformational questions

^(15)N and ^(13)C Nuclear Magnetic Resonance Spectra of Diazo and Diazonium Compounds

Nitrogen-15 and carbon-13 nuclear magnetic resonance (NMR) spectra have been taken of a number of diazo, benzenediazonium, and azido compounds. The ^(13)C chemical shifts of C1 of these substances are at rather high fields (23-112 ppm for diazo compounds compared to 155-185 ppm for imines, and 102-123 ppm for diazonium salts compared to 148 ppm for nitrobenzene) and do not seem explicable in terms of high charge densities on these carbons. The ^(15)N resonances of diazomethane shift downfield when phenyl groups are substituted on carbon and upfield when the carbon is incorporated as C5 of a cyclopentadiene ring. Substitution of electron-donating and electron-attracting groups in the 4 position of benzenediazonium salts shifts the ^(15)N resonances downfield and upfield, respectively

Natural-Abundance Nitrogen-15 Nuclear Magnetic Resonance Spectroscopy. Steric and Electronic Effects on Nitrogen-15 Chemical Shifts of Piperidines and Decahydroquinolines

Natural-abundance ^(15)N-NMR chemical shifts of closely related methyl-substituted piperidines, decahydroquinolines, and their N-methyl derivatives have been measured in cyclohexane and methanol. For both solvents, the secondary amines and two groups of tertiary amines give separate linear correlations with the ^(13)C chemical shifts of their hydrocarbon analogues. Additive shift parameters for carbon substituents near nitrogen, similar to those which correlate ^(13)C chemical shifts, have been determined. Except for the N-alkylation parameters, these parameters are relatively solvent insensitive, at least for cyclohexane and methanol. Nonetheless, ^(15)N chemical-shift comparisons are best made for the same solvent or very similar solvents. A large shift effect results when substituents are changed which are antiperiplanar to the orbital of the unshared electrons of tertiary amines. The use of the additive shift parameters and the general correlation between ^(15)N and ^(13)C shifts with respect to analysis of conformational and structural changes is illustrated using N-methylpiperidine and cis-2,3-dimethylpiperidine as specific examples