From high-resolution proton, ^(13)C, and ^(15)N nuclear magnetic resonance spectra of ^(15)N-enriched pyridine and its hydrochloride, all of the chemical shifts, ^(15)N-H, and ^(l5)N-^(13)C coupling constants have been obtained. Attention is centered on the coupling constants, which are compared with other values and discussed in light of the known inapplicability to pyridine of the average energy approximation. The coupling ^2J_(NH)exhibits the normal dependence on geometry, and the algebraic increase observed on protonation parallels the behavior exhibited by other acyclic compounds with unsaturated nitrogen. The coupling ^1J_(NC) in pyridine increases 30-fold on protonation, which accords with qualitative predictions. In contrast to the absence of C-N coupling over more than one bond in saturated systems (glycine, alanine, 1,l-dimethylhydrazine), pyridine and pyridinium ion display substantial C-N couplings over two and three bonds. Because large two-bond C-N couplings are also observed in methylnitrosamine as well as in some amides, the presence of a π system is felt to be important for the longer range couplings. Substantial solvent effects are observed on these couplings which may be attributed to increasing hydrogen bonding at nitrogen. On the basis of changes in ^(15)N chemical shifts and ^(15)N-H coupling constants, ^lJ_(NC) is shown to very likely undergo a change in sign when the nitrogen is protonated.
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Lichter et al. (1971) studied this question.