A theoretical and experimental 14N NMR study of association of pyridine

The 14N NMR spectra of liquid pyridine and its mixtures with n-heptane were measured over a full concentration range. The chemical shifts of the pyridine nitrogen were found to vary in the spectra of the mixtures as compared with the value for the pure base. These variations were compared with theoretical results, i.e. with the difference in shielding constant for the nitrogen nucleus in the isolated molecule and its associates (calculated by GIAO/CHF). The geometry of pyridine molecules and of its associates was optimised at RHF/6-31+G** level. On this basis the most probable structures for the associates in liquid pyridine were proposed. The computer simulation of liquid pyridine at a temperature of 300 K, by the molecular dynamics method using the Amber force field, lent validity to the existence of the proposed dimers.