Studies of Nuclear Magnetic Resonance Spectra of Positive Halogen Salts of Pyridine and Substituted Pyridines

Abstract

In the NMR studies of positive halogen salts of pyridine and substituted pyridines, it is observed that the effect of the substituents on the chemical shift is quite remarkable and perturbed significantly. It appears that in shifting from pyridine to the pyridinium cation, there is quite a large decrease in electron-density at the 4-position and smaller loss at 2- and 3-positions. It is also observed that the unit of electron density corresponds to a shift, which amounts to 0.22, 0.38 and 0.63 in γ-scale at 2-, 3- and 4-position respectively. Examination of the results shows that there is more downfield shift of the ring protons on complexation with bromine than that of its iodine counterpart. Therefore, it would appear to be reasonable to interpret a more prominent downfield shift as arising from the deshielding effect of π-electron density redistribution in the bromine complex which is greater than that of the iodine complex.