14N Nuclear Quadrupole Resonance in Substituted Pyridines

Nitrogen-14 nuclear quadrupole resonance has been observed in 15 substituted pyridines, including cyano, acetyl, and chloro derivatives and the methyl esters of the pyridine monocarboxylic acids. The substituents produce variations of 5%–10% in the field gradient at the nitrogen. The σ and π charge densities at the ring nitrogen are discussed in terms of the relative electrophilic properties of the substituents and excellent qualitative agreement with the classical ideas of chemistry is obtained. Correlation of the nitrogen NQR data with the Hammett σ values and with chlorine-35 NQR results is also considered. The charge distribution of the cyano group is analyzed and discussed in connection with the ring nitrogen. The ionic character of the cyano C–N bond is found to be approximately 17%.