Nitrogen-14 nuclear quadrupole resonance study of substituted nitrobenzenes

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Abstract
Nitrogen‐14 nuclear quadrupole resonance (NQR) spectra of 26 C–nitro compounds are reported and analyzed in the framework of the Townes and Dailey theory. Six different possibilities for the principal axis system of the electric field gradient (efg) tensor at the nitrogen of the NO2 group are considered. The results lead to the conclusion that the z direction of the principal axes is in the plane of the molecule and perpendicular to the C–N bond, whereas the x direction is along the C–N bond. The variations of the π electron density are in very good agreement with the ideas of classical chemistry. Satisfactory correlations of NQR data with the Hammett σ and σ R constants have been found. σ p and σ R for the cholesteryl group are determined which suggest that the cholesteryl group, COOC27H45, is an electron attracting group. The nitrogen‐14 quadrupole coupling constant in the solid phase of nitromethane (1695 kHz) is found to be substantially larger than that found in the gaseous phase (1187 kHz) by microwave measurements. This result cannot be explained by invoking the presence of hydrogen bonds in the solid phase of nitromethane.