Nuclear relaxation and molecular properties

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Abstract
The quadrupolar relaxation of the nitrogen nucleus has been studied in organic nitro compounds of the type X-NO2 (with X = C, N, O). Fine structure due to vicinal 14N, 1H coupling has been found in the proton spectra of nitramines and of alkyl nitrates. Complete line shape analysis of the proton resonances leads to the quadrupolar relaxation times T q of the nitrogen nucleus. Quadrupolar coupling constants have been estimated from these T q values. The results indicate that the symmetry of the electron distribution at the NO2 nitrogen site increases in the sense X = C < X = O < X = N. This trend may be rationalized in terms of the effect of the substituent X, within the framework of the Townes and Dailey model for quadrupolar interactions. The major factor leading to low field gradients at the NO2 nitrogen site is found to be inductive electron withdrawal from the nitrogen orbital in the X-NO2 bond by electronegative X substituents. The temperature dependence of T q has also been studied by line shape analysis and leads to the activation parameters for molecular reorientation.