A solid-state deuterium NMR study of internal rotation in p-nitroaniline

Abstract

Deuterium quadrupole echo spectra have been obtained at 55.3 MHz and several temperatures between 26 and 133°C for samples of solid p-nitroaniline, deuterated either in the amino group or in the 2,6 positions of the aromatic ring. At all temperatures, the aromatic deuterons exhibit a simple rigid lattice powder pattern which is accurately simulated using a quadrupole coupling constant of 174 ± 1 kHz and asymmetry parameter 0.06 ± 0.02. Quadrupole echo spectra for the amino deuterons show features characteristic of 180° flips about the D-N-D bisector, and the activation energy for this motion is found to be 49 ± 6 kJ/mol, with a rate k = (5.2 ± 0.3) × 10 5 s −1 at 133°C. For the amino deuterons, the dependence of echo amplitude and spectral lineshape on pulse spacing includes contributions from heteronuclear dipolar coupling as well as from internal rotation. This makes the extraction of accurate rates difficult, unless the lineshape data are supplemented by independent information. The spin-lattice relaxation rate, which is dominated by the flipping motion at high temperature, is very useful in this regard.