PMR study of intramolecular motions in solid p-toluidine

Wide-line PMR studies on solid p-toluidine have been made in the temperature range 77–318 K at 7.5 MHz. Analysis of second moment data suggests a rigid and non-rotating state of the crystal lattice at 77 K. The molecular and crystal structure is confirmed. The second moment gradually decreases up to 240 K (a fall of 2.8 G²) which is indicative of quantum mechanical tunnelling of methyl protons. An observed abrupt fall in p.s.m. beyond 240 K is followed by a phase transition and is attributed to methyl and amino group reorientation upto the melting point. The potential barrier to the reorientation is found to be approximately 8.88 kcal mol^?1 in the solid phase.