Etude par resonance magnetique des mouvements moléculaires dans l'acrylonitrile solide

Abstract

Phase transitions and molecular motions in solid acrylonitrile and its deuterated homologue CH2=CDCN, have been studied between 100 and 191 °K (m.p.) by wide line n.m.r. and by T1 relaxation time measurements. Phase I (164 °K < T < 191 °K) is trapped and becomes metastable by quick cooling of acrylonitrile at 77 °K. It changes into the phase II, stable between 113 °K and 164 °K by a long duration annealing at 155–160 °K. The phase II → phase III transition occurs at 113 °K. It may be assumed that phase III, stable below this temperature, is rigid at T < 105 °K. Phase II may be characterized by a rotational oscillation of molecules around an axis defined by the N atom and the middle of the vinyl double bond. In phase I, acrylonitrile molecules undergo a binary reorientation motion around this axis with an activation energy of 4.2 kcal mol−1. The motion of peroxy radicals, trapped in acrylonitrile has been also studied by e.s.r. These radicals were produced by oxygen addition to free radicals previously formed by γ irradiation of acrylonitrile at 77 °K. The g anisotropy variation with temperature, shows no discontinuities at phase transitions, the activation of reorientation of peroxy radicals being 0.65 kcal mol−1. This result suggests that we are dealing in fact with macroradicals, the internal rotation of which is only observable in a solid matrix.