SELF-DIFFUSION IN THE PLASTIC PHASE OF SUCCINONITRILE BY H-2 NMR QUADRUPOLE ECHO TRAIN

Deuterium NMR measurements at 46.07 MHz on the solid phases of succino nitrile are reported. In the low temperature phase II (< -40-degrees-C ) the spectrum is characteristic of a rigid system, although with a re latively short T1 (almost-equal-to 0.1 s). The plastic phase, which is stable between -40-degrees-C and 58-degrees-C, exhibits a narrow line and T1 behaviour consistent with fast gauche-trans isomerization and fast molecular reorientation. The estimated correlation times for thes e processes in the plastic phase range between 10(-11) s and 3 x 10(-1 0) s, and correspond to the extreme narrowing regime, omega0tau(c) << 1. The measured linewidths in the low temperature range of this phase are, however, significantly broader than those expected from the T1 va lues, indicating the occurrence of an additional relaxation mechanism. This excess width decreases with increasing temperature. It is propos ed that random local deviations of the molecular ordering from pure cu bic symmetry result in incomplete averaging of the deuterium quadrupol e interaction and consequently to inhomogeneous line broadening. The d ecrease in the excess linewidth upon heating within the plastic phase is ascribed to self-diffusion which averages out the residual anisotro py. To separate the effects of local ordering and self-diffusion, quad rupole echo train experiments were performed as a function of the puls e interval in the sequence. Analysis of the results yields quantitativ e information about the temperature dependence of the kinetic paramete rs of the self-diffusion process, as well as about the local ordering. The latter can be described in terms of an average order parameter of the order of 10(-3), while the correlation time for the self-diffusio n process obeys the equation tau(d)(seconds) = 3.7 x 10(-14) exp (46.9 /RT), where R is in kJ mol-1.