NMR relaxation study of molecular motions between unequal potential wells in solid t r a n s,t r a n s-muconodinitrile

Abstract

We report observations of extremely unusual proton NMR relaxation rates in solid trans,trans-muconodinitrile (TMD, N≡C–CH=CH–CH=CH–C≡N). In particular we measured, over the temperature range 77–423 °K, proton dipolar relaxation times T1D and spin lattice relaxation times T1 (at 24 and 58 MHz). The relaxation pattern is characterized by the following features: (a) very long motional T1 and T1D even at their respective minima, (b) no detectable motional narrowing of the line even at the T1D minimum, (c) unequal slopes at temperatures below and above the minimum of T1 (and T1D) vs 1/T, and (d) significant deviations from the usual linear dependence on resonance frequency of the values of the relaxation times at their respective minima. We extended an earlier NMR theory to the case of spin lattice relaxation due to molecular reorientations between the extremely unequal potential energy wells of TMD. We were able to explain all features of the above data in terms of this theory. By comparing our data to the results of several calculations of intermolecular potential energy which used different interatomic force parameters, we were able to rule out some of these, thereby determining the best choice for the parameters in this crystal. The detailed structure of this potential profile (i.e., relative depths of the wells and energy barriers hindering rotation) was then determined from the T1 and T1D data. We thus have observed and characterized in TMD a low concentration of orientational defects which occur when a molecule occupies a higher energy well. Our observations are probably the first of such extreme NMR relaxation effects due to motions between significantly inequivalent sites.