Potential-energy calculations for molecular reorientations between unequal wells in solid t r a n s,t r a n s-muconodinitrile

Abstract

The possible occurrence of large-amplitude molecular reorientations in solid trans,trans-muconodinitrile (TMD), C6H4N2, is investigated by calculating the dependence of the van der Waals potential energy on molecular orientation in the crystal. The ’’6-exp’’ potential function is used with atom–atom interaction parameters from different sources. In spite of some diversity among the various resultant potential-energy profiles, due mainly to uncertainties in the parameters involving nitrogen, there are some important features common to all the profiles: (1) relatively low energy barriers for hindered rotation of the TMD molecule around its long axis; (2) existence of two subminima associated with the highly anistropic motion at molecular orientations of 120°–130° and 210°–230° from the low-energy equilibrium minimum. The magnitudes of the energy barriers involved indicate that molecular reorientations can occur at different rates between the unequal potential wells in TMD. The calculations show that at room temperature not more than half a percent of misoriented TMD molecules occupy the metastable states. These predicted dilute orientational defects, which did not affect the x-ray diffraction analysis made earlier, were detected and studied by NMR relaxation measurements which we reported in a recent paper.