Molecular Conformational Changes in the Mesophases of TBBA

Abstract

In a previous paper [1] we have presented results concerning the molecular orientational order in the mesophases of TBBA from a consistent analysis of neutron, ¹⁴N NQR and DMR data. We consider here the problem of the relative temperature dependence of various DMR splittings, in the light of these results. Two extreme models have been considered (i) the most probable conformation changes with temperature [2,3] and (ii) the most probable conformation does not change with temperature, but more than one order parameter is needed to describe the molecular orientational order [3,4]. In the calculation, we introduce explicitly the internal motions, essentially rotations of the phenyl rings around their para-axis. In model (i), the molecule is assumed to rotate uniformly around its long axis Oz_o. This long axis is defined in the frame Ox_my_mz_m attached to the most probable orientation of the central ring, with Oz_m along the para-axis and Ox_m in the plane of the ring, by its polar and azimuthal angles ε and φ, respectively. It is found that the relative temperature dependence of five splittings is satisfactorily explained, assuming a variation of φ, presumably due to variations of the mean dihedral angles between rigid molecular fragments, while ε is kept constant as suggested by the results of ref.[1]. On the contrary we show that explanation (ii) cannot be the main phenomenon since it predicts relative variations which are definitely too small.

The corresponding publication has been submitted to J. Physique (1980)