The deuterium NMR spectrum of 4-methoxy-da-3,5-dz-4'-cyanobiphenyl has been recorded as a function of temperature in its monotropic nematic phase. The particularly simple molecular structure for this nematogen allows three elements of the Saupe ordering matrix to be determined from the spectral splittings. The temperature dependence of the major element of the ordering matrix is well predicted by theory provided allowance is made for the molecular biaxiality.
There have been many attempts to develop molecular theories for the order-disorder transition exhibited by nematic liquid crystals [ 1 ]. In the majority of these theories the molecules are assumed to be rigid and cylindrically symmetric while, in practice, nematogenic molecules are flexible and of low symmetry. As a consequence, an unambiguous test of such theories against the properties of real nematics is rarely possible. However, the advent of computer simulation studies of model liquid crystals with rigid particles of high symmetry has meant that the validity of the approximations employed in the theories may now be checked [2]. Nonetheless, such comparisons provide no direct information on the form of the pair potential for real nematogens. Therefore, it is important to be able to investigate the behaviour of materials with relatively simple molecular structure. For example, p-quinquephenyl does possess a nematic phase but the transition temperatures (TcN = 388°C; TNI = 423°C [3]) are inconveniently high and its thermal stability is uncertain [4] ; it is not therefore a particularly easy material to study. The mesomorphic range and possibly the structural simplicity of compounds exhibiting liquid crystal phases may be enhanced by working at high pressures. This possibility obtains because the nematic-isotropic transition increases more rapidly