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With the aid of DTA the phase diagram of 4-(trans-4-heptyl-cyclohexyl)-benzonitrile, 7PCH, was redetermined. Dielectric studies on the static and complex permittivity have been performed on 7PCH in the pressure range 0.1 -220 MPa, the frequency range 1 kHz-13 MHz and the temperature range 290-380 K. For the slow relaxation process in the nematic phase, characterized by the relaxa­tion time tII , we have calculated the activation volume, Δ ≠VII, of about 80 to 60 cm3/mol (decreasing with increasing temperature), and the activation enthalpy, Δ ≠HII, of approximately 70 kJ/mol. The isochoric activation energy, Δ ≠UII, amounts to about 36 kJ/mol. The pressure dependence of Δ ≠HII is less pronounced compared with previous findings for 5PCH and 7CB. Estimates of t0 (extrapo­lated from the relaxation time in the isotropic phase) allow the calculation of the retardation factor gII = tII/t0 that is analysed in terms of theories of Martin-Meier-Saupe and Coffey in order to evalu­ate the nematic potential q. Using data for the order parameter 5 from NMR studies it was possible to determine the strength of the interaction potential in the nematic phase and to check the relation q~S which is valid at p = const., but not at V = const. The results are compared with recent findings for other homologous series.

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Differential thermal analysis and dielectric studies under pressures up to 300 MPa and temperatures of about 200 to 350 K have been performed on 2-methyl-2-nitro-propane (TBN). TBN displays an orientationally disordered phase (ODIC), solid I, and two non-plastic phases, solids II and III. The coexistence region of the plastic phase I increases with increasing pressure, whereas the low-temperature phase II apparently vanishes at a triple point I, II, III, above 300 MPa. The static permittivity increases on freezing, characterizing the solid I as an ODIC phase. In the frame of the Kirkwood-Onsager-Fröhlich theory the g-factor is about unity, discounting specific dielectric correlations. The dielectric behaviour of TBN is similar to previously studied related compounds, such as 2-chloro-2-methyl-propane or 2-brome- 2-methyl-propane