Dielectric absorption in pentachlorobenzene compounds

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Abstract
The dielectric properties of pressed discs of pentachloronitrobenzene (pure, and contained in a naphthalene matrix), pentachloronitrosobenzene, pentachloroaniline (discs plus powder), pentachlorophenol, pentachloroanisole and N-methylpentachloroaniline have been investigated in the frequency range 20 Hz to 10 MHz and at ca. 298 K (230–294 K for pentachloroaniline powder). Dielectric absorption in pentachloronitrobenzene and pentachloroaniline is attributed to molecular rotation in the solid state. Pentachlorophenol does not show dielectric absorption under these conditions, but a sharp increase in dielectric constant at a temperature (350 K) well below the melting point (449 K) probably reflects the onset of molecular rotation. The absence of evidence for rotation in pentachloronitrobenzene, pentachloroanisole and N-methylpentachloroaniline is attributed to dimerisation in the case of the nitroso compound and to the blocking effect of the methyl group in the case of the other two. There is no obvious correlation between the apparent ability of the molecule to rotate and its polarity, as measured by the dipole moment.

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