Unusual Dielectric Behavior of Solid Cyclopentanol and Molecular Motion

Dielectric and visual studies of cyclopentanol show the presence of a “rotational” form I stable from the melting point at − 17.6[ddot]C to a transition point at − 37.5[ddot]C, and a “rotational” form II from there to a second transition point at − 72[ddot]C. There are two “non-rotational” forms: form IV produced on quenching II with liquid nitrogen transforms upon warming to form III, which is the form usually produced when II is cooled, in the vicinity of -80[ddot]C. The dielectric constant of form III is about 2.5 and does not change rapidly with temperature; that of form IV is consistently 1% below that of form III. The dielectric constant of I is about 30. That of II produced from I grown from the melt is variable and ranges as high as 80; but if II is produced from III, the dielectric constant will be only about 30. In both I and II there is an association of about three molecules in chains to produce the high dielectric constants. In I the chains are distributed isotropically; whereas, in II there is a preference, increasing at lower temperatures, to form chains so that the resultant dipole moment is oriented along the c-axis of the hexagonal crystal structure. Crystals of I tend to grow from the melt with their c-axes normal to surrounding surfaces. Since the transformation to II is barely first order and does not involve restructuring, the dielectric anisotropy of II results in abnormally high values of capacitance being measured. The randomly oriented crystallites of II produced from III result in the measurement of an average value.