John le G. Gilchrist

Dielectric relaxation of Amines by inversion and internal rotation

Fifteen aliphatic and alicyclic secondary amines dispersed in polyethylene matrices exhibited dielectric relaxations at 4.2 K which are attributed to inversion of the nitrogen coordination pyramids by tunnelling of the amino hydrogens. Two primary phosphines exhibited broad dispersion curves which are attributed to relaxation by PH2 group rotations and twelve primary amines exhibited relaxations with characteristically skew dispersion curves. This characteristic is attributed to interference between tunnel couplings corresponding to internal rotation and to inversion. Comparison is made with published microwave and infrared spectroscopic data of these and similar molecular species and conclusions are drawn, where possible, about the barriers to inversion and to internal rotation. All the dielectric dispersions are attributed to Debye relaxation with a spread of time constants, and this is explicitly justified in one case.

Low-temperature dielectric losses of hydrocarbon media containing polar additives

Abstract Polypropylene films and other hydrocarbon media containing various dipolar additives were studied mainly between 4.2 and 105 K, and at 1.2 kHz. Five di

Vitamin E and related ether-phenols: A low-temperature dielectric relaxation study

Abstract Within a classs of phenols comprising durenols and methylated chromenols, chromanols and coumaranols, hydrogen-abstraction rate constants, and therefore radical-scavenging and antioxidant effectiveness are closely paralleled by low-temperature relaxation rates, and are apparently influenced in similar proportions by the inductive and resonance effects of para-alkoxy groups. Relaxation rates of 2,3,5,6-tetrachlorophenol and 2,6-di-tertiary-butyl phenol with and without 4-methoxy substituents are also reported and discussed.

Low temperature dielectric relaxation spectroscopy

Abstract It has been found during the last 2 1 2 years that dielectric relaxations associated with many organic compounds could be observed by dissolving or dispersing them in hydrocarbon solvents or matrices and cooling them to 4.2 K. Such relaxations were nearly always attributable to conformational changes of independent solute or dopant molecules and could be used as a means of spectroscopic investigation. Two ways of using such data are outlined and the advantages and limitations are discussed using ROH, RSH and R 1 R 2 NH compounds as examples.

Anomalous dielectric permittivity of ammonium perchlorate below 40 K

Abstract Polycrystalline NH 4 ClO 4 and ND 4 ClO 4 samples have anomalously increased dielectric permittivity below ≈ 40 K which depends on measuring voltage, on sample grain size and thermal history. Any sudden change in temperature or bias field provokes an immediate increase in permittivity followed by a slow decrease. The behaviour has analogies with a dipole glass, an incommensurate phase, spin glasses and ferromagnetic powders.

Pyrrolidine conformation: A low-temperature dielectric relaxation study

Abstract The 4.2 K dielectric relaxation rates of pyrrolidine, several of its derivatives and other secondary amines absorbed in polyethylene and polypropylene are compared and it is concluded that pyrrolidine prefers the C s (“envelope”) conformation in which the CNC bond angle is smallest and the nitrogen inversion barrier highest, 3-methyl-3-propylpyrrolidine also prefers this conformation, though less markedly. Incorporation into a bicyclic structure can cause the pyrrolidine ring to be distorted beyond the range of forms observed in the monocyclic species.

Dielectric absorption in hydrocarbon polymers with non-polar additives

Abstract Polypropylene films containing various non-polar additives including CCl 4 , tetramethylsilane and CS 2 (99.9 or 99.8%) exhibit weak dielectric absorption at the temperatures where molecular tumbling motions are expected on the relevant timescale. The absorption possibly corresponds to rotational jumps of up to 90° between inequivalent orientations of the additive molecules as has been suggested for ammonium in alums.

Hydroxyls in polyethylene and in alkanes. A low-temperature dielectric relaxation study

Abstract A dielectric relaxation of oxidised polyethylene originally observed by Carson was reproduced both by synthetically introducing hydroxyl groups into the polymer molecules, and by adding long-chain secondary alcohols. By comparing the 4.2 K relaxation strengths and rates of polyethylenes containing these and other additives the crystal structure is shown to be of primary importance only in the case of long-chain secondary alcohols. These also produced weak relaxations when introduced into crystals of odd alkanes, but in even alkanes relaxed strongly, and at a different frequency. 8-aminopentadecane only relaxed strongly in odd alkanes. An attempt is made to correlate these findings with the intermolecular steric hindrances in the respective crystals.

Nonlinear dielectric properties of glasses at low temperatures

Abstract Disordered dielectrics exhibit a variety of nonlinear responses at temperatures below 4 K, sometimes in remarkably small fields. Four characteristic types of behaviour are identified. Two of these are well understood in terms of the ‘standard’ two-level system model, but two require further experimental study and theoretical development. One type is similar to piezoelectric ceramic behaviour.

Dielectric relaxations in partly deuterated ammonium dichromate

Abstract Two dielectric relaxations in partly deuterated ammonium dichromate are attributed to reorientations of mixed-isotope ammonium ions. Loss peaks were observed between 20 and 40 K and obey the Arrhenius law with activation energy 1.5 kcal/mol for the stronger relaxation. The dipole moment is of the order of 0.015 D.