Molecular motion in binary systems of molecular crystals

Abstract

Nuclear magnetic resonance (NMR) relaxation has been used to study molecular motion in binary systems of the molecular crystal hexamethylethane with several chemically distinct solutes in concentrations up to 10%. The anisotropic reorientation of HME molecules in the brittle phase is generally enhanced, but endospherical reorientation in the plastic phase is only disturbed by one of the solutes. Self-diffusion of HME molecules is reduced by one solute and enhanced by the others, but effects are small and the activation enthalpy is independent of solute concentration for the range studied. Possible explanations of the observed effects are discussed. The effects of the solvents on the phase transitions are also examined.

On the Crystallography and Thermodynamics in Orientationally Disordered Phases in Two-Component Systems

“…It should be mentioned that for all the mixtures used isomorphism was supposed to exist; i.e., one Gibbs energy curve was sufficient to describe the behavior of the solid solutions. This is not the case for the systems presented in this paper, so then, the empirical relation [8] must be used carefully. In particular, the enthalpy-entropy compensation temperature is associated with each solid solution domain, and then, expression [8] should be applied by using the stable and metastable temperatures of the solid solutions considered.…”

“…This is not the case for the systems presented in this paper, so then, the empirical relation [8] must be used carefully. In particular, the enthalpy-entropy compensation temperature is associated with each solid solution domain, and then, expression [8] should be applied by using the stable and metastable temperatures of the solid solutions considered. If such a method is followed the ¹ calculated by Eq.…”

“…The high temperature orientationally disordered form, which is stable between 356.6$1.0 K and the melting point (473.7$1.0 K), has face-centered cubic symmetry with Z"4 (denoted as C $ ) with a lattice parameter of 8.876 (8) A s at 363 K (20,21).…”

“…An empirical estimation method for the enthalpy-entropy compensation temperature in terms of a quasi-subregular solution model including a compensation law was published (32). From the experimental evidence of a large variety of mixtures relating molecular mixtures (such as p-dihalobenzenes, alkali halides, alkaline earth oxides, and cryogenic noble gases), the equation [8] in which ¹ and ¹ are the melting temperatures of the first and second components in the mixture, was derived. It should be mentioned that for all the mixtures used isomorphism was supposed to exist; i.e., one Gibbs energy curve was sufficient to describe the behavior of the solid solutions.…”

“…Another powerful way to modify the molecular surroundings is to introduce a foreign substance (6)(7)(8). In experimental practice this comes down to the study of binary systems.…”