A molecular mechanics calculation of conformational energies and barrier heights in haloalkanes

Orientational relaxations in solid (1,1,2,2)tetrachloroethane

We employ dielectric spectroscopy and molecular dynamic simulations to investigate the dipolar dynamics in the orientationally disordered solid phase of (1,1,2,2)tetrachloroethane. Three distinct orientational dynamics are observed as separate dielectric loss features, all characterized by a simply activated temperature dependence. The slower process, associated to a glassy transition at 156 ± 1 K, corresponds to a cooperative motion by which each molecule rotates by 180° around the molecular symmetry axis through an intermediate state in which the symmetry axis is oriented roughly orthogonally to the initial and final states. Of the other two dipolar relaxations, the intermediate one is the Johari-Goldstein precursor relaxation of the cooperative dynamics, while the fastest process corresponds to an orientational fluctuation of single molecules into a higher-energy orientation. The Kirkwood correlation factor of the cooperative relaxation is of the order of one tenth, indicating that the molecular dipoles maintain on average a strong antiparallel alignment during their collective motion. These findings show that the combination of dielectric spectroscopy and molecular simulations allows studying in great detail the orientational dynamics in molecular solids.

A modification to the COSMIC parameterisation using ab initio constrained potential functions

The H..H non-bonded potential employed in the current COSMIC force field has been contrasted with H..H potentials used in a number of other force fields. Initial conversion of the variety of functions to a Morse format, achieved using a simple graphical fitting procedure, allowed a direct comparison to be made, showing the COSMIC potential to differ considerably from the other potentials. This difference was reflected in the failure of COSMIC to reproduce ab initio and experimental energies for molecules with significant H..H interactions, with particular reference to the energy curves of benzophenone and diphenyl ether. Considerable improvement in these energies is produced by the use of a Morse function originally based on the H..H potential used in MM3.