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Abstract: The role of the torsional potential on the glass transition of a linear polymer has been investigated by means of Molecular Dynamics simulations. The study has been performed on a polyethylene model polymer, which has been simulated at decreasing temperatures towards its glass transition. The united atom description has been adopted, with every methyl and methylene group along the chains being regarded as an individual interacting site. Three rotational isomers are allowed by the reference torsional potential, which has been parameterized so that the two torsional barriers can be tuned separately. Then, two sets of torsional parameters have been considered. The first set is characterized by increasing values for the trans–gauche (t–g) barrier and by no gauche–gauche (g–g) barrier. In the second set, the t–g barrier is kept constant, and the g–g barrier takes increasing values. The zero potential situation has also been analyzed. The glass transition temperature (T g) has been obtained by following the specific volume of the system as a function of temperature. It has been found that T g increases with increasing values of the t–g barrier. Nevertheless, T g becomes independent of the t–g barrier for very high barriers. As for the role of the g–g barrier, T g is almost independent of it, provided that the t–g barrier already exists. [Copyright &y& Elsevier]

We present here a systematic study by quantum mechanical methods of structural and energetic properties for a series of substitutions by alkyl groups of the hydrogens in hydrogen peroxide. The emphasis is on the torsion around the peroxidic bond, which leads to the chirality changing stereomutation. The dihedral angle dependence of the geometrical features and of the dipole moment is discussed with reference to previous experimental and theoretical information, and with respect to the preceding paper on hydrogen peroxide (Maciel et al., Chem Phys Lett, 2006, 432, 383). This information is of interest for chiral separation experiments as well as in view of a possible dynamical mechanism for chirality exchange by molecular collisions. The cis and trans barriers appear to vary remarkably upon substitution by alkyl groups (methyl, ethyl, n- and iso-propyl, sec- and tert-butyl hydroperoxides), the most important property being their geometrical dimensions. As the latter increase, tendency for the equilibrium configuration towards the trans structure increases, so that the trans barrier becomes negligible for dimethyl and diethyl peroxides and for n- and iso-butyl hydroperoxides, giving essentially achiral molecules. For the chiral ones (HOOH, CH3OOH, and C2H5OOH) torsional level energies and eigenfunctions are calculated and their distribution as a function of temperature determined. Their use is exemplified by a calculation of the dipole moment of hydrogen peroxide at room temperature, reconciling previous disagreement between theory and experiment. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [ABSTRACT FROM AUTHOR]

Abstract: The Fourier transform microwave spectrum of dimethyl methylphosphonate studied by us previously is reanalyzed here to obtain more physically reasonable parameters describing the various Coriolis-like couplings between overall rotation and internal rotation of the two methoxy methyl tops. In particular, we use exactly the same frequencies and spectral assignments as in our previous study, but the least squares fit is started from a rather different set of initial molecular parameters and is carried out with a slightly smaller set of adjustable parameters. The standard deviation of the fit is not significantly changed, but convergence to a rather different minimum in parameter space is obtained. This new minimum does not change the three rotational constants significantly, but values for the 12 Coriolis coupling constants are dramatically rearranged, so that parameters arising from coupling between the two internal rotation motions are greatly reduced in magnitude. These new Coriolis constants bring the derived direction cosines for the methoxy methyl groups in the principal axis system into much better agreement with ab initio predictions. We have used our new parameters to derive internal rotation barrier heights for the two methyl groups of 280 and 188cm−1. [Copyright &y& Elsevier]