Skeletal rotations energy barriers

High static and dynamic flexibility of the polysiloxane chain, associated with a very low energy barrier to rotation around their skeletal bonds and a low energy of deformation of the SiOSi bond angle, make the polymer soluble in many solvents. The catalyst attached to such a mobile polymer chain, which can adopt many conformations, is available for the interaction with reactants in a... 

Many applications of Kilpatrick and Pitzer s procedure for calculating thermodynamic properties of molecules with compound rotation have been reported. In all cases possible potential energy cross-terms between rotating tops have been neglected. Contributions from internal rotation of symmetric tops have been calculated using the appropriate tables." These tables have also been used in calculations for the internal rotation of asymmetric tops hindered by a simple -fold cosine potential. 3-Fold potential barriers have been assumed in calculations for the —OH rotations in propanol and 1-methylpropanol, the —SH rotations in propane-1-thiol, butane-2-thiol, 2-methylpropane-l-thiol, and 2-methylbutane-2-thiol, the C—S skeletal rotations in ethyl methyl sulphide, diethyl sulphide, isopropyl methyl sulphide, and t-butyl methyl sulphide, and the C—C skeletal rotations in 2,3-dimethylbutane, and 2-methylpropane-l-thiol. 2-Fold cosine potential barriers have been assumed in calculations in the S—S skeletal rotations in dimethyl disulphide and diethyl disulphide. ... 

Eton describes the contribution to the total energy due to hindered rotation around skeletal bonds. The formulas generally used for the torsional potentials are those of Brant and Flory [2,6-10] where the torsional barriers used can be of different values [2,6-10]. It is necessary to point out, however, the limiting condition that must be imposed on the rotational degrees of freedom. Rotations around bonds that have very high torsional barriers (C=C, C=0), and single bonds between them affected by their conjugation, as in the case of polypeptides, must not be considered [11]. 

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