Mean-square optical anisotropies and molar Kerr constants of cyclic and linear dimethyl siloxanes

Mean-square optical anisotropies, 〈γ2〉0, and mean molar Kerr constants, 〈mK〉0, have been calculated for cyclic dimethyl siloxane oligomers having from n=8 to n=20 skeletal bonds. Stringent criteria were used to select ‘‘cyclic’’ conformations of the equivalent open chains based on the relative orientations and rotational states of the terminal bonds. The values obtained for the cyclic oligomers are compared with values calculated by implicit matrix methods for open chain dimethyl siloxanes of equivalent length. Substantial differences between calculated values for cyclic and linear oligomers are observed, and are discussed in terms of the restricted conformational population of the cyclic species. Calculated mean molar Kerr constants are also compared with experimental values measured for cyclic and linear oligomers in the undiluted state at 298 K. It is found that although there is good correlation between the trends calculated and those observed experimentally, discrepancy exists between the absolute values.