Molecular motion in cyclic siloxanes studied by 17O, 29Si, and 13C nuclear magnetic resonance spectroscopy

Abstract

NMR studies of nine cyclosiloxanes, D_n, have been carried out, and measurements made of ¹⁷O NMR line-widths and chemical shifts, and ²⁹Si and ¹³C NMR relaxation parameters. The results obtained give information on the changes in molecular motion with increasing ring size. It is proposed that the smaller D_n compounds, up to D₈, can be assumed to be undergoing isotropic motion as neat liquids and in solution in C₆D₆. At ∼ D₈, in 1 mol dm^–3solution, isotropic motion is no longer sufficient to describe the relaxation parameters, and for the larger rings a novel conformational process involving SiMe₂: groups passing through the D_nring is proposed to account for the results.