Molecular motion in cyclic siloxanes studied by 17O, 29Si, and 13C nuclear magnetic resonance spectroscopy
Abstract
NMR studies of nine cyclosiloxanes, Dn, 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 Dn 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 Dnring is proposed to account for the results.