A study of molecular re-orientation in liquid acetonitrile by nuclear spin-lattice relaxation

Abstract

Studies of the nitrogen, deuteron and proton relaxation rates of liquid acetonitrile at various temperatures were carried out by pulsed N.M.R. techniques. Re-orientation of the methyl group about its symmetry axis is found to be approximately ten times faster than the re-orientation of the nitrile group. Analysis of the data provides an example of a large roomtemperature contribution to the intramolecular relaxation component by a spin-rotation interaction. Previously sizeable spin-rotation interactions were considered to be insignificant for organic molecules at room temperature.

Electron resonance spectra of some radicals derived from acetylenes and nitriles

“…At -30 ~ in methyl-d3-acetylene the re-orientation about the symmetry axis is about fourteen times faster than the motion perpendicular to the symmetry axis [24], and it seems reasonable to assume that the motion of the propargyl radical follows a similar pattern. Similar results were found for CD3CN [22,23].…”

“…This implies that the motion of these radicals in the liquid is anisotropic. There have been several studies of anisotropic molecular motion in liquids based on the measurement of nuclear spin relaxation times [21] and it is of interest that two of the first molecules investigated were acetonitrile [22,23] and methylacetylene [24]. At -30 ~ in methyl-d3-acetylene the re-orientation about the symmetry axis is about fourteen times faster than the motion perpendicular to the symmetry axis [24], and it seems reasonable to assume that the motion of the propargyl radical follows a similar pattern.…”

Nuclear magnetic resonance relaxation and anisotropic reorientation in liquid dimethylmercury

“…H~( c D ,~, The usual method of obtaining the anisotropic rotational diffusion tensor of a symmetric top molecule involves the measurement of the relaxation time of two quadrupolar nuclei which have different bond angles with respect to the symmetry axis of the molecule (24)(25)(26)(27). Thus in our molecule we could determine the anisotropic l-otational diffusioil tensor for dimethylmercury-d, from quadrupolar 1-elaxation of both D and 201Hg.…”

Extraction of T2 from NMR linewidths in simple spin systems by use of reference deconvolution

“…In freely rotating groups the competing spin-rotation relaxation attempts to mask this effect. For this reason, at and above room temperature these groups often show monoexponential decay; the non-exponentiality may be more obvious at lower temperatures 20,21 and in groups with hindered rotation.…”