Absolute temperature dependence of chemical shielding of some reference nuclei

Abstract

In order to interpret adequately any temperature dependence measurements of NMR shifts it is necessary to have an independent measurement of the temperature dependence of the chemical shielding of the reference substance itself. Such data have previously been unavailable due to lack of a single reference substance whose temperature dependence is absolutely known. We have measured the 129Xe resonance frequency in xenon gas extrapolated to zero density using six external references ( 1H in TMS and in CH 3 groups of p-xylene, 19F in C 6F 6, in CFCl 3, in 1,4-ditrifluoromethylbenzene and in 1,4-dibromotetrafluorobenzene). Because of the nature of the isolated xenon atom it should have a rigorously temperature-independent chemical shielding. Hence the measured temperature dependence is due entirely to the external reference used. The temperature dependent chemical shifts of the reference nuclei obtained in this manner were found to be internally self-consistent, agreeing with independent measurements of pairs of the neat liquids in concentric capillary samples. The latter measurements provide evidence that the absolute temperature dependence of any other reference substance can be obtained from the absolute temperature dependence of shifts reported here by making similar simple measurements of new references against these six.