A new type of sample tube for reducing convection effects in PGSE-NMR measurements of self-diffusion coefficients of liquid samples

Pulsed gradient spin-echo (PGSE) NMR measurements of the self-diffusion coefficients of low viscosity liquids are greatly hampered by the effects of convection especially away from ambient temperature. Here we report on a new NMR tube designed to minimize the deleterious effects of convection. In this tube, which derives from a Shigemi symmetrical NMR tube, the sample is contained in an annulus formed from a concentric cylinder of susceptibility matched glass. The performance of this tube was demonstrated by conducting measurements on the electrochemically important LiN(SO₃CF₃)₂ (LiTFSI)–diglyme (DG) system. Calibrations were first made using DG at column heights of 2, 3, and 4-mm in the temperature range between −40 and 100 °C. Measurements of the diffusion coefficients of the lithium, anion, and DG were then performed to probe the solvent–ion and ion–ion interactions in the DG doped with LiTFSI. Changes in the ¹H, ⁷Li, and ¹⁹F PGSE-NMR attenuation curves at −40 °C provided clear evidence of interactions between the DG and lithium ion.