Determination of self-diffusion coefficients in mixtures with benchtop 13C NMR spectroscopy via polarization transfer

Abstract

Nuclear magnetic resonance (NMR) is an established method to determine self-diffusion coefficients in liquids with high precision. The development of benchtop NMR spectrometers makes the method accessible to a wider community. In most cases, ¹H NMR spectroscopy is used to determine self-diffusion coefficients due to its high sensitivity. However, especially when using benchtop NMR spectrometers for the investigation of complex mixtures, the signals in ¹H NMR spectra can overlap, hindering the precise determination of self-diffusion coefficients. In ¹³C NMR spectroscopy, the signals of different compounds are generally well resolved. However, the sensitivity of ¹³C NMR is significantly lower than that of ¹H NMR spectroscopy leading to very long measurement times, which makes diffusion coefficient measurements based on ¹³C NMR practically infeasible with benchtop NMR spectrometers. To circumvent this problem, we have combined two known pulse sequences, one for polarization transfer from ¹H to the ¹³C nuclei (PENDANT) and one for the measurement of diffusion coefficients (PFG). The new method (PENPFG) was used to measure the self-diffusion coefficients of three pure solvents (acetonitrile, ethanol and 1-propanol) as well as in all their binary mixtures and the ternary mixture at various compositions. For comparison, also measurements of the same systems were carried out with a standard PFG-NMR routine on a high-field NMR instrument. The results are in good agreement and show that PENPFG is a useful tool for the measurement of the absolute value of the self-diffusion coefficients in complex liquid mixtures with benchtop NMR spectrometers.