Non-contact in situ multi-diagnostic NMR/dielectric spectroscopy

Abstract

Introduction of a dielectric material in an NMR probe head modifies the frequency response of the probe circuit, a phenomenon revealed by the detuning of the probe. For NMR spectroscopy, this detuning is corrected for by tuning and matching the probe head prior to the NMR measurement. The magnitude of the probe detuning - the dielectric shift - provides direct access to the dielectric properties of the sample, enabling NMR spectrometers to simultaneously perform both dielectric and NMR spectroscopy. By measuring sample permittivity as function of frequency, permittivity spectroscopy can be performed using the new methodology. As a proof concept, this was evaluated on methanol, ethanol, 1-propanol, 1-pentanol and 1-octanol using a commercial CPMAS NMR probe head. The results accurately match literature data collected by standard dielectric spectroscopy techniques. Subsequently, the method was also applied to investigate the solvent-surface interactions of water confined in the micropores of an MFI-type, hydrophilic zeolite with Si/Al ratio of 11.5. In the micropores, water adsorbs to Br{\o}nsted acid sites and defect sites, resulting in a drastically decreased dielectric permittivity of the nano-confined water. A theoretical background for the new methodology is provided using an effective electric circuit model of a CPMAS probe head with solenoid coil, describing the detuning resulting from insertion of dielectric samples in the probe head.