Simultaneous Dielectric and NMR Spectroscopy Unveiled

A recent study reveals how nuclear magnetic resonance (NMR) spectrometers can perform dielectric and NMR spectroscopy simultaneously, representing a shift in the capabilities of NIR spectrometers according to Analytical Chemistry (1).

Nuclear magnetic resonance (NMR) spectroscopy is a powerful analytical technique used to study the structure, dynamics, and composition of molecules and organic compounds (2). It relies on the interaction between the magnetic properties of atomic nuclei and an external magnetic field. When subjected to radio frequency radiation in the presence of this field, nuclei absorb and emit electromagnetic radiation at characteristic frequencies, providing valuable information about their chemical environment (2). By analyzing the resulting NMR spectra, researchers can elucidate molecular structures, determine the purity and concentration of compounds, and investigate molecular interactions in various fields ranging from chemistry and biochemistry to medicine and materials science (2).

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Recently, a research team, led by Eric Breynaert from the Centre for Surface Chemistry and Catalysis − Characterization and Application Team (COK-KAT), explored how to advance NMR spectroscopy further by presenting and describing a novel methodology that allows NMR spectrometers to perform both dielectric and NMR spectroscopy simultaneously (1). The innovation of their method lies in the incorporation of a dielectric material into the NMR probe head, which alters the frequency response of the probe circuit, a phenomenon known as probe detuning (1).

Visualization of a radio signal | Image Credit: © AndSus - stock.adobe.com

Traditionally, probe detuning has been used to reduce radiation damping to combat the signal-to-noise ratio (3). Normally, detuning is corrected prior to NMR measurements. However, Breynaert and his team recognized that the magnitude of this detuning, termed the dielectric shift, offered valuable insight into the dielectric properties of the sample (1).

Therefore, by measuring the sample's dielectric permittivity as a function of frequency, the team successfully conducted dielectric permittivity spectroscopy using commercial cross-polarization magic angle spinning (CPMAS) NMR probe heads (1). Their proof of concept involved evaluating various alcohols, obtaining results that closely matched data obtained through standard dielectric spectroscopy techniques (1).

The researchers also tested their methodology by investigating solvent–surface interactions. They analyzed the behavior of water confined in the micropores of an MFI-type, hydrophilic zeolite (1). In these micropores, water adsorbs to Bro̷nsted acid sites and defect sites, leading to a significant decrease in dielectric permittivity (1). This observation opens new avenues for studying nanoconfined fluids and their interactions with solid surfaces.

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The team also provided theoretical underpinnings for their methodology. They used an electric circuit model of a CPMAS probe head with a solenoid coil to describe the detuning caused by dielectric samples' insertion (1).

The researchers demonstrated in their study that NMR spectroscopy could see its capabilities increase. Their approach allowed them to gain simultaneous insights into both the chemical and dielectric properties of samples (1). Such advancements hold promise for a wide range of fields, from chemistry and materials science to pharmaceuticals and beyond, where NMR spectroscopy plays a pivotal role in analysis and research.

References

(1) Morais, A. F.; Radhakrishnan, S.; Arbiv, G.; et al. Noncontact In Situ Multidiagnostic NMR/Dielectric Spectroscopy. Anal. Chem. 2024, 96 (13), 5071–5077. DOI: 10.1021/acs.analchem.3c03007

(2) Michigan State University, Nuclear Magnetic Resonance Spectroscopy. MSU.edu. Available at: https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/spectrpy/nmr/nmr1.htm (accessed 2024-05-13).

(3) Krishnan, V. V.; Murali, N. Radiation Damping in Modern NMR Experiments: Progress and Challenges. Prog. Nucl. Magn. Reson. 2013, 68, 41–57. DOI: 10.1016/j.pnmrs.2012.06.001