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Electric field gradient and nuclear quadrupole moment: How do these two determine the properties of matter?
Nuclear quadrupole resonance spectroscopy (NQR) is a chemical analysis technique that is closely related to nuclear magnetic resonance (NMR). Unlike NMR, NQR's nuclear resonance can be detected without an external magnetic field, so NQR spectroscopy is also called "zero-field NMR". The resonance of NQR is mediated by the interaction between the electric field gradient (EFG) and the quadrupole moment of the nuclear charge distribution. In contrast to NMR, NQR is applicable only to solids and not to liquids, because in liquids the electric field gradient near the nuclei averages out to zero and the EFG tensor has a zero trace. Since the EFG of a particular nucleus's location in a substance is determined primarily by the valence electrons that participate in specific bonds with other nearby nuclei, the NQR frequency of the transition is unique in that substance.
The frequency of NQR in a particular compound or crystal is proportional to the product of the nuclear quadrupole moment (a property of the nucleus) and the EFG near the nucleus.
A similar, but not identical, phenomenon in NMR is the coupling constant, also a result of internuclear interactions in the analyte. Any nucleus with more than one unpaired nuclear particle (proton or neutron) will have a charge distribution that results in an electric quadrupole moment. The allowed nuclear energy levels shift unevenly due to the interaction of the nuclear charge with the electric field gradient provided by the uneven distribution of electron density.
Energy directed toward the nucleus via electromagnetic radiation may cause the nucleus to absorb some energy, which can be seen as a disturbance in the quadrupole energy levels. Unlike the case of NMR, absorption in NQR occurs in the absence of an external magnetic field. Applying an external static field to the quadrupole nuclei will split the quadrupole energy levels according to the Zeeman interaction.
NQR technology is very sensitive to the nature and symmetry of bonding around the core and can characterize phase changes in solids at different temperatures.
Due to symmetry, these shifts average out to zero in the liquid phase, so NQR spectra can only be measured in the solid phase. In the case of NMR, nuclei with spin ≥ 1/2 possess a magnetic dipole moment, so that their energy can be partitioned by the magnetic field, resulting in a resonant absorption of energy related to the Larmor frequency. In the case of NQR, nuclei with spin ≥ 1, such as 14N, 17O, 35Cl and 63Cu, also have electric quadrupole moments. The nuclear quadrupole moment is related to the non-spherical nuclear charge distribution, which indicates the degree to which the nuclear charge distribution deviates from a spherical shape, that is, the ellipsoid or disk shape of the nucleus.
NQR is a direct observation of the interaction between the quadrupole moment and the local electric field gradient (EFG) of its environment. The NQR transition frequency is proportional to the electric quadrupole moment of the nucleus and the strength of the local EFG. However, in solids, the strength of the EFG reaches several kilovolts per meter square, so it is not feasible to perform NQR experiments with a specific EFG, as in NMR, by selecting the external magnetic field.
NQR spectra are specific to a substance and are therefore called "chemical fingerprints."
Due to the strong dependence of NQR frequency on temperature, NQR can be used as an accurate temperature sensor with a resolution of up to 10^−4 °C. The application of NQR spectrum also has broad prospects and has great potential to play a role in the pharmaceutical industry. In particular, the application of 14N-NQR enables the discrimination of enantiomeric compounds in racemic mixtures, such as D-serine and L-serine. Although these two compounds have similar compositions, they have completely different properties. D-serine may become a biomarker for Alzheimer's disease and a drug for the treatment of schizophrenia, while L-serine is a drug that is undergoing FDA human A drug in clinical trials known for its potential to treat amyotrophic lateral sclerosis.
NQR also has the ability to distinguish crystalline polymorphs. For example, compounds containing sulfonamide drugs have shown sensitivity to polymorphism. The difference in NQR frequency, as well as the differences in quadrupole coupling constants and asymmetry parameters, enables the differentiation of polymorphs, an ability that makes NQR a powerful tool for drug authentication and combating counterfeiting.
Multiple research teams around the world are working to develop NQR technology to detect explosives. Equipment designed to detect mines and explosives hidden in luggage has been tested. Such a detection system consists of a radio frequency (RF) power source, a coil that generates a magnetic excitation field, and detection circuitry that monitors the RF NQR response of the explosive. . Even a fake device called ADE 651 claims to use NQR to detect explosives, but it can't actually do that.
Nevertheless, the device was successfully sold for millions of dollars to dozens of countries, including the Iraqi government.
The main limitation of NQR comes from the abundance of isotopes. NQR requires the presence of a non-zero quadrupole moment, which is only observed in nuclei with spin greater than or equal to one and whose local charge distribution deviates from spherical symmetry. Although existing NQR techniques have low signal intensity due to low isotopic abundance of most NQR-active nuclei, NQR spectroscopy still shows its utility in many practical scenarios.
Faced with the infinite possibilities of NQR, can we break through the existing technological limitations in the future and allow this technology to play a greater role in more fields?
