Nuclear quadrupole resonance
The NQR resonance is mediated by the interaction of the electric field gradient (EFG) with the quadrupole moment of the nuclear charge distribution.
It is this product which is termed the nuclear quadrupole coupling constant for a given isotope in a material and can be found in tables of known NQR transitions.
In NMR, an analogous but not identical phenomenon is the coupling constant, which is also the result of an internuclear interaction between nuclei in the analyte.
Any nucleus with more than one unpaired nuclear particle (protons or neutrons) will have a charge distribution which results in an electric quadrupole moment.
Allowed nuclear energy levels are shifted unequally due to the interaction of the nuclear charge with an electric field gradient supplied by the non-uniform distribution of electron density (e.g. from bonding electrons) and/or surrounding ions.
Unlike the NMR case, NQR absorption takes place in the absence of an external magnetic field.
Application of an external static field to a quadrupolar nucleus splits the quadrupole levels by the energy predicted from the Zeeman interaction.
Due to symmetry, the shifts become averaged to zero in the liquid phase, so NQR spectra can only be measured for solids.
As such it is a measure of the degree to which the nuclear charge distribution deviates from that of a sphere; that is, the prolate or oblate shape of the nucleus.
NQR is a direct observation of the interaction of the quadrupole moment with the local electric field gradient (EFG) created by the electronic structure of its environment.
The NQR transition frequencies are proportional to the product of the electric quadrupole moment of the nucleus and a measure of the strength of the local EFG:
just as the NMR experimenter is free to choose the Larmor frequency by adjusting the magnetic field.
This potential may be produced by the electrons as stated above, whose probability distribution might be non-isotropic in general.
This method corresponds to the multipole expansion in cartesian coordinates (note that the equations below use the Einstein sum-convention):
This leads to a simplification because the equation for the potential energy now contains only the second derivatives in respect to the same variable:
The remaining terms in the integral are related to the charge distribution and hence the quadrupole moment.
NQR probes the interaction between the nuclear quadrupole moment and the electric field gradient at the nucleus.
Such sensitivity makes NQR spectroscopy a useful method for the study of bonding, structural features, phase transitions, and molecular dynamics in solid-state compounds.
More specifically, the application of 14N-NQR has allowed for the differentiation of enantiomeric compounds from racemic mixtures; namely in, D-serine and L-serine.
On one hand, D-serine is a potential biomarker for Alzheimer’s disease as well as a treatment for schizophrenia.
L-serine, on the other hand, is a drug undergoing FDA-approved human clinical trials due to its potential in treating amyotrophic lateral sclerosis.
Differences in NQR frequencies, along with the quadrupole coupling constants and asymmetry parameters, allow differentiation between polymorphs as can be done with enantiomeric compounds.
[3] Distinguishing between polymorphs in such a manner makes NQR a powerful tool for authenticating drugs against counterfeits.
Units designed to detect landmines[8] and explosives concealed in luggage have been tested.
A detection system consists of a radio frequency (RF) power source, a coil to produce the magnetic excitation field and a detector circuit which monitors for a RF NQR response coming from the explosive component of the object.
A fake device known as the ADE 651 claimed to exploit NQR to detect explosives but in fact could do no such thing.
Nonetheless, the device was successfully sold for millions to dozens of countries, including the government of Iraq.
NQR requires the presence of a non-zero quadrupole moment, which is only observed in nuclei with a nuclear spin greater than or equal to one (I ≥ 1) and whose local charge distribution deviates from spherical symmetry.
[10][11][1] NQR requires fairly large sample sizes due to the signals being of very low intensity.
[2][3] This poses experimental obstacles due to a large majority of NQR-active nuclei having low isotopic abundances.
