Nuclear quadrupole resonance

(physics) The phenomenon in which certain nuclei in a static, inhomogeneous electric field absorb energy from a radio-frequency field.

A selective absorption phenomenon observable in a wide variety of polycrystalline compounds containing nonspherical atomic nuclei when placed in a magnetic radio-frequency field. Nuclear quadrupole resonance (NQR) is very similar to nuclear magnetic resonance (NMR), and was originated as an inexpensive (no stable homogeneous large magnetic field is required) alternative way to study nuclear moments. It later gained a modest popularity. See also Magnetic resonance; Nuclear magnetic resonance (NMR).

In the simplest case, for example, ³⁵Cl in solid Cl₂, NQR is associated with the precession of the angular momentum I (and the nuclear magnetic dipole moment μ) of the nucleus, depicted in the illustration as a flat ellipsoid of rotation, around the symmetry axis (taken as the z axis) of the Cl₂ molecule fixed in the crystalline solid. The precession, with constant angle θ between the nuclear axis and symmetry axis of the molecule, is due to the torque which the inhomogeneous molecular electric field exerts on the nucleus of electric quadrupole moment eQ. The absorption occurs classically when the frequency of the rf field and that of the precessing motion of the angular momentum coincide.

Interaction of ³⁵CI nucleus with the electric field of a Cl₂ molecule.

NQR spectra have been observed in the approximate range 1–1000 MHz. Most of the NQR work has been on molecular crystals. For such crystals the coupling constants found do not differ very much from those measured for the isolated molecules in microwave spectroscopy. The most precise nuclear information which may be extracted from NQR data are quadrupole moment ratios of isotopes of the same element. If values for the axial gradient of the molecular electric field can be estimated from atomic fine structure data, then fair values of the quadrupole moment may be obtained. However, it has also proved very productive to use the quadrupole nucleus as a probe of bond character and orientation and crystalline electric fields and lattice sites, and extensive data have been accumulated in this area. See also Microwave spectroscopy.