Nuclear Quadrupole Coupling Constant

Definition and meaning of Nuclear Quadrupole Coupling Constant in chemistry.

The nuclear quadrupole coupling constant is a parameter that quantifies the interaction between the electric quadrupole moment of an atomic nucleus and the electric field gradient surrounding it. It is expressed in frequency units (typically kilohertz to megahertz, depending on the nucleus) and determines important spectroscopic properties of nuclei with non-spherical charge distributions.

In more detail

This constant is relevant only for nuclei with nuclear spin quantum number I greater than 1/2, which possess a non-spherical nuclear charge distribution. The coupling constant depends on both the nuclear quadrupole moment Q and the electric field gradient (EFG) created by electrons and neighboring atoms. It is crucial in nuclear quadrupole resonance (NQR) spectroscopy and significantly affects NMR line widths and splittings, providing detailed information about the electronic environment and chemical bonding around the nucleus. The magnitude of the coupling reveals the asymmetry of the charge distribution surrounding the nucleus. Because the size of the quadrupole moment varies enormously between nuclei, coupling constants span a wide range: deuterium (2H) has an unusually small quadrupole moment and correspondingly small coupling constants (tens to hundreds of kilohertz), whereas nuclei such as 35Cl or 14N typically show coupling constants of several to over a hundred megahertz.