Nuclear Quadrupole Coupling in Polar Molecules

The effect of an external charge on the nuclear electric quadrupole coupling has been investigated for various ions. These results presumably apply to the case of polar molecules. The quadrupole moment induced by the nuclear quadrupole moment $Q$ is $46.5Q$, $8.7Q$, $50.2Q$, and $86.8Q$ for the ${\mathrm{Cl}}^{-}$, ${\mathrm{Cu}}^{+}$, ${\mathrm{Rb}}^{+}$, and ${\mathrm{Cs}}^{+}$ ions, respectively. When an external charge interacts with the ion as in a polar molecule, the induced moment is added to $Q$, so that the quadrupole coupling is multiplied by a factor of order 10-100. This model of a polar molecule disregards the exchange repulsion between the ions. Evidence is presented from several molecules which seems to confirm the existence of the induced quadrupole moment, although there is only partial agreement with the experimental values. It is shown that the contribution to the quadrupole coupling due to the induced dipole moment in second order is small compared to the induced quadrupole effect.