Calculation of Nuclear Quadrupole Effects in Molecules

Methods are given for the computation of nuclear quadrupole interactions in molecules. They apply primarily to the interpretation of microwave spectra of linear and symmetric top molecules in which either one or two nuclei on the molecular axis show quadrupole coupling to the molecular field. Tables are given of the energy values, based on Casimir's formula, for the quadrupole coupling of a single nucleus, and also of transformation coefficients for application to the case of intermediate coupling when two nuclei are involved. Spectral intensities are discussed briefly. Examples are given to illustrate various aspects of the theory. The molecules BrCN and CICN show quadrupole effects resulting from the nitrogen nucleus as well as to the halogen. In the latter case, the intermediate coupling theory is required for a complete explanation of the observed microwave-absorption lines, as there are significant deviations from the first-order linear approximation.