. Can. J. Chem. 52,2143Chem. 52, (1974. The electric field gradient q has been calculated from Hartree-Fock wavefunctions for the molecules LiF, LiCI, LiBr, NaF, NaCI, NaBr, KF, and KCI. It is found that o and TC electrons behave quite differently. Exchange distortions are of great importance for a proper understanding of o contributions to q. Polarizations of n electrons into the bonding region are also considerable. These effects are responsible for failures of classical models and are further discussed using calculated density difference diagrams. Chem. 52, 2143Chem. 52, (1974. Le present article rapporte les resultats de calculs ab irzitio du gradient de champ electrique y pour la strie de molecules ioniques-type LiF, LiCI, LiBr, NaF, NaCI, NaBr, K F et KCI. Ces gradients ont ete obtenus a I'aide de fonction d'onde de type Hartree-Fock. Ces rksultats demontrent clairement que les electrons o et TC se comportent d'une facon tres differente dans ces molecules. Les distorsions d'echange presentes se rtvelent aussi d'importance lnajeure pour une comprihension adequate de la contribution des electrons o a la valeur de q. La polarisation des electrons n dans la region de liaison est a~~s s i considerable. L'ensemble de ces effets s'avere donc responsable de la faillite des modeles classiques pour I'interpretation de q. La disc~~ssion est appuyee des diagrammes de difference de densite correspondants. IntroductisnThe theoretical interpretation of the electric field gradient q in a molecule is of interest as it is a direct measure of the charge distribution. This follows from the fact that the interaction e2qQ, called the quadrupole coupling constant, between a nuclear quadrupole moment Q and the field gradient q of a molecular charge distribution p, is a weak interaction such that first-order perturbation theory is adequate (1). Experimentally eZgQ is determined from various hyperfine structures in resonance experiments of the microwave type (l,2). The field gradient q is also an intrinsic part of the theoretical expression for the force constant of a diatomic molecule (3).In this article we examine various models used to interpret q in ionic molecules, in particular, the alkali halides. Considerable theoretical work (4) has been done using a simple model based on ions with polarizable charge distributions. The problem then becomes the perturbation of an ion with a nuclear quadrupole moment by the field of an external point charge. The expression for the field gradient of simple ions with closed-shells configurations appears as 'NRCC Postgraduate Fellow. where superscripts f indicate positive and negative ions correspondingly. The factor y,' represents the effect of the deformation of the closed shells of the ion by the external charge on the value of q. If the distorted electron distribution opposes the field gradient of the external charge, then ymi is positive and there is a skieldingeffect, while if the distorted distribution enhances the field gradient, then y , ' is negative and there is an antishieldin...

A general theory of spectroscopic interferences for predissociating systems is developed using scattering theory. The introduction of complex bound states with complex eigenvalues results in a simple method for the derivation of total absorption, predissociation, and photon scattering (Raman) cross sections. The validity of other formalisms for these interferences, including Fano lineshapes, is examined and cases of spectroscopic interest are pointed out.