Electric Field Gradients in Alkali Halide Molecules

Abstract

. 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...

XVI. Representations of the Electron Density and its Topographical Features

“…10 for LiF. 36 We note that in the case of LiH, Politzer and Brown" found very little difference between li.p"" and li.p p , while Banyard et al" found them to be quite different.…”

Multinuclear Residual Quadrupolar Couplings for Structure and Assignment

“…[87] Quantitative use of 7 Li RQCs for structure determination is challenging as -unlike in the case of 2 H -most lithium compounds do not contain more than 1-2 independent 7 Li sites, [88] and the sign of Δν Q is usually not accessible due to absence of resolved 7 Li scalar or dipolar couplings. Recently we proposed a method for structure verification of lithium compounds with rigid organic ligands entailing the following steps: (i) measurement of ligand 13 C, 1 H RDCs and independent determination of A, (ii) construction of model structures with optimised geometry and calculation of V using DFT and (iii) calculation of Δν Q for each model according to Equation 3 and ranking of the structures by comparison to the experimental 7 Li RQC. [89] The method was employed to determine the solvation of lithium chelated by monoanionic bis(benzoxazole-2-yl), bis-(benzothiazole-2-yl) and bis(pyrid-2-yl) methanide ligands in THF-d 8 (Figure 6).…”

“…In bonded hydrogen atoms, for example, the 1 s electron can be regarded as spherically symmetric, and the nuclear EFG -and hence the 2 H quadrupole coupling -depends mainly on the effective charge and electronic structure of the direct neighbour. [11] For highly ionic lattices -such as alkali or heavier earthalkali halides -point charges placed at the position of the neighbouring anions give a reasonable qualitative picture of the cationic nuclear EFG, [12] a picture that may be also valid for molecular complexes of these metals with electronegative ligands: [13] Figure 2a shows the lithium EFG of the monomeric lithium amide Li[N(SiMe 3 ) 2 ], where a point charge calculation with a full negative charge on N at a distance of 1.774 Å (3.35 a. u.)…”

“…NMR spectroscopy is one of the key methods in chemistry to determine the structure and dynamics of compounds and materials, ranging from small organic and organometallic molecules via synthetic and biological polymers to minerals, glasses and other solids. While most small molecule routine NMR spectroscopy is done on nuclei with a nuclear spin (I) of 1/2 (e. g. 1 H, 13 C, 15 N, 19 F, 31 P), only 32 of all 124 stable or longlived magnetic nuclei have this property. [1] The remaining majority of 92 -among them many important elements like oxygen, halides and most metals -are quadrupolar nuclei with I > 1/2.…”

“…This information may also be exploited for well-defined molecular species under solution-like conditions and using conventional solution NMR methods in the form of residual quadrupolar couplings (RQCs). Like residual dipolar couplings (RDCs) and residual chemical shift anisotropies (RCSAs) that are well established for spin-1/2 nuclei such as 1 H, 13 C and 15 N, the measurement of RQCs requires weakly orienting media such as liquid crystals (LCs) or uniaxially stretched/compressed gels.…”