Intermolecular interactions in nuclear magnetic resonance. X. A site-specific continuum model for the gas-to-liquid shifts of nonpolar solutes. Applications to proton medium shifts and the determination of cavity radii

Abstract

. RUMMENS. Can. J. Chem. 54, 254 (1976). Based on the Onsager reaction field theory of the dielectric polarization of a continuum, a-model has been developed to calculate the Van der Waals contribution U , to the gas-to-liquid medium shifts of nonpolar solutes. The addition of a site factor correction, to account for the off-center location of the measured nucleus in the solute molecule, provides a marked improvement. The agreement between experimental and calculated u,,~'s is good (k0.05 ppm), but a separate scale factor is required for each solute. This nonuniversality is shown to be related to the Onsager approximation for the solute cavity radius; a3 = 3VI/4rN2,. It is shown that Van der Waals shifts can be used for the determination of the Onsager cavity radii; for a number of molecules these cavity radii are given. It is claimed that this method is as precise, but much easier to apply, than the Bottcher-Onsager optical method.No evidence could be found for the existence of a u,? contribution to the medium shifts due to permanent electric dipoles of solvent molecules. Combination of experimental medium shifts in anisotropic solvents with calculated U , contributions allows the evaluation of the neighbor anisotropy contribution u,. The U , effect is found to be solute dependent; there is a V1-1/6 proportionality related to the molar volume Vl of the solute and in addition there are site effects with site factors both greater and less than unity, depending on molecular shape. The solvent dependence of U , includes not only the magnetic susceptibility anisotropy ax2 and the molar volume (V2-I/3) but also a term corresponding to the geometric anisotropy of the solvent molecule.The solvent CS2 is found to behave as an isotropic molecule.FRANS H. A. KUMMENS. Can. J. Chem. 54, 254 (1976). Fond6 sur la thCorie du champ de reaction d'onsager, un rnodele a CtC dCveloppe perrnettant le calcul de la contribution Van der Waals u , , sur les diplacements chirniques des solutCs nonpolaires en passant de1'Ctat gazeux B 1'Ctat liquide. L'iiiclusion d'un facteur de correction de site, tenant compte de la position non-centrale du noyeau considirk, apporte une arnilioration sensible. L'accord entre les U , calculCs et les U , observes est bon ( k0.05 ppm), mais une constante de multiplication, differente pour chaque solutC, est nicessaire. 11 a CtC dCmontrC que ce manque &universalit6 est cause par l'approximation d'onsager pour le rayon de la cavitC du solutC; a3 = 3 VI/4rNJ,. I1 est dCmontrC de plus que les dkplacernents chimiques Van der Waals peuvent &tre utilisCs pour la dktermination des rayons de cavitC Onsager; ces rayons de cavitC ont CtC donnCs pour un certain nornbre de molCcules. I1 est tenu pour certain que cette methode est aussi prCcise, rnais plus facile B utiliser, que la rnCthode optique de Bottcher-Onsager.11 n'a pas CtC possible de trouver une preuve quelconque en faveur ou contre l'existence d'une contribution u,? aux deplacernents chimiques de milieu due aux moments Clectriques permanents des rnolCcules d...

Reaction field model with free volume for the NMR chemical shift of129Xe dissolved in organic solvents

“…This reaction produces a field -the reaction field -at the solute, the average square of which is non-zero. This field, which is the source of the van der Waals dispersion forces, is equated with F. In principle B depends only on the solute, however, in practice a solvent dependence has been observed [27].…”

“…Following notation of Rummens [1,27], the subscripts 1 and 2 will denote the solute xenon and the solvent respectively. Then ty w can be written as [11]:…”

“…The sensitivity of the large 129Xe chemical shift to the physical environment of the atom makes xenon NMR a stringent test of solute/solvent interaction models. Here a theory for the temperature dependence of the chemical shift of solutes in solvents is developed as an extension of the reaction field theory of Onsager [26] as developed by Rummens [10,11,27]. The present theory incorporates the concept of free volume in liquids.…”

“…(1)Here o-a is the contribution from the magnetic anisotropy of the solvent, o'e2 is the shielding from the permanent dipole moment of the solvent, cr e is the reaction field of the solvent due to the permanent dipole moment of the solute, tr w comes from the dispersion term of the Van der Waals interaction, and Crup is from the repulsive portion of the Van der Waals interaction. Rummens[27,29] showed that for many polar solvents the o'e2 contribution is negligible. Since the local dipole moments in the polymers studied here are relatively weak, this contribution will be neglected, cr e is zero in this case because xenon has no permanent dipole moment.…”

Nuclear magnetic resonance studies of domain growth in the late stage of phase separation of a binary liquid mixture

“…(A) ϭϪ0.067 ppm ͑for the N-methylaniline͒ were taken from the literature 31. It appears that over the entire composition range, the variation of the 1 H NMR chemical shift of cyclohexane in a one-phase C/A mixture can be accurately described assuming the additivity of the solvent effects of each component ͑seeFig.…”