Nuclear magnetic resonance solvent shifts of xenon. A test of the reaction field model

Abstract

The medium shift of 129Xe was measured in a number of liquid alkanes, monosubstituted benzenes, and a set of ten test solvents where the magnetic anisotropy is negligible. The shifts arise from van der Waals shielding, but in the test solvents they do not correlate well with the predicted reaction field. A good correlation is found for the n-alkanes where the repulsive part of the shift is assumed constant. Branched alkanes show positive deviations which are ascribed to enhanced repulsive interactions. Shifts in monosubstituted benzenes correlate well with prediction except for solvents with low-lying excited states which show negative deviations. It is concluded that the reaction field model is valid within a group of closely related solvents, but it fails when extended to collections of randomly selected solvents. IntroductionRecently we undertook an NMR study of the interaction of xenon with biological systems. In the course of this work we expect to utilize the chemical shift of lBXe to determine the locus of xenon when it is present in complex systems such as cell membrane suspensions. For this work, it is important to have an understanding of the factors responsible for xenon shifts in the absence of specific interactions. In order to elucidate the mechanism of the nonspecific medium shift, we measured the 129Xe shift in a number of simple solvents, and we have attempted to explain the results in terms of a reaction field model.The magnetic shielding of a molecule dissolved in a solvent differs significantly from its value in the isolated state. Part of this difference is connected with the bulk magnetic susceptibility of the sample. The remaining

NMR chemical shifts of129Xe dissolved in liquid haloalkanes and their mixtures

“…As noted earlier, van der Waals contributions to solvent-induced chemical shifts are deshielding in nature so that ir is convenient to record such effects as the negative of the van der Waals shielding constant, -tr W, in columns 4 of Tables 1 and 2. Inspection of the chemical shifts and van der Waals shielding constants listed in Tables 1 and 2 reveals that the values of 6(-cr) and -cr w measured here agree well with the meager data available in the literature for similar systems [15][16][17]. A comparison of -cr w with -tr b clearly shows that van der Waals deshielding dominates bulk susceptibility contributions in the solvent-induced chemical shifts observed with 129Xe and the solvents of interest here.…”

Solvent-induced H/D isotope effect on129Xe chemical shifts

“…This problem is associated with the diversity of manifestation of solute-solvent interaction as well as with significant distinction in properties of solvents. Existing models of the Van der Waals contribution to shielding constants cannot explain the overall picture of 129Xe solvent shifts [1][2][3][4][5][6][7][8][9][10][11][12]. We also found that there is no satisfactory description of these shifts in the frame of multifactor solvent scales derived from data for media effect on the number of spectral and physicochemical properties of substances by means of the factor analysis of representative data matrix [13].…”

“…The transition of a xenon atom from the gaseous state into liquid solution is accompanied by strong deshielding of up to 300 ppm. This property is one of the most important reasons for numerous investigations of intermolecular interactions by 129Xe NMR method [1][2][3][4][5][6][7][8][9]. In solutions, the 129Xe isotope with natural abundance of 26.4% and spin 1/2 gives easily detectable narrow single lines.…”

“…Nevertheless, some homologous series of solvents with closely related structure reveal characteristic regularity in the variation of shielding of dissolved 129Xe. For instance, an effect of solvent on xenon shifts was found to be explained in terms of Van der Waals interactions for sets of unbranched alkanes [2,4,6,9], alcohols [3,8], amines [8] as well as for some liquefied gases [5]. We thought in this connection that finer transformation of solvent properties would provide us with additional information about the nature of the interaction of xenon atoms with the solvent.…”

129Xe NMR chemical shift in Xe@C60 calculated at experimental conditions: Essential role of the relativity, dynamics, and explicit solvent

“…[53] The dynamically averaged isotropic 129 Xe shift for Xe atom dissolved in benzene at 300 K was calculated to be 191.4 6 2.5 ppm, reaching the excellent agreement with previously reported experimental data ranging from 188 to 195 ppm. [51,54,55] The models used in Refs. [52,53] are connected in this article to study the Xe@C 60 molecule dissolved in liquid benzene.…”