Abstract
The 129Xe chemical shift in an aqueous solution exhibits a non-monotonic temperature dependence, featuring a maximum at 311 K. This is in contrast to most liquids, where the monotonic decrease of the shift follows that of liquid density. In particular, the shift maximum in water occurs at a higher temperature than that of the maximum density. We replicate this behaviour qualitatively via a molecular dynamics simulation and computing the 129Xe chemical shift for snapshots of the simulation trajectory. We also construct a semianalytical model, in which the Xe atom occupies a cavity constituted by a spherical water shell, consisting of an even distribution of solvent molecules. The temperature dependence of the shift is seen to result from a product of the decreasing local water density and an increasing term corresponding to the energetics of the Xe-H2O collisions. The latter moves the chemical shift maximum up in temperature, as compared to the density maximum. In water, the computed temperature of the shift maximum is found to be sensitive to both the details of the binary chemical shift function and the coordination number. This work suggests that, material parameters allowing, the maximum should be exhibited by other liquids, too.
Highlights
Nuclear magnetic resonance (NMR) spectroscopy using 129Xe is commonly practiced to investigate many different materials such as gases[1], liquids[2,3,4], liquid crystals[5,6], and porous solids[7,8,9,10]
We describe the system from first principles, by simulating the molecular dynamics using the AMOEBA water model[31]
We described the system with a semianalytical model, in which the solvated Xe atom was placed in a water cavity
Summary
Nuclear magnetic resonance (NMR) spectroscopy using 129Xe is commonly practiced to investigate many different materials such as gases[1], liquids[2,3,4], liquid crystals[5,6], and porous solids[7,8,9,10]. An analytical empirical model by Lounila et al.[16] relates the local density of the solvent medium to the temperature dependence of the xenon shift in the different phases of thermotropic, uniaxial liquid crystals. This model reproduces the linear behavior of the shift in the normal, isotropic liquid phase. Is a direct proportionality of the Xe shift to the number of nearest-neighbor atoms or molecules This may be viewed as resulting from the increasing availability of vacant electronic states[27,28], through which the negative, so-called paramagnetic shielding contributions[29] (positive chemical shift contributions) may operate. We parameterized the cavity model entirely empirically by demanding that the experimental results are reproduced
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