We report (129)Xe NMR experiments showing that a Fe4L6 metallosupramolecular cage can encapsulate xenon in water with a binding constant of 16 M(-1). The observations pave the way for exploiting metallosupramolecular cages as economical means to extract rare gases as well as (129)Xe NMR-based bio-, pH, and temperature sensors. Xe in the Fe4L6 cage has an unusual chemical shift downfield from free Xe in water. The exchange rate between the encapsulated and free Xe was determined to be about 10 Hz, potentially allowing signal amplification via chemical exchange saturation transfer. Computational treatment showed that dynamical effects of Xe motion as well as relativistic effects have significant contributions to the chemical shift of Xe in the cage and enabled the replication of the observed linear temperature dependence of the shift.

Twist-bend nematic phase of the liquid crystal dimer CB7CB: orientational order and conical angle determined by 129 Xe and 2 H NMR spectroscopy, Liquid Crystals, 42:5-6, 708-721,The liquid crystal dimer 1ʺ,7ʺ-bis(4-cyanobiphenyl-4ʹ-yl) heptane (CB7CB) has been shown to possess a nematicnematic phase transition at 376 K. The phase below this temperature has been identified as a globally uniaxial twistbend nematic phase, N TB . Within the temperature range 376-388 K, a classic uniaxial nematic phase, N, appears. The N TB phase has a helical structure and the liquid crystal director, n, is tilted with respect to the helix axis; these are formed into a conglomerate of degenerate domains having opposite handedness. In a magnetic field, the helix axis orients along the field. In the present studies, the properties of CB7CB are investigated using 129 Xe NMR spectroscopy of dissolved xenon and 2 H NMR of CB7CB-d 4 and the probe 4-octyl-4ʹ-cyanobiphenyl-d 2 also dissolved in CB7CB. In a uniaxial liquid crystalline environment, the 129 Xe shielding tensor is cylindrically symmetric due to the deformation of the electron cloud by anisotropic forces associated with the director. The anisotropic part of the shielding tensor depends upon the orientational order parameter of the liquid crystal with respect to the applied magnetic field and the conical (aka tilt) angle of the director. The temperature dependence of the orientational order parameter and of the conical angle has been determined independently from 129 Xe and previous 2 H NMR experiments. In the N TB phase, the averaged Saupe ordering matrix contains three off-diagonal elements. The temperature dependence of one of these, resulting from the phase chirality, has been determined from the 2 H quadrupolar splittings.

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.

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