¹²⁹Xe Nuclear Shielding and Diffusion in the A and C* Phases of a Chiral Smectogen

Abstract

The 129Xe NMR shielding and diffusion of xenon dissolved in the chiral liquid-crystal 1-methylheptyl 4‘-(4-n-decyloxybenzoyloxy)biphenyl-4-carboxylate (10B1M7) were studied over the temperature range covering the isotropic (I), smectic A (SmA), and ferroelectric, ferrielectric, and antiferroelectric smectic C*(SmC*) phases. The 129Xe shielding reveals clearly the I−SmA and SmA−SmC*(ferroelectric) phase transitions. The SmC* sub-phase transitions can also be detected by smaller but distinguishable shifts of the shielding accompanied by line width changes. A theoretical model developed earlier is applied to the shielding data, confirming a negative anisotropic contribution to the shielding as the SmA phase forms and allowing the evaluation of the tilt angle in the SmC*. Diffusion experiments were mostly performed in the direction parallel to the layer normal, i.e., along the external magnetic field, D ∥, in the smectic phases but a few experiments were carried out in the in-plane direction, i.e., perpendicular to the external magnetic field, D ⊥, as well. These experiments indicate large anisotropy, D ⊥/D ∥, of the 129Xe diffusion tensor that increases as the temperature decreases. Application of the Arrhenius equation to the temperature-dependence of D ∥ reveals different activation energies for each studied phase. This can be correlated to changes in the smectic layer structure that occur between the different smectic phases.

Cage-Like Local Structure of Ionic Liquids Revealed by a129Xe Chemical Shift

“…13,14 Among other gases, xenon solubility in several ILs has been measured experimentally over a range of temperatures, 15 and the phase behavior of binary systems of IL/gas (CO 2 , H 2 , and Xe) in selected imidazolium-based ILs has been investigated by modeling techniques. 16 Since the seminal papers of Fraissard 17 and Ripmeester, 18 NMR spectroscopy of 129 Xe has been applied to characterize a large variety of systems from simple liquids 19 to host molecules in solution, 20 from liquid crystals 21 to microporous 22−24 and mesoporous materials, 25 and from polymers 26 to hybrid materials 27 and biological fluids. 28 129 Xe NMR spectroscopy exploits the amplified shielding response of Xe atoms to its physicochemical surrounding, giving rise to large chemical shift variations induced by intermolecular interactions.…”

The role of NMR in the study of partially ordered materials: Perspectives and challenges

“…Just in the recent year, a new method based on a combination between PFG NMR and MAS NMR [147] and a multiquantum stimulated echo-based pulsed-gradient spin-echo sequence (MQ-PGSTE) have been developed [148]. 1 H NMR and 129 Xe NMR techniques have also been applied to diffusion measurements in the SmA, ferroelectric, and antiferroelectric smectic LC phases [149], giving rise to basic information useful for both theoretical and applicative points of view. 2D exchange 2 H NMR and line-shape analysis is applied to determine jump diffusion of molecules in mesophases with spatial discontinuities (i.e., ferrielectric and TGB phases) [150], while 1 H NMR relaxometry [41,115] successfully described molecular rotations mediated by translational displacements (see Scheme 1f) in chiral LC [151] and TGB phases [152].…”

Xenon NMR as a Probe for Microporous and Mesoporous Solids, Polymers, Liquid Crystals, Solutions, Flames, Proteins, Imaging

“…Additional information about TLCs can be obtained by applying a theoretical model according to which the 129 Xe shielding can be represented in the form [49]: (7) where ρ(T) is the LC density at temperature T, and the temperature dependence of the shielding constant and shielding anisotropy are assumed to be linear and are described by the coefficients ε and ∆ε, respectively [50]. In equation (7), σ 0 is the shielding constant of the reference, T 0 the reference temperature, σ d ' and ∆σ d ' are the temperatureindependent isotropic shielding constant and anisotropy of the shielding tensor (in ppm/g -1 cm 3 ), respectively.…”