Preferential solvation of carbohydrates in water-trifluoroethanol mixtures: a solvent detected heteronuclear NMR approach

The present study aims to establish a simple approach involving multi-field multinuclear longitudinal relaxation (R1) analysis of the solvents to decipher solute-solvent interactions during the solvation of model carbohydrates in aqueous trifluoroethanol (TFE) co-solvent systems (TFE:D2O). The behavior of D2O and TFE is monitored around β-CD (β-cyclodextrin) and glucose through R1D (2H) and R1F (19F), respectively. Correlation times (τc) are estimated for D2O and TFE for various % (v/v) compositions of TFE:D2O mixtures. The differential trends of the R1 or τc ratio for D2O and TFE (in the presence and absence of carbohydrates) revealed that both β-CD and glucose undergo selective solvation by TFE in comparison to D2O. Owing to its encapsulation properties, β-CD exhibited a comparatively higher tendency to undergo solvation by TFE than glucose. The maximum transfer of solute bound water to bulk solvent appears in the 20-30% (v/v) TFE range. The current approach emerges as being straightforward in contrast to traditional methods that primarily focus on solute behavior to unravel the preferential solvation dynamics.

Acetone mobility in zeolite cages with new features in the deuteron NMR spectra and relaxation

We studied deuteron NMR spectra and spin-lattice relaxation of deuterated acetone-d₆, adsorbed into zeolites NaX (1.3) and NaY(2.4) at 100% coverage of sodium cations. At temperatures roughly below 160 K the deuterons are localized and their NMR characteristics are determined by CD₃ rotation and rotational oscillations of acetone molecules. In NaX the CD₃ rotation and rotational oscillations about the twofold axis of acetone dominate the spectra below 100 K, while above it oscillations also about other axes become important. In NaY dominant features are related to methyl tunnelling and to a smaller extent to rigid acetones, before the rotational oscillations about twofold axis start to prevail above 40 K. The analysis of the strongly non-exponential magnetization recovery was done by applying the recently introduced method (Ylinen et al., 2015 [12]), improved here to take into account the limited fast recovery at the level crossings, 10% at ω_t=ω₀ and 28% at ω_t=2ω₀. At first the experimental recovery is fitted by three exponentials with adjustable weights and decay rates. Then these quantities are calculated from activation energy distributions and known expressions for the deuteron relaxation rate. In NaY two distinctly separate activation energy distributions were needed, the dominant one being very broad. The use of three distributions, two of them covering practically the same energies as the broad one, lead to a somewhat better agreement with experiment. In general the theoretical results agree with experiment within experimental scatter. As the final result the mean activation energies and widths are obtained for activation energy distributions.

Ultra-Fast Molecular Rotors within Porous Organic Cages

Using variable temperature 2 H static NMR spectra and 13 C spin-lattice relaxation times (T1 ), we show that two different porous organic cages with tubular architectures are ultra-fast molecular rotors. The central para-phenylene rings that frame the "windows" to the cage voids display very rapid rotational rates of the order of 1.2-8×106  Hz at 230 K with low activation energy barriers in the 12-18 kJ mol-1 range. These cages act as hosts to iodine guest molecules, which dramatically slows down the rotational rates of the phenylene groups (5-10×104  Hz at 230 K), demonstrating potential use in applications that require molecular capture and release.

Capturing Guest Dynamics in Metal-Organic Framework CPO-27-M (M = Mg, Zn) by (2)H Solid-State NMR Spectroscopy

Metal-organic frameworks (MOFs) are promising porous materials for gas separation and storage as well as sensing. In particular, a series of isostructural MOFs with coordinately unsaturated metal centers, namely, CPO-27-M or M-MOF-74 (M = Mg, Zn, Mn, Fe, Ni, Co, Cu), have shown exceptional adsorption capacity and selectivity compared to those of classical MOFs that contain only fully coordinated metal sites. Although it is widely accepted that the interaction between guest molecules and exposed metal centers is responsible for good selectivity and large maximum uptake, the investigation of such guest-metal interaction is very challenging because adsorbed molecules are usually disordered in the pores and undergo rapid thermal motions. (2)H solid-state NMR (SSNMR) spectroscopy is one of the most extensively used techniques for capturing guest dynamics in porous materials. In this work, variable-temperature (2)H wide-line SSNMR experiments were performed on CPO-27-M (M = Mg, Zn) loaded with four prototypical guest molecules: D2O, CD3CN, acetone-d6, and C6D6. The results indicate that different guest molecules possess distinct dynamic behaviors inside the channel of CPO-27-M. For a given guest molecule, its dynamic behavior also depends on the nature of the metal centers. The binding strength of guest molecules is discussed on the basis of the (2)H SSNMR data.

The first example of a crystalline guest-free form of the tris(5-acetyl-3-thienyl)methane (TATM) host material

The first example of a guest-free crystalline form of tris(5-acetyl-3-thienyl)methane (TATM), in both powder and single crystal form, was obtained by leaching water-soluble guests out of the guest-host inclusion compounds with acetone, ethanol and methanol.

A deuterium NMR study of guest molecular dynamics in tris(5-acetyl-3-thienyl) methane inclusion compounds

Deuterium nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation times (T1) are used to investigate the guest molecular dynamics of tris(5-acetyl-3-thienyl) methane (TATM) inclusion compounds. The seven guests: acetonitrile, nitromethane, dimethyl sulfoxide, benzene, mesitylene, ortho-xylene, and para-xylene show a wide variety of motional behaviour. The reorientation of acetonitrile in TATM and nitromethane in TATM were both modelled as precession on a cone, the base of which is more elliptical in shape for nitromethane, as would be expected considering their molecular symmetries. DMSO in TATM does not undergo any reorientation other than methyl rotation at the temperatures investigated. At low temperatures, the 2H lineshape and the deuterium spin-lattice relaxation time both depend on the rate of methyl rotation. Activation barriers of 11.7 (±0.4) kJ/mol and 11.2 (±0.5) kJ/mol were found from the two techniques, respectively. Benzene undergoes sixfold reorientation about the principal molecular axis; however, the rate is still greater than 108 Hz down to 113 K. The spin-lattice relaxation time profile does not reach a minimum on decreasing temperature to 112 K, while the slope of this plot provides an activation energy of 4.1 (±0.4) kJ/mol for the sixfold reorientation. The deuterium NMR spectra of mesitylene-d9 in TATM can be simulated using a model where the guest occupies two different sites in the TATM lattice, with the guest molecules performing in-plane C3 rotation either very rapidly (k > 108 Hz) or very slowly (k < 103 Hz), with the relative populations of each changing with temperature. Another model proposed suggests the possibility that there is a continuous distribution of motional rates, the median of which is increasing with temperature. This second model is more realistic; however, too many parameters are present to consider a detailed fit. Finally, both o-xylene and p-xylene are rigidly held in the TATM clathrate, while rotation of the methyl groups is rapid. Key words: inclusion compounds, solid state deuterium NMR spectroscopy, molecular dynamics.