Discotic Mesophases of the Hexakis(alkylsulfono)benzene Series: Characterization by Differential Scanning Calorimetry, Optical Microscopy and Nuclear Magnetic Resonance Spectroscopy

Members of the Hexakis(alkylsulfono)benzene series 4, containing 7 to 15 carbon atoms per alkyl chain (4 c-k), exhibit one and possibly several liquid crystalline discotic mesophases. The mesophases are uniaxial and most probably have a columnar structure. Their anisotropic magnetic susceptibility is negative and in sufficiently strong magnetic fields they align with the director perpendicular to the field direction. Deuterium NMR spectra of C6D6 dissolved in the mesophases exhibit quadrupolar splittings which are strongly temperature dependent. In analogy with the conventional discotics this suggests the presence of multiple solvation sites for solute molecules

The Crystal and Mesophase Structure of Hexakis(alkylsulfono)- benzene Homologues by X-Ray Diffractometry

A powder X-ray diffractometer study of hexakis(tridecylsulfono)benzene (HASB13) has been carried out over the temperature range 20 to above 80 °C. In this range three phase transitions are observed by sharp discontinuities in the diffraction pattern indicating a solid-solid, solidmesophase, and mesophase-liquid transition. The mesomorphic phase is identified as a hexagonal columnar discotic mesophase, Dhd, with intercolumnar spacing of 25.7 Å and average stacking distance of 4.9 Å. Both distances are independent of temperature but there appears to be a gradual increase in the stacking disorder as the temperature is increased. The magnitude of the intercolumnar distance suggests a considerable degree of side chain disorder. A detailed X-ray diffraction study was also performed at room temperature on a single crystal of hexakis(propylsulfono) benzene (HASB3), which is not mesogenic. The results provide detailed information on the structure of HASB 3 which is used in the interpretation of HASB 13 results

Resolution and Polarization Distribution in Cryogenic DNP/MAS Experiments

This contribution addresses four potential misconceptions associated with high-resolution dynamic nuclear polarization/magic angle spinning (DNP/MAS) experiments. First, spectral resolution is not generally compromised at the cryogenic temperatures at which DNP experiments are performed. As we demonstrate at a modest field of 9 T (380 MHz [superscript 1]H), 1 ppm linewidths are observed in DNP/MAS spectra of a membrane protein in its native lipid bilayer, and <0.4 ppm linewidths are reported in a crystalline peptide at 85 K. Second, we address the concerns about paramagnetic broadening in DNP/MAS spectra of proteins by demonstrating that the exogenous radical polarizing agents utilized for DNP are distributed in the sample in such a manner as to avoid paramagnetic broadening and thus maintain full spectral resolution. Third, the enhanced polarization is not localized around the polarizing agent, but rather is effectively and uniformly dispersed throughout the sample, even in the case of membrane proteins. Fourth, the distribution of polarization from the electron spins mediated via spin diffusion between [superscript 1]H–[superscript 1]H strongly dipolar coupled spins is so rapid that shorter magnetization recovery periods between signal averaging transients can be utilized in DNP/MAS experiments than in typical experiments performed at ambient temperature.National Institutes of Health (U.S.) (Grant EB002804)National Institutes of Health (U.S.) (Grant EB003151)National Institutes of Health (U.S.) (Grant EB002026)National Institutes of Health (U.S.) (Grant EB001965)National Institutes of Health (U.S.) (Grant EB004866)National Science Foundation (U.S.). Graduate Research Fellowship Progra

Solid-State Dynamics in the closo-Carboranes:A (11)B MAS NMR and Molecular Dynamics Study

This work explores the dynamic behaviour of the three closo-carborane isomers (formula C2B10H12) using modern solid-state magic angle spinning (MAS) NMR techniques and relates the experimental measurements to theoretical results obtained using molecular dynamics simulations. At high temperatures and at B0 = 9.4 T, the 11B MAS linewidths are narrow (40-90 Hz) for the three isomers. The rotational correlation times (tauc) calculated by molecular dynamics are on the picosecond timescale, showing a quasi-isotropic rotation at these temperatures, typical for liquid systems. For all three isomers, the values of the 11B spin-lattice relaxation times (T1) show discontinuities as the temperature is decreased, confirming the phase changes reported in the literature. At low temperatures, the 11B MAS spectra of all three isomers exhibit much broader lines. The simulations showed that the molecular reorientation was anisotropic around different symmetry axes for each isomer, and this was supported by the values of the reduced quadrupolar parameter PQeff derived from “dynamic shift” measurements using 11B MQMAS NMR spectroscopy. The behaviour of PQeff as a function of temperature for para-carborane suggests that molecular reorientation is about the C5 symmetry axis of the molecule at low temperatures and this was supported by the molecular dynamics simulations

The determination of all six spectral densities of motion for partially oriented dichloromethane-d2

Measurements of spin-lattice relaxation, combined with 2-dimensional measurements of single-and double-quantum spin-echo decay rates, were used to detn. the complete set of spectral d. parameters which characterize D relaxation of CD2Cl2 partially oriented in a nematic liq. crystal (Merck Licristal Phase V). At 31.5 Deg and 9.2 MHz, terms describing cross-correlation between 2 D quadrupole coupling tensors are comparable to the autocorrelation terms. A partial interpretation of these results in terms of anisotropic mol. reorientation and order-director fluctuations is presented and is confirmed by addnl. measurements of the temp. and field dependence of spin-lattice relaxation rates. [on SciFinder (R)

Synthesis and dynamic nmr of hexathiadodecamethoxymetacyclophane

A “one-pot” synthesis of the title compound (1) is described, which might also be suitable for other related compounds. It is shown by 1H NMR that (1) has a saddle structure with C2 symmetry. At room temperature it undergoes fast dynamic interconversion between two symmetry related structures resulting in an overall four-fold symmetry. The activation parameters for this reaction are: ΔE#=10.5 kcal/mole, ΔS#=7.1 e.u. At ~-100°C the reaction is frozen out on the NMR time scale