Residual Dipolar Couplings—A Valuable NMR Parameter for Small Organic Molecules

Extending molecular dynamics with dipolar NMR tensors as constraints to chiral phosphorus compounds

Molecular dynamics with orientational constraints (MDOC) simulations use NMR parameters as tensorial constraints in the stereochemical analysis of small molecules. 13C-31P Residual dipolar couplings-aided MDOC simulations of small phosphorus molecules determined the relative configurations of rigid molecules after including 3JH-H-couplings as additional constraints. However, flexible molecules remain a problem.

Ti3C2Tx MXene Liquid Crystal: Access to Create Background-Free and Easy-Made Alignment Medium

The formation of lyotropic liquid crystals (LCs) in two-dimensional (2D) colloidal dispersions enables the production of mesoscopic/macroscopic ordered materials from nanoscale building blocks. In contrast to graphene oxide (GO) LCs, the practical applications of MXene LCs are less exploited. This study bridges the gap by utilizing a simple and versatile fabrication method to prepare Ti₃C₂T_x MXene LC that can be applied as a background-free alignment medium for the residual dipolar coupling (RDC) measurement of organic molecules. Ti₃C₂T_x LC displays the size- and concentration-dependent alignment degree. Ti₃C₂T_x nanoflakes with an average size of around 600 nm can provide the quadrupolar ²H splitting of 71 Hz at a concentration of 50 mg/mL and show excellent fluidity at such a high concentration. Compared with other alignment media, Ti₃C₂T_x LC exhibits the features of no-background and narrow line broadening, which actualizes the acquirement of clean and high-quality NMR spectra for the accurate RDC extraction. Notably, the alignment of LCs is determined to be maintainable in the redispersed solution after freeze-drying, providing the great convenience for the preparation of alignment Ti₃C₂T_x media, long-term sample preservation, and quantitative evaluation of alignment degree. Meanwhile, the alignment LC media for RDC measurement can be established in other MXenes such as Ti₂CT_x and Ti₃CNT_x. Collectively, our findings demonstrate the potential of creating various alignment media from the fascinating MXene family.

Distinct alignment of benzene derivatives in stretched polystyrene and polybutylacrylate gels: Specific polymer-solute interactions

We have measured the alignment of a range of benzene derivatives in cross-linked polystyrene and poly(butyl acrylate) using a small number of residual dipolar couplings and simple geometric considerations. For apolar solutes in polystyrene and protic solutes in poly(butyl acrylate), the preferred molecular orientation does not coincide with the longest molecular axis (steric aligment). This behavior may be explained by specific π-π and hydrogen bonding interactions between solute and polymer, respectively, the latter being confirmed by molecular dynamics simulations.

Configuration verification via RDCs on the example of a tetra-substituted pyrrolidine ring

The configurational analysis of organic compounds is an important application for high resolution NMR spectroscopy. In the present study, a tetra-substituted pyrrolidine with four chiral carbon atoms is analyzed using classical methods based on (3) J and NOE data in solution and compared and verified with recently introduced alternative approaches via residual dipolar couplings (RDCs) in two weak anisotropic alignment media. The molecule shows sufficient rigidity in the five-membered ring for the configurational characterization with the various techniques. However, the flexibility caused by the many freely rotating bonds potentially poses problems for the interpretation of data. It is shown that RDCs measured in poly-γ-benzyl-l-glutamate and a stretched polydimethylsiloxane gel provide useful information for the distinction of diastereomers, but the success varies with the data interpretation strategy used. Although a general improvement of corresponding correlation factors is observed when limiting data to a subset of dipolar couplings directly connected to the central ring, the distinction power is reduced because of the smaller number of RDCs available for potential model falsification. Singular value decomposition for fitting experimental RDCs is able to distinguish in most cases the correct from incorrect configurations, but the differences in correlation factors can be relatively small. Surprisingly, predicting RDCs using the rod model as implemented in PALES gives best results in distinguishing the eight possible diastereomers. It is also found that the use of proton-phosphorus and carbon-phosphorus RDCs helps with the configurational analysis of the model compound.

Room-temperature and low-ordered, amphotropic-lyotropic ionic liquid crystal phases induced by alcohols in phosphonium halides

Tri- n-decylmethylphosphonium chloride and bromide ( 1P10X) salts are not liquid crystalline. However, mesophases are induced by adding very small amounts of an alcohol or water. The temperature ranges of the induced smectic A 2 (SmA 2) liquid-crystalline phases can be very broad and the onset temperatures can be below room temperature depending upon the concentration of the alcohol or water and the structure of the alcohol. At least one molar equivalent of hydroxyl groups is necessary to convert the 1P10X completely into a liquid crystal. Strong association between the hydroxyl groups of an alcohol or water and the head groups of the 1P10X is indicated by spectroscopic, diffraction, and thermochemical data. Unlike many other smectic phases, those of the 1P10X/alcohol complexes are easily aligned in strong magnetic fields and the order parameters of selectively deuterated alcohols as measured by (2)H NMR spectroscopy, approximately 10 (-2), are much lower than the values found when the host is a commonly employed thermotropic liquid crystal. The dependence of the specific values of the order parameters on temperature, the nature of the halide anion, and the structure and concentration of the alcohol are reported. In sum, a detailed picture is presented to explain how and why an alcohol or water induces liquid crystallinity in the 1P10X salts. The data also provide a blueprint for designing media with even lower order parameters that can be hosts to determine the conformations and shapes of guest molecules.