Enantiomeric excess measurements in weakly oriented chiral liquid crystal solvents through 2D H selective refocusing experiments

In this article, a simple and robust method is proposed for simplifying the analysis of proton spectra of molecules dissolved in weakly oriented chiral media. The NMR approach investigated is based on the use of proton selective refocusing 2D experiments (SERF) to measure proton–proton dipolar couplings from unresolved lines. This technique is applied to the case of enantiomers dissolved in chiral polypeptide liquid crystals. It is shown that an accurate determination of enantiomeric excess is possible within a short experimental time.     

Theoretical and experimental investigation of (13)C relayed (2)H-(2)H-COSY 2D experiments: application to the analysis of weakly aligned solutes

We describe new NMR 2D experiments denoted DECADENCY for DEuterium CArbon DEuterium Nuclear Correlation spectroscopY dedicated to the analysis of anisotropic deuterium spectra. They belong to the class of X-relayed Y,Y-COSY 2D experiments that was initially explored in the case of a (1)H-X-(1)H fragment (I(X)=1/2) in isotropic medium. DECADENCY 2D experiments permit to correlate the quadrupolar doublets associated with two inequivalent deuterium nuclei in an oriented CD(2) fragment through heteronuclear polarization transfers. Two kinds of pulse sequences are described here using either a double INEPT-type or DEPT-type process. DECADENCY 2D experiments provide an interesting alternative to (2)H-(2)H COSY experiments when the geminal (2)H-(2)H total coupling (scalar and dipolar) is null or too small to provide visible cross-correlation peaks. Such a situation is typically observed for geminal deuteriums in prochiral or chiral molecules dissolved in chiral liquid crystals. The efficiency of these techniques is illustrated using dideuterated prochiral molecules, the phenyl[(2)H(2)]methanol and the 1-chloro[1-(2)H(2)]nonane, both dissolved in organic solutions of poly-gamma-benzyl-l-glutamate. The advantages of each sequence are presented and discussed. It is shown that the relative sign of the quadrupolar doublets can be determined.     

Homo- and heteronuclear 2D NMR approaches to analyse a mixture of deuterated unlike/like stereoisomers using weakly ordering chiral liquid crystals

We describe several homo- and heteronuclear 2D NMR strategies dedicated to the analysis of anisotropic (2)H spectra of a mixture of dideuterated unlike/like stereoisomers with two remote stereogenic centers, using weakly orienting chiral liquid crystals. To this end, we propose various 2D correlation experiments, denoted "D(H)(n)D" or "D(H)(n)C" (with n=1, 2), that involve two heteronuclear polarization transfers of INEPT-type with one or two proton relays. The analytical expressions of correlation signals for four pulse sequences reported here were calculated using the product-operators formalism for spin I=1 and S=1/2. The features and advantages of each scheme are presented and discussed. The efficiency of these 2D sequences is illustrated using various deuterated model molecules, dissolved in organic solutions of polypeptides made of poly-gamma-benzyl-L-glutamate (PBLG) or poly-epsilon-carbobenzyloxy-L-lysine (PCBLL) and NMR numerical simulations.     

Spin-spin coupling edition in chiral liquid crystal NMR solvent

The application of the G-SERFph pulse sequence is presented on enantiomeric mixtures dissolved in a chiral liquid crystal. It aims at editing, within one single 2D spectrum, every proton coupling which is experienced by a given proton site in the molecule, and leads to real phased T-edited spectroscopy (T=J+2D). This NMR experiment is based on the combination of homonuclear semi-selective refocusing techniques with a spatial frequency encoding of the sample. This approach, which consists in handling selectively each coupling in separate cross sections of the sample, is applied to the visualization of enantiomers dissolved in a chiral liquid crystalline phase. Advantages and limits of this methodology are widely discussed.     

piN and pipiN couplings of the Delta(1232) and its chiral partners

We investigate the interactions and chiral properties of the four spin-3/2 baryons N(-)(D13), N+(P13), Delta+(P33), and Delta(-)(D33) together with the nucleon. We construct the SU(2)R x SU(2)L invariant interactions between the spin-1/2 and spin-3/2 baryons with the aid of a new, specially developed spin and isospin projection technique for these baryon fields, where the chiral invariant interactions contain one- and two-pion couplings. We obtain simple relations for the coupling constants of the one- and two-pion spin-1/2-3/2 transitions terms. The relation for the one-pion interactions reasonably agrees with the experiments, which suggests that these spin-3/2 baryons are chiral partners.     

SERF-filtered experiments: new enantio-selective tools for deciphering complex spectra of racemic mixtures dissolved in chiral oriented media

The use of chiral liquid crystals as NMR solvent is a powerful tool to visualize and quantify enantiomers. This technique is much more robust than traditional liquid state techniques. Recent developments have been performed to simplify proton spectra obtained in these solvents which are usually complex to analyze. In the different published sequences the enantiomeric differentiation can be visualized but none of them rely the different (1)H-(1)H couplings to have a coherent set for each enantiomer especially in cases of racemic mixtures. In this work to overcome such limitations, we present a new NMR pulse sequence using semi-selective pulses to assign for each enantiomer the different (1)H-(1)H couplings. The aim is to obtain the signal of only one enantiomer, through a first Selective ReFocussing (SERF) filtering block implemented for the enantio-selection followed by a second SERF block for measuring a chosen coupling. The whole sequence is called (1)H SERF-filtered-SERF. These techniques permit for the first time to assign all dipolar couplings to each enantiomer of racemic mixtures dissolved in chiral oriented media. We demonstrate that using this experiment it is possible to visualize enantio-differentiation even in the case where the multiplets are not resolved in the SERF experiments, pushing back the limits of the enantiomeric visualization.     

Construction and calibration of a 50 T/m z-gradient coil for quantitative diffusion microimaging

q-Space imaging is capable of providing quantitative geometrical information of structures at cellular resolution. However, the size of restrictions that can be probed hinges on available gradient amplitude and places very high demands on gradient performance. In this work we describe the design and construction of a small, high-amplitude (50 T/m) z-gradient coil, interfaced with a commercial 9.4 T microimaging system. We also describe a method to calibrate the coil for quantitative measurements of molecular diffusion at very high-gradient amplitudes. Calibration showed linear current response up to 50 T/m, with a gain=1.255 T/m/A. The z-gradient coil was combined with the commercial x- and y-gradients for tri-axial imaging, and its performance was demonstrated by ADC maps of free water and by q-space experiments on water sequestered around polystyrene microspheres (4.5 microm diameter), which showed the expected diffraction peak. In addition, diffusion-weighted images of a fixed mouse spinal cord illustrated the capability of this coil for quantitative imaging of tissue microstructure.     

Design of a biplanar gradient coil using a genetic algorithm

A biplanar z-gradient coil has been designed using a genetic algorithm, and its efficiency for producing a gradient along the axis of a solenoid magnet compared to that of a conventional Maxwell coil set. Coils of 21.8 cm by 20.9 cm area and 10 cm separation give 0.37 m Tm⁻¹ A⁻¹ with standard and maximum deviations of 2.6 and 13.1% of this value over an optimised cuboid region of 12 by 15 by 1.8 cm. The experimentally useable linear volume extends beyond this to 50% of the separation between the planes. Design data are also given for a transverse gradient set.     

Quantum corrections of the Dzyaloshinskii-Moriya interaction on the spin-$\frac{1}{2}$ AF-Heisenberg chain in an uniform magnetic field

We have investigated the ground state phase diagram of the 1D AF spin- Heisenberg model with the staggered Dzyaloshinskii-Moriya (DM) interaction in an external uniform magnetic field H. We have used the exact diagonalization technique. In the absence of the uniform magnetic field (H=0), we have shown that the DM interaction induces a staggered chiral phase. The staggered chiral phase remains stable even in the presence of the uniform magnetic field. We have identified that the ground state phase diagram consists of four Luttinger liquid, staggered chiral, spin-flop, and ferromagnetic phases.     

1H and 13C NMR studies of glycine in anisotropic media: double-quantum transitions and the effects of chiral interactions

The (1)H NMR spectrum of glycine in stretched gelatin gel and in cromolyn liquid crystal displays a well-resolved doublet due to (1)H-(1)H dipolar interaction. Multiple spectra were obtained within a wide range of offset frequencies of partially saturating radio-frequency (RF) radiation to generate steady-state irradiation envelopes or z-spectra of glycine. Maximal suppression of the doublet occurred when the irradiation was applied exactly at the centre frequency, between the two glycine peaks. This phenomenon is due to double-quantum transitions and is similar to our previous work on quadrupolar nuclei (2)H (HDO) and (23)Na(+). When the (13)C isotopomer glycine-2-(13)C was used, the same effect was found in twice, split by (1)J(CH)+2D(CH). Additional signals in (1)H and (13)C NMR due to prochiral-chiral interactions were found when glycine-2-(13)C was dissolved in chiral anisotropic gelatin and κ-carrageenan gels. The NMR spectra were successfully simulated assuming a (2)J(HH) coupling constant of -16.5Hz and two distinct dipolar coupling constants for the -(13)CH(2)- group (D(C,HA), and D(C,HB)).     

Resolution enhancement in spectra of natural products dissolved in weakly orienting media with the help of 1H homonuclear dipolar decoupling during acquisition: application to 1H-13C dipolar couplings measurements

In weakly orienting media such as poly-gamma-benzyl-L-glutamate (PBLG) a polymer that forms a chiral liquid crystal in organic solvents, the spectral resolution for embedded molecules is usually poor because of numerous (1)H, (1)H dipolar couplings that generally broaden proton spectra. Therefore (1)H, (13)C dipolar couplings are difficult or impossible to measure. Here, we incorporate Flip-Flop decoupling during detection into an HSQC experiment. Flip-Flop removes the (1)H, (1)H dipolar couplings and scales the chemical shifts of the protons as well as the (1)H, (13)C dipolar couplings during detection. A resolution gain by a factor 1.5-4.2 and improved signal intensity by an average factor of 1.6-1.7 have been obtained. This technique is demonstrated on (+)-menthol dissolved in a PBLG/CDCl(3) phase.     

新型酰胺型手性大环多胺的合成及光谱表征

以L-脯氨酸为原料,通过酯化、桥连和关环三步反应合成了两种新的酰胺型手性大环多胺。它们可作为NMR手性溶解剂用于羧酸类手性化合物的手性识别。通过多因素正交实验,确定了最佳反应条件： (8S,18S)-1,4,10,13,16-五氮杂三环[16.3.0.04,8]二十一烷-9,17-二酮(2-a)反应温度50 ℃,反应时间10 d,氮气保护,无水甲醇,产率为34.9%,熔点278～280 ℃;13-苯基-(8S,18S)-1,4,10,13,16-五氮杂三环[16.3.0.04,8]二十一烷-9,17-二酮(2-b)反应温度为回流,反应时间20 d,氮气保护,无水甲醇,产率为2.6%,熔点243～245 ℃。通过红外光谱、质谱、元素分析、一维、二维核磁共振波谱等手段对目标化合物的分子结构进行了表征,对化合物的红外光谱吸收峰及核磁谱线进行了归属分析,为进一步研究主客体分子之间的相互作用机理,开发更多更有效的NMR手性溶解剂提供了光谱依据。     

Visualisation of enantiomers via insertion of a BIRD module in X-H correlation experiments in chiral liquid crystal solvent

Several 13C-1H NMR techniques are derived simplifying the visualisation of enantiomers in chiral ordering solvents. They proceed through various heteronuclear 2D experiments where a bilinear rotation decoupling sequence (BIRD) is inserted in the middle of the t1 evolution period. In this way, the small couplings are refocused while the large couplings are preserved. The methods allow extracting precise values of one-bond carbon-proton residual dipolar couplings for each enantiomer out of unresolved proton-coupled 13C or carbon-coupled 1H spectra. Illustrative examples are analysed and discussed using various pulse sequences.     

A one-dimensional continuous model for carbon nanotubes

The two-dimensional (2D) continuous elastic energy model for isotropic tubes is reduced to a one-dimensional (1D) curvature elastic energy model strictly. This 1D model is in accordance with the Kirchhoff elastic rod theory. Neglecting the in-plane strain energy in this model, it is suitable to investigate the nature features of carbon nanotubes (CNTs) with large deformations and can also reduce to the string model in [Z.C. Ou-Yang et al., Phys. Rev. Lett. 76, 4055 (1997)] when the deformation is small enough. For straight chiral shapes, this general model indicates that the difference of the chiral angle between two equilibrium states is about π/6, which is consistent with the lattice model. It also reveals that the helical shape has lower energy for per atom than the straight shape has in the same condition. By solving the corresponding equilibrium shape equations, the helical tube solution is in good agreement with the experimental result, and super helical shapes are obtained and we hope they can be found in future experiments.     

Tailoring enhanced chiroptical effect with Fabry–Perot cavity-coupled sandwich chiral metamaterials

Cavity-coupled plasmonic structure is demonstrated to be a simple and effective tool to manipulatelight,enhance the biosensing figure of merit, and control the polarization state. In this Letter, we demonstrate the tunability of the chiroptical effect of cavity-coupled chiral structure, i.e., sandwich chiral metamaterials(SCMs), in whichradiation coupling dominates the interaction between particles. Two types of SCMs whose building blocks are 3D chiral and 2D chiral, respectively, are numerically studied. Distinct responses are observed in these two materials. The chiroptical effect can be effectively manipulated and enhanced in the 2D case, while the SCMs consisting of 3D chiral layers keep the chiroptical effecta constant. A theoretical analysis based on matrix optics is developed to explain the corresponding phenomena, which gives a reasonable agreement with numerical simulations.     

β-环糊精手性识别D/L-樟脑磺酸的NMR研究

利用高分辨液体NMR技术考察了D₂O中β-环糊精与手性药物D/L-樟脑磺酸对映体之间的相互作用,结果表明樟脑磺酸对映体能够进入β-环糊精的疏水空腔并与之发生手性相互作用. β-环糊精能够手性识别樟脑磺酸对映体. 化学计量关系实验和2D ROESY实验结果表明,樟脑磺酸对映体的疏水结构只能从β-环糊精疏水空腔的宽口端进入,并形成化学计量比为1∶1的非对映体络合物.     

β-环糊精手性识别D/L-樟脑磺酸的NMR研究

利用高分辨液体NMR技术考察了D_2O中β-环糊精与手性药物D/L-樟脑磺酸对映体之间的相互作用, 结果表明樟脑磺酸对映体能够进入β-环糊精的疏水空腔并与之发生手性相互作用. β-环糊精能够手性识别樟脑磺酸对映体. 化学计量关系实验和2D ROESY实验结果表明, 樟脑磺酸对映体的疏水结构只能从β-环糊精疏水空腔的宽口端进入, 并形成化学计量比为1∶1的非对映体络合物.     

Chirality of molecular nanostructures on surfaces via molecular assembly and reaction: manifestation and control

The formation of chiral nanostructures via molecular assembly and reaction on solid surfaces is a ubiquitous surface process due to the symmetry-breaking at 2D surface. Studying chirality during the adsorption, assembly, and reaction of molecules on 2D solid surfaces at molecular level not only sheds deep insights into the enantioselective heterogeneous catalysis, chiral recognition, origin and evolution of chirality, and many important physical chemistry processes but also provides an important strategy to create chiral nanostructures. Here, we give a survey of recent advances in chiral expression and control in molecular assemblies and reactions on surfaces. We firstly give a brief introduction to the general concepts of chiral molecular nanostructures on surfaces. And then we focus on the induction and control of chirality expressed in molecular assemblies. The recent developments in the control strategies such as chiral co-adsorber, chiral auxiliary, chiral solvent, chiral templated surfaces, as well as the underlying mechanism to achieve the chiral induction and amplification, are reviewed. After that, we review the studies of chirality expressed in on-surface synthesis which has been proved to be a promising strategy to fabricate covalently bonded low-dimensional nanostructures and materials. In this respect, we introduce the chiral expression in the intramolecular and intermolecular coupling reactions on surfaces. In addition, we survey the methods to steer the stereoselectivity of on-surface reactions including the design of precursor structure, steric hindrance effect, substrate, kinetic parameters et al. Finally, the future outlook in this field is discussed.     

Holographic correspondence in topological superconductors

We analytically derive a compatible family of effective field theories that uniquely describe topological superconductors in 3D, their 2D boundary and their 1D defect lines. We start by deriving the topological field theory of a 3D topological superconductor in class DIII, which is consistent with its symmetries. Then we identify the effective theory of a 2D topological superconductor in class D living on the gapped boundary of the 3D system. By employing the holographic correspondence we derive the effective chiral conformal field theory that describes the gapless modes living on the defect lines or effective boundary of the class D topological superconductor. We demonstrate that the chiral central charge is given in terms of the 3D winding number of the bulk which by its turn is equal to the Chern number of its gapped boundary.     

Processing DOSY spectra using the regularized resolvent transform

A new method for processing diffusion ordered spectroscopy (DOSY) data is presented. This method, the regularized resolvent transform (iRRT-the i denoting the adaptation of the method to evaluate the inverse Laplace transform), is better than conventional processing techniques for generating 2D DOSY spectra using data that has poor chemical shift resolution. From the same data, it is possible to use the iRRT to generate 1D subspectra corresponding to different components of the sample mixture; these subspectra compare favorably to 1D spectra of the pure substances. Both the 2D spectra and the 1D subspectra offer a vast improvement over results generated using a conventional processing technique (non-linear least-squares fitting). Consequently, we present the iRRT as a stable and reliable tool for solving the inverse Laplace transform problem present in experiments such as DOSY.