Chirality Synchronization in Trifluoroethanol Dimer Revisited: The Missing Heterochiral Dimer

Chirality self-recognition in the dimer of transient chiral 2,2,2-trifluoroethanol (TFE) is studied using chirped pulse and cavity-based Fourier transform microwave spectroscopy with the aid of ab initio calculations. The broad-band and extreme high-resolution capabilities enable us to assign rotational spectra of the most stable homo- and heterochiral dimers and analyze their structural and dynamical properties in detail. A strong preference for the homochiral over the heterochiral diastereomers is observed. The current study unambiguously identifies the structure of the most stable homochiral dimer and supports the identification by the previous low-resolution infrared study. More importantly, it also indisputably detects the so far elusive, most stable heterochiral dimer

Strong Solvent-Dependent Preference of Δ and Λ Stereoisomers of a Tris(diamine)nickel(II) Complex Revealed by Vibrational Circular Dichroism Spectroscopy

In the present study, we use vibrational circular dichroism (VCD) spectroscopy to investigate the metal-centered Δ and Λ chirality of a tris­(diamine)­nickel­(II) complex. Chiral diphenylethylenediamine is chosen as the ligand, which puts the Δ and Λ isomers of the complex in a diastereomeric relationship. X-ray crystallography indicates an equal preference of both stereoisomers in the solid state. This equal preference is also supported by the related density functional theory calculations. A comparison between the experimental and calculated VCD spectra also proves the existence of both isomers in an acetonitrile solution. However, a significant shift of the equilibrium toward the Λ diastereomer is found for the complex in dimethyl sulfoxide. This solvent-induced preference for a particular absolute configuration is hypothesized to arise from a stronger and more effective solvation of the Λ isomer. The observation that the solvent can significantly influence and shift an equilibrium between two diastereomeric forms is expected to have important implications on structural analysis and on how reaction mechanisms are rationalized

Conservation of Helicity in a Chiral Pyrrol-2-yl Schiff-Base Ligand and Its Transition Metal Complexes

Tetradentate enantiopure Schiff-base ligand (<i>R</i>,<i>R</i>) and (<i>S</i>,<i>S</i>)-bis­(pyrrol-2-ylmethyleneamine)-cyclohexane (H₂L) and its five transition metal complexes with Ni­(II), Cu­(II), Zn­(II), Pd­(II), and Pt­(II) were synthesized. Their structural properties, in particular, the ligand chirality, coordination topology, and the resulting helicity in solution, were investigated by using IR, vibrational circular dichroism (VCD), UV–vis, and electronic circular dichroism (ECD) spectroscopies, complemented with density functional theory calculations. Conformational searches and the associated spectral simulations for the ligands and the complexes were performed at the B3LYP/Gen level. Comparison of the experimental and theoretical IR and VCD spectral signatures of these complexes reveal that the Zn complex takes on a dinuclear, distorted tetrahedral coordination topology around the metal centers, whereas the other four metal complexes adopt the mononuclear, distorted square-planar coordination arrangement in solution. The helicity of all systems studied was identified to be <i>M</i> with the (<i>R</i>,<i>R</i>) ligand and <i>P</i> with the (<i>S</i>,<i>S</i>) ligand, dictated by the ligand chirality and the strong preference for the chair configuration by the cyclohexane moiety. Furthermore, the resulting helicity was found to dominate the ECD spectral features, even though the helicity-determining angles are close to zero for the nearly square-planar metal complexes. The related VCD spectral features are sensitive to both helicity of the complex and the chirality of the ligands, as well as the coordination topology. The simulated ECD spectra for the <i>P</i> and <i>M</i> helicity of the [Zn-(<i>R</i>,<i>R</i>)-L]₂ complex shows almost mirror-imaged ECD spectral features, whereas very similar ECD spectra were recently reported for the <i>P</i>- and <i>M</i>-dinuclear Mn complexes with a di-μ-oxo dimetal core as a linker. We highlight the advantages of utilizing multiple chiroptical techniques and theoretical spectral simulations to correlate chiroptical spectral features with multiple chirality and helicity elements in the systems

Rotational Spectra of Two Hydrogen-Bonded Methyl Salicylate Monohydrates: Relative Stability and Tunneling Motions

The mechanism for the dual fluorescence of methyl salicylate (MS) is a long-standing research topic. Two distinctly different hydrogen-bonded MS–water complexes, one with the carbonyl O and the other with the hydroxy O serving as the hydrogen-bond acceptor, were detected using chirped pulse and cavity-based Fourier transform microwave spectroscopy and found to be of comparable stability. This observation provides direct experimental evidence for the previously undetected hydroxy O-bonded isomer, supporting the recently proposed new mechanism for the dual fluorescence of MS in protic solvents. An interesting interplay between the methyl internal rotation and water tunneling motions was also investigated. The existence of an unconventional O···H–C hydrogen-bond was verified using the atoms-in-molecules theory. Thorough searches for the elusive ketoA, a higher energy form of MS, were unsuccessful. This result supports recent theoretical reports that only one MS species exists in the ground state

Absolute Configuration and Conformation of Two Fráter–Seebach Alkylation Reaction Products by Film VCD and ECD Spectroscopic Analyses

Two chiroptical spectroscopic techniques, namely, electronic and vibrational circular dichroism (ECD and VCD), as well as NMR spectroscopy have been utilized to determine the absolute configurations and geometries of two Fráter–Seebach alkylation reaction products with long hydrocarbon chains. The experimental studies have been complemented with density functional theory calculations. Strong characteristic bisignate VCD signatures in the carbonyl stretching region have been observed for both compounds in film state. Truncated models, i.e., without the long CH₂ chains, have been utilized to examine different hydrogen-bonding topologies between two monomeric moieties and to simulate the corresponding IR and VCD spectra of the dimers. In addition, the exciton coupling model has also been applied to the CO groups of the two monomeric moieties, which can be coupled through intermolecular hydrogen-bonding. On the basis of these simplified approaches, the absolute configurations of the compounds have been unambiguously assigned using VCD and ECD spectroscopy. Spectral simulations in the IR and UV–vis regions have also been carried out with the full dimers to validate the fitness of the truncated model. The study shows that the combination of the film VCD and ECD techniques is a relatively straightforward method to determine the absolution configurations of such synthetic compounds

Contrasting Reactivities of Silicon and Germanium Complexes Supported by an <i>N</i>‑Heterocyclic Guanidine Ligand

We report the synthesis of an acyclic two-coordinate germylene supported by two bulky and electron donating <i>N</i>-heterocyclic guanidine [IPrN]⁻ ligands (IPr = [(HCNDipp)₂C:]; Dipp = [2,6-ⁱPr₂C₆H₃]), and its reactivity with molecular hydrogen to form IPrNH, which presumably proceeds via the unstable intermediate [H₂Ge­(NIPr)₂]. Our attempts to isolate the corresponding silylene [:Si­(NIPr)₂] led to an unexpected ligand activation/rearrangement process involving N–C­(aryl) bond cleavage within the <i>N</i>-heterocyclic guanidine ligand; this transformation was also studied by computational methods

IR and Vibrational Circular Dichroism Spectroscopy of Matrine- and Artemisinin-Type Herbal Products: Stereochemical Characterization and Solvent Effects

Five Chinese herbal medicinesmatrine, oxymatrine, sophoridine, artemisinin, and dihydroartemisininwere investigated using vibrational circular dichroism (VCD) experiments and density functional theory calculations to extract their stereochemical information. The three matrine-type alkaloids are available from the dry roots of <i>Sophora flavescens</i> and have long been used in various traditional Chinese herbal medicines to combat diseases such as cancer and cardiac arrhythmia. Artemisinin and the related dihydroartemisinin, discovered in 1979 by Professor Youyou Tu, a 2015 Nobel laureate in medicine, are effective drugs for the treatment of malaria. The VCD measurements were carried out in CDCl₃ and DMSO-<i>d</i>₆, two solvents with different dielectric constants and hydrogen-bonding characteristics. A “clusters-in-a-liquid” approach was used to model both explicit and implicit solvent effects. The studies show that effectively accounting for solvent effects is critical to using IR and VCD spectroscopy to provide unique spectroscopic features to differentiate the potential stereoisomers of these Chinese herbal medicines

Summary of the SRM Results.

<p>Summary of the SRM Results.</p

Probability of Making Social Connections by Hours of the Day.

<p>Probability of Making Social Connections by Hours of the Day.</p

CQ inhibited autophagy and enhanced ROS in QBC939 cells.

<p>(A) Cells were treated with CQ (50 μM) or 3-MA (5 mM) and/or cisplatin (10 or 20 μg/ml) for 24 h and then cell viability was measured by MTT assays. (B) Immunoblot analysis of intracellular p62 and LC3-II/I in cells treated with CQ (50 μM) or 3-MA (5 mM) for 24 h. Overall ROS and mtROS were measured in (C) QBC939 cells and HepG2 cells treated with 100 μM Mito-TEMPO and/or CQ (50 μM) or 3-MA (5mM) for 12 h (×200). Quantitation of the cell average fluorescence intensity of (D) DCFH-DA and (E) MitoSOX under the same treatment conditions as in (C). All values are the mean±SE.</p