Chiralitätserkennung zwischen neutralen Molekülen in der Gasphase

Chirality Discrimination at Binary Organic|Water Interfaces Monitored by Interfacial Tension Measurements with Preliminary Comparison with Molecular Dynamics Simulations

Chirality discrimination at a binary toluene (organic)/water(aqueous) interface between R- or S-Tol-BINAP (2,2'-Bis(di-p-tolylphosphino)-1,1'-binaphthyl) molecules and the water-soluble serine chiral specie is examined for the first time, using a combination of interfacial tension measurements and molecular dynamic simulations. Experimental interfacial measurements exhibit a clear chirality-controlled difference when a homochiral versus a heterochiral enantiomeric pairs are introduced at the interfaces. The related molecular dynamics simulations support the experimental results and provide further molecular insight of intermolecular interactions at the interfaces. The results indicate that interfacial tension measurements can capture the preferential interactions which exist between different pairs of enantiomers at the binary interfaces, opening up a new way for probing chirality discrimination at liquid-liquid interfaces.

Triptycene-based Chiral Porous Polyimides for Enantioselective Membrane Separation

Enantiomers of 2, 6-diaminotriptycene (R, R-1 and S, S-1) are split by chiral-phase HPLC and their absolute configurations are identified by single-crystal X-ray diffraction technology. Using the enantiomers as monomers, a couple of chiral porous polyimides (R-FTPI and S-FTPI) are prepared by polycondensation reactions and display good heat stability, high BET surface area and good solubility in organic solvents. Moreover, both of R-FTPI and S-FTPI can be cast into robust, free-standing films suitable for enantioselective separation with symmetrical chiral selectivity.

Water Triggers Hydrogen-Bond-Network Reshaping in the Glycoaldehyde Dimer

Carbohydrates are ubiquitous biomolecules in nature. The vast majority of their biomolecular activity takes place in aqueous environments. Molecular reactivity and functionality are, therefore, often strongly influenced by not only interactions with equivalent counterparts, but also with the surrounding water molecules. Glycoaldehyde (Gly) represents a prototypical system to identify the relevant interactions and the balance that governs them. Here we present a broadband rotational‐spectroscopy study on the stepwise hydration of the Gly dimer with up to three water molecules. We reveal the preferred hydrogen‐bond networks formed when water molecules sequentially bond to the sugar dimer. We observe that the dimer structure and the hydrogen‐bond networks at play remarkably change upon the addition of just a single water molecule to the dimer. Further addition of water molecules does not significantly alter the observed hydrogen‐bond topologies.

On the applicability of functional-group symmetry-adapted perturbation theory and other partitioning models for chiral recognition - the case of popular drug molecules interacting with chiral phases

The applicability of symmetry-adapted perturbation theory (SAPT) and functional-group SAPT (F-SAPT) to study chiral recognition is investigated on an example of three popular chiral drug molecules: ibuprofen, norepinephrine, and baclofen, interacting with phenethylamine or proline - two molecules that are often used as chiral phases in chromatography. The comparison of the F-SAPT with the interacting quantum atoms (IQA) approach is also provided. Accurate estimation of energetic differences of the non-covalent intermolecular complexes composed of two chiral molecules is a necessary prerequisite for the possibility of a prediction of the chiral recognition. The SAPT method with interacting molecules described on the density functional theory level provides accurate total interaction energies, while the F-SAPT approach is the most useful in determining which functional groups are responsible for strengthening or weakening of the interaction between chiral molecules. The largest difference in the interaction energies has been calculated for the baclofen-phenethylamine and norepinephrine-phenethylamine pairs, while the smallest for the ibuprofen-proline and baclofen-proline ones. In most cases, the intermolecular interaction is found to be composed of a strong directional hydrogen bond, which was stabilized by two or more weaker non-covalent interactions between groups (in accordance with the phenomological three-point rule), but in several cases more subtle factors are responsible for larger stability of one diastereoisomer, like the stabilization of the conformation involving two noninteracting functional groups attached to a chiral atom through intramolecular attraction. Additionally, the simulated IR spectra were analyzed for all pairs of diastereoisomeric complexes and the red- and blue-shifts of characteristic bond vibrations were discussed in the context of inter-group interactions.

The Chiral Trimer and a Metastable Chiral Dimer of Achiral Hexafluoroisopropanol: A Multi-Messenger Study

1,1,1,3,3,3-hexafluoro-propan-2-ol aggregates preferentially into an achiral dimer of achiral monomers, but the trimer is found to prefer three metastable chiral monomer units arranged into a strained OH⋅⋅⋅O hydrogen-bonded ring, which is reinforced by secondary CH⋅⋅⋅FC interactions. This is shown by a combination of infrared, microwave, and Raman spectroscopy in supersonic jet expansions and supported by high-level quantum chemical calculations. It involves an activation of the monomers by >15 kJ mol^-1 , clearly driven by the much stronger hydrogen-bond interaction available to the gauche and even more to the cis monomer units.

The Interplay Between Conformation and Absolute Configuration in Chiral Electron Dynamics of Small Diols

A competition between chiral characteristics alternatively attributable to either conformation or to absolute configuration is identified. Circular dichroism associated with photoexcitation of the outer orbital of configurational enantiomers of 1,3- and 2,3-butanediols has been examined with a focus on the large changes in electron chiral asymmetry produced by different molecular conformations. Experimental gas-phase measurements offer support for the theoretical modeling of this chiroptical effect. A surprising prediction is that a conformationally produced pseudo-enantiomerism in 1,3-butanediol generates a chiral response in the frontier electron dynamics that outweighs the influence of the permanent configurational handedness established at the asymmetrically substituted carbon. Induced conformation, and specifically induced conformational chirality, may thus be a dominating factor in chiral molecular recognition in such systems.

Spontaneous Mirror-Symmetry Breaking in Isotropic Liquid Phases of Photoisomerizable Achiral Molecules

Spontaneous mirror-symmetry breaking is of fundamental importance in science as it contributes to the development of chiral superstructures and new materials and has a major impact on the discussion around the emergence of uniform chirality in biological systems. Herein we report chirality synchronization, leading to spontaneous chiral conglomerate formation in isotropic liquids of achiral and photoisomerizable azobenzene-based rod-like molecules. The position of fluorine substituents at the aromatic core is found to have a significant effect on the stability and the temperature range of these chiral liquids. Moreover, these liquid conglomerates occur in a new phase sequence adjacent to a 3D tetragonal mesophase.

London dispersion in molecular chemistry--reconsidering steric effects

London dispersion, which constitutes the attractive part of the famous van der Waals potential, has long been underappreciated in molecular chemistry as an important element of structural stability, and thus affects chemical reactivity and catalysis. This negligence is due to the common notion that dispersion is weak, which is only true for one pair of interacting atoms. For increasingly larger structures, the overall dispersion contribution grows rapidly and can amount to tens of kcal mol(-1) . This Review collects and emphasizes the importance of inter- and intramolecular dispersion for molecules consisting mostly of first row atoms. The synergy of experiment and theory has now reached a stage where dispersion effects can be examined in fine detail. This forces us to reconsider our perception of steric hindrance and stereoelectronic effects. The quantitation of dispersion energy donors will improve our ability to design sophisticated molecular structures and much better catalysts.

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.

Molecular recognition in glycolaldehyde, the simplest sugar: two isolated hydrogen bonds win over one cooperative pair

Carbohydrates are used in nature as molecular recognition tools. Understanding their conformational behavior upon aggregation helps in rationalizing the way in which cells and bacteria use sugars to communicate. Here, the simplest α-hydroxy carbonyl compound, glycolaldehyde, was used as a model system. It was shown to form compact polar C₂-symmetric dimers with intermolecular O–H⋅⋅⋅O=C bonds, while sacrificing the corresponding intramolecular hydrogen bonds. Supersonic jet infrared (IR) and Raman spectra combined with high-level quantum chemical calculations provide a consistent picture for the preference over more typical hydrogen bond insertion and addition patterns. Experimental evidence for at least one metastable dimer is presented. A rotational spectroscopy investigation of these dimers is encouraged, also in view of astrophysical searches. The binding motif competition of aldehydic sugars might play a role in chirality recognition phenomena of more complex derivatives in the gas phase.

The last globally stable extended alkane

Mother of all folding: cold isolated linear alkanes C(n)H(2n+2) prefer an extended all-trans conformation before cohesive forces between the chain ends induce a folded hairpin structure for longer chains. It is shown by Raman spectroscopy at 100-150 K that the folded structure becomes more stable beyond n(C) = 17 or 18 carbon atoms. High-level quantum-chemical calculations yield n(C) = 17 ± 1 as the critical chain length.

Chirality effects on the IRMPD spectra of basket resorcinarene/nucleoside complexes

The IRMPD spectra of the ESI-formed proton-bound complexes of the R,R,R,R- and S,S,S,S-enantiomers of a bis(diamido)-bridged basket resorcin[4]arene (R and S) with cytosine (1), cytidine (2), and cytarabine (3) were measured in the region 2800-3600 cm(-1). Comparison of the IRMPD spectra with the corresponding ONIOM (B3LYP/6-31(d):UFF)-calculated absorption frequencies allowed the assessment of the vibrational modes that are responsible for the observed spectroscopic features. All of the complexes investigated, apart from [R⋅H⋅3](+), showed similar IRMPD spectra, which points to similar structural and conformational landscapes. Their IRMPD spectra agree with the formation of several isomeric structures in the ESI source, wherein the N(3)-protonated guest establishes noncovalent interactions with the host amidocarbonyl groups that are either oriented inside the host cavity or outside it between one of the bridged side-chains and the upper aromatic nucleus. The IRMPD spectrum of the [R⋅H⋅3](+) complex was clearly different from the others. This difference is attributed to the effect of intramolecular hydrogen-bonding interactions between the C(2')-OH group and the aglycone oxygen atom of the nucleosidic guest upon repulsive interactions between the same oxygen atom and the aromatic rings of the host.