Chiral Character Research Articles

Helicity of magnetic fields associated with non-relativistic electron vortex beams

The helicities of the magnetic fields associated with non-relativistic electron vortex beams are considered for radially extended Bessel modes. Investigating the helicity density of this system lies in realizing the unique electric and magnetic properties of electron vortex beams that influence their interactions with matter. We have evaluated the vector potential components needed for the derivation of the magnetic helicity density and found that this has a non-zero distribution. This is attributed entirely to the cylindrically symmetric density flux of the electron beam, which scales with the winding number ℓ . It is also related to the magnetic moments of the electron oriented along the z-direction, giving rise to the z-component of the magnetic field. We obtain different helicity distributions for different signs of winding number ±ℓ , which confirms the chiral character of the magnetic fields associated with the electron vortex beam. The physical consequences of taking the spin current density into consideration are also examined. This allows a comparison to be made between the evaluated total current helicities, one with and one without including the spin-polarization.

Nonreciprocity of spin waves by chiral fluctuations induced in the conical state

Nonreciprocity in chiral magnets is promising for applications in chiral spintronics. Spin-wave nonreciprocity happens when applying an external magnetic field to a chiral magnet while a stream of particles parallel to the field flows through it. Here, we found through micromagnetic simulations that, in the absence of lattice chirality, the excitation spectra of spin waves in the conical magnetic state depend on the relative orientations of the spin-polarized current and the external magnetic field applied along the magnetic helix axis. When both are in the same direction, the spin component of the magnet along the helical axis increases with time while the chiral index decreases. Statistical analysis shows that the spin fluctuations are anisotropic, where the fluctuations along the helical axis have chiral characters as multi-modal, high-frequency, and low-intensity. We give an intuitive interpretation of these observations from the point of view of symmetry breaking and explore the factors affecting the nonreciprocity of the spin-wave spectrum, guiding experimental observations.

Synthesis and interest in medicinal chemistry of β-phenylalanine derivatives (β-PAD): an update (2010-2022).

β-Phenylalanine derivatives (β-PAD) represent a structural family of therapeutic interest, either as components of drugs or as starting materials for access to key compounds. As scaffolds for medicinal chemistry work, β-PAD offer the advantage of great diversity and modularity, a chiral pseudopeptidic character that opens up the capacity to be recognized by natural systems, and greater stability than natural α-amino acids. Nevertheless, their synthesis remains a challenge in drug discovery and numerous methods have been devoted to their preparation. This review is an update of the access routes to β-PAD and their various therapeutic applications.

Response of the mechanical and chiral character of ethane to ultra-fast laser pulses.

A pair of simulated left and right circularly polarized ultra-fast laser pulses of duration 20 femtoseconds that induce a mixture of excited states are applied to ethane. The response of the electron dynamics is investigated within the next generation quantum theory of atoms in molecules (NG-QTAIM) using third-generation eigenvector-trajectories which are introduced in this work. This enables an analysis of the mechanical and chiral properties of the electron dynamics of ethane without needing to subject the C-C bond to external torsions as was the case for second-generation eigenvector-trajectories. The mechanical properties, in particular, the bond-flexing and bond-torsion were found to increase depending on the plane of the applied laser pulses. The bond-flexing and bond-torsion, depending on the plane of polarization, increases or decreases after the laser pulses are switched off. This is explainable in terms of directionally-dependent effects of the long-lasting superpositions of excited states. The chiral properties correspond to the ethane molecule being classified as formally achiral consistent with previous NG-QTAIM investigations. Future planned investigations using ultra-fast circularly polarized lasers are briefly discussed.

Two-level atom dynamics induced by a spin-orbit coupled optical vortex: dressed states formulation

We consider here the interaction of a two-level atom with a tightly focused paraxial optical vortex beam in the dressed states formalism. The interaction is characterized by a term that couples the photon spin angular momentum (SAM) with its orbital angular momentum (OAM). This term affects all the physical quantities related to the dressed states, like their energies, populations, and relaxation rates among them. We also show that the Mollow triplet associated with the resonance fluorescence spectrum of a two-level atom acquires a chiral character. We give numerical examples based on experimentally accessible values of the various parameters, and we show that the chirality can give rise to observable modifications of the various physical quantities under consideration.

Chiral discrimination in helicity-preserving Fabry-Pérot cavities

We theoretically study circular dichroism of chiral molecules embedded inside a helicity-preserving Fabry-P\'erot cavity. We find an increase of the intrinsic chiroptical response of the molecules by 2 orders of magnitude and report the first clear signature of chiral cavity polaritons upon entering the regime of strong light-matter coupling. We study a cavity design based on two dielectric photonic crystal mirrors acting, in a narrow frequency range, as efficient polarization cross-converters in transmission for one polarization and almost perfect reflectors for the other polarization. We show that a Pasteur medium hosted inside such a cavity can couple efficiently to both the outside of the cavity and to the helicity-preserving mode, inheriting an enhanced chiral character. We expect such a device to be useful in the future to design ultrasensitive chiral sensors for optics and stereochemistry.

Tools for overcoming reliance on energy-based measures in chemistry: a tutorial review.

The vast majority of literature in the chemical sciences describes fundamental chemical and physical phenomena using scalar measures, such as the energy, even though many phenomena are beyond the scope of scalar-based considerations. This problem exists no matter how accurately the associated energies are calculated. The solution that is explained in this work is to remove the reliance on scalar quantum chemical measures and instead utilize the vector-based and full symmetry-breaking nature of next generation quantum theory of atoms in molecules (NG-QTAIM). The connection with experiment on neutral chiral molecules is explained. A selection of non-energy-based explanations are provided: the functioning of molecular devices, why the cis-effect is the exception rather than the rule, stereochemical phenomena including chiral discrimination, quantifying chiral character of formally achiral molecules, mixed S and R stereoisomer character and the effect of an applied electric field. Current and future developments along with suggestions for future avenues of investigation are discussed. This tutorial review provides the practical details required to implement NG-QTAIM for a range of phenomena that are not accessible with energy-based measures. Step-by-step worked examples are included with data sets and instructions for use of commercial and open-source software along with examples of how to interpret the results.

Mixed chiral and achiral character in substituted ethane: A next generation QTAIM perspective

We use the newly introduced spanning stress tensor trajectory Uσ-space construction within next generation quantum theory of atoms in molecules (NG-QTAIM) for a chirality investigation of singly and doubly substituted ethane with halogen substituents: F, Cl, Br. Singly substituted ethane was overall achiral comprising cancelling chiral components in Uσ-space. The resultant axial bond critical point (BCP) sliding responded more strongly to the increase in atomic number of the substituted halogen than the chirality. The presence of the very light F atom was found responsible for a very high degree of achiral character of the doubly substituted ethane.

Chirality–helicity of cumulenes: A non‐scalar charge density derived perspective

AbstractWe investigate the presence of helical character and chirality using a vector‐based charge density perspective instead of energetic or structural measures. The vector‐based perspective of the chemical bonding, constructed using the most preferred direction of charge density accumulation, finds the presence of induced symmetry‐breaking for α,ω‐disubstituted [4]cumulenes as the end groups are torsioned. The stress tensor trajectories Tσ(s) are used to provide the additional symmetry‐breaking required to quantify the degree and nature of the chirality and helical character. The S‐1,5‐dimethyl‐[4]cumulene contains a very low degree of chiral character but significant axiality(helicity) resulting in a weakly helical morphology of the corresponding Tσ(s). The (‐)S(‐), (+)S(‐) and (+)S(+) conformations of S‐1,5‐diamino‐[4]cumulene contain very significant degrees of both chirality and helical character resulting in helical morphology of the corresponding Tσ(s). We discuss the consequences for the Tσ(s) in locating chiral character in these molecules in future experiment investigations.

Unveiling the Emergent Traits of Chiral Spin Textures in Magnetic Multilayers.

Magnetic skyrmions are topologically wound nanoscale textures of spins whose ambient stability and electrical manipulation in multilayer films have led to an explosion of research activities. While past efforts focused predominantly on isolated skyrmions, recently ensembles of chiral spin textures, consisting of skyrmions and magnetic stripes, are shown to possess rich interactions with potential for device applications. However, several fundamental aspects of chiral spin texture phenomenology remain to be elucidated, including their domain wall (DW) structure, thermodynamic stability, and morphological transitions. Here the evolution of these textural characteristics are unveiled on a tunable multilayer platform—wherein chiral interactions governing spin texture energetics can be widely varied—using a combination of full‐field electron and soft X‐ray microscopies with numerical simulations. With increasing chiral interactions, the emergence of Néel helicity, followed by a marked reduction in domain compressibility, and finally a transformation in the skyrmion formation mechanism are demonstrated. Together with an analytical model, these experiments establish a comprehensive microscopic framework for investigating and tailoring chiral spin texture character in multilayer films.

Helicity-selective Raman scattering from in-plane anisotropic α-MoO3

Hyperbolic crystals, such as α-MoO3, can support large wavevectors and photon density as compared to the commonly used dielectric crystals, which makes them a highly desirable platform for compact photonic devices. The extreme anisotropy of the dielectric constant in these crystals is intricately linked with the anisotropic character of the phonons, which along with photon confinement leads to the rich physics of phonon polaritons. However, the chiral nature of phonons in these hyperbolic crystals have not been studied in detail. In this study, we report our observations of helicity selective Raman scattering from flakes of α-MoO3. Both helicity-preserving and helicity-reversing Raman scattering are observed. Our studies reveal that helical selectivity is largely governed by the underlying crystal symmetry. This study shed light on the chiral character of the high symmetry phonons in these hyperbolic crystals. It paves the way for exploiting proposed schemes of coupling chiral phonon modes into propagating surface plasmon polaritons and realizing compact photonic circuits based on helical polarized light.

Chiral Bloch–Zener oscillations of spin–orbit coupled cold atoms in an optical superlattice

We study the Bloch oscillation (BO) dynamics of spin–orbit coupled cold atoms in a one-dimensional Su–Schrieffer–Heeger optical lattice. The Hamiltonian of our system can be mapped into a tilted SSH ladder pierced by an effective magnetic flux. Based on the calculation of Wannier–Stark states, we investigate dynamical properties of BOs with the chiral character of spin-momentum locking. Importantly, the chiral Bloch–Zener oscillations can be induced by adjusting system parameters. The corresponding Landau–Zener (LZ) tunneling probability is analytically derived by the LZ model and we found a good agreement with the numerical simulation.

Temperature ranges of the chiral polar smectic phases

The paper presents the analysis of static and dynamic properties of achiral polar smectic doped with chiral dopant. In this system the two phases with equal symmetries exist, the and the phase, which continuously transform one to the other upon temperature changes. The analysis of dynamical properties allowed for the development of the criterion which determines the temperature ranges of the two phases. Besides, the analysis of collective modes revealed that they are either amplitude or phase. However, in many of them the character changes from polar to tilt or vice versa. The collective modes occur in pairs of opposite chiralities. The chiral character of modes explains also the gaps which occur between the amplitude modes.

Determination of enantiomeric purity of esomeprazole by capillary electrophoresis

Abstract Proton pump inhibitors are the most effective agents used in gastric hyper-acidity-related disorders. Omeprazole is a benzimidazole-derivative compound with an asymmetric sulphur in its structure, which generates its chiral character. Esomeprazole (S-omeprazole) was the first proton pump inhibitor introduced as an enantiomerically pure compound in therapy, after the successful chiral switch of the racemic omeprazole. This work is aimed at performing a complementary study to an already published chiral separation method of omeprazole. As preliminary analysis, the electrophoretic behavior of omeprazole enantiomers and the possible mechanism of the chiral resolution was studied using different background electrolytes containing different β-cyclodextrin derivatives, as chiral selectors. The robustness of the chiral separation method was tested by applying a Plackett-Burman design. The method was validated according to current ICH guidelines and proved to be reliable, linear, precise and accurate for the determination of 0.2% R-omeprazole as chiral impurity in esomeprazole samples. The validated method was successfully used for the analysis of esomeprazole-containing gastro-resistant tablets. According to our results, valuable information on the mechanism of chiral separation of omeprazole was gained and the application area of the previously developed method was successfully enlarged. The presented rapid and cost-effective capillary electrophoresis method proved to be suitable for the determination of enantiomeric purity of esomeprazole from pharmaceutical preparations and could represent an alternative for the available compendial methods.

Exchange Interactions Drive Supramolecular Chiral Induction in Polyaniline

AbstractThe focus of this paper is on the intermolecular interaction active between polyaniline (PANI) and 10‐camphorsulfonic acid (10CSA). Enantiopure 10CSA, present in the electropolymerization solution, promotes chiral induction in the supramolecular polyaniline polymer (cPANI). Tight integration of experimental data (circular dichroism, CD, near edge X‐ray absorption spectra, NEXAFS, conductive probe atomic force microscopy, CP‐AFM) and theoretical [density functional theory, (DFT)] results allows to unfold the nature of the electronic interaction between PANI and 10CSA and to shed light on the physical interactions inducing the chiral character to bulk pristine non‐chiral PANI: eventually yielding cPANI. The electropolymerization follows a “wet chemistry” method: electrochemical polymerization of aniline in the co‐presence in bulk solution of enantiopure 10‐camphorsulfonic acid (10CSA). The latter is exploited as chirality inductor. The method of integration between experimental results with ab‐initio theoretical calculations, strongly suggests that the chiral induction exerted by the CSA stems from exchange interaction between CSA and PANI.

Rigidly Fused Spiro-Conjugated π-Systems.

Spiro-fused π-systems have gained considerable attention for their application as semiconductors in molecular electronics. Here, a synopsis regarding recent breakthroughs in ladderized spirobifluorenes and indeno-spirobifluorenes, along with further spiro-condensed heteroatomic hydrocarbons with donor-acceptor moieties, is provided. Additionally, an extended range of rigid spirobifluorene polymers and specific doubly linked spiro-systems with partial chiral character is discussed. The diverse applications of the aforementioned structures are thoroughly evaluated.

Theoretical studies of the ultrafast deactivation mechanism of 8-oxo-guanine on the S1 and S2 electronic states in gas phase.

The 8-oxo-deoxyguanosine is the most abundant specie of the DNA oxidative damage. Despite the deleterious effects such as gene mutation it may cause, the 8-oxodG was also reported to have beneficial effect such as repairing the nearby cyclobutane pyrimidine dimer (CPD) after photoexcitation. Due to its strong biological relevance, the photoinduced excited state dynamics behavior of 8-oxo-deoxyguanosine is of particular interest. In this work, a theoretical investigation by combination of complete active space self-consistent field (CASSCF) ab initio calculations and on-the-fly nonadiabatic dynamics simulations are implemented to provide intrinsic deactivation mechanism of its free base 8-oxoguanine after being excited to the S1 and S2 states. Two minimum energy conical intersections (MECIs) characterized by the C3-puckered motion with attractive chiral character are located, which contribute appreciably to the S1 state deactivation process. When the system is being excited to the S2 state directly, a S2→S1→S0 two-step decay pattern is proposed. A nearly planar S2/S1 intersection plays a significant role in the S2→S1 decay process. The subsequent S1 state relaxation process is also dominated by the C3-puckered deformation motion. One decay time is estimated to be 704fs, which compares well with the experimental observation of 0.9±0.1ps in solvents. Particular illustration is the fact that the MECIs configurations we located bear an exceptional resemblance with previous reported thymine, cytosine and guanine, suggesting that the current work could lend support for better understanding of the non-natural nucleobases and derivatives.

Synthesis of two chiral surfactants by a simple method and the surface activity evaluation

In order to expand the application range of surfactants, two chiral surfactants named CTACYT and CTATYS were synthesized by reacting cetyltrimethylammonium chloride (CTAC) with L-cysteine and L-tyrosine, respectively by a simple method. Studies show that the foaming ability, foam stability and emulsifying properties of both CTACYT and CTATYS are superior to CTAC. The corrosion inhibition efficiencies of CTACYT and CTATYS are 96.72% and 90.22%, respectively, which are higher than that of CTAC. The new chiral surfactants remain the chiral character, and the specific optical rotations of CTACYT and CTATYS were 8.1° and 11.6°, respectively. This work provides a new and easy way to the synthesis of chiral surfactants, which will benefit the related research in this field.

Extension of the standard Heisenberg Hamiltonian to multispin exchange interactions

An extension of the Heisenberg Hamiltonian is discussed, that allows to go beyond the standard bilinear spin Hamiltonian taking into account various contributions due to multispin interactions having both chiral and non-chiral character. The parameters of the extended Hamiltonian are calculated from first principles within the framework of the multiple scattering Green function formalism giving access to an explicit representation of these parameters in real space. The discussions are focused on the chiral interactions, i.e.\ biquadratic and three-spin Dzyaloshinskii-Moriya like vector interactions $\vec{\cal{D}}_{ijij}$ (BDMI) and $\vec{\cal{D}}_{ijkj}$ (TDMI), respectively, as well as the three-spin chiral interaction (TCI) $J_{ijk}$. Although all parameters are driven by spin-orbit coupling (SOC), some differences in their properties are demonstrated by calculations for real materials. In particular it is shown that the three-spin chiral interactions $J_{ijk}$ may be topology as well as SOC induced, while the TDMI is associated only with the SOC. As the magnitude of the chiral interactions can be quite sizable, they can lead to a stabilization of a noncollinear magnetic texture in some materials that is absent when these interactions are neglected.

Oriented propagation of magnetization due to chiral edge modes in Kitaev-type models

Detecting chiral edge modes in topological materials has been intensively pursued in experiments. However, the phenomena caused by the modes are not yet elucidated theoretically. We study the dynamics of chiral spinon wave packets at the edge in Kitaev-type magnets. More precisely, by relying on the exact solvability of the models, we construct a spinon wave packet, localized edge magnetization, which shows oriented propagation along the edge, whose behavior is expected from the chiral character of the dispersion relation of the chiral edge modes. In general, this approach enables us to study not only spin transport in anisotropic magnets but also charge transport in Bogoliubov-de Gennes-type superconductors because it does not rely on a conserved quantity.