Physical Chemistry Chemical Physics最新文献

Quantum anomalous Hall (QAH) insulators with dissipation-less chiral edge channels provide ideal platforms for the exploration of topological materials and low-power spintronic devices. However, the ultralow operation temperature and small nontrivial gaps are the bottlenecks for QAH insulators towards future applications. Here, a new family of QAH insulators, that is, Janus M₂XS₂Se₂ (M = V, Ti; X = W, Mo) monolayers, are proposed to be ferromagnets with large perpendicular magnetic anisotropy (PMA) and high Curie temperature above room temperature. Moreover, the present M₂XS₂Se₂ monolayers hold sizable nontrivial topological gaps, resulting in the 1^st chiral edge state with Chern number C = −1. Unexpectedly, there also exists an occupied 2^nd chiral edge state below the Fermi level. Although all M₂XS₂Se₂ monolayers retain their PMA characteristics on application of biaxial strain, various topological phase transitions are present. The V₂WS₂Se₂ monolayer preserves the QAH state regardless of strain, while the V₂MoS₂Se₂ and Ti₂WS₂Se₂ monolayers transform from QAH states to metallic states under tensile strains. The present M₂XS₂Se₂ monolayers show competitive advantages among the reported materials for the development of topological electronic devices.

Quantum mechanical tunneling (QMT) is a well-documented phenomenon in the C–H bond activation mechanism and is commonly identified by large KIE values. Herein we present surprising findings in the kinetic study of hydrogen tunneling in the Co⁺ mediated decomposition of acetic acid and its perdeuterated isotopologue, conducted with the energy resolved single photon initiated dissociative rearrangement reaction (SPIDRR) technique. Following laser activation, the reaction proceeds along parallel product channels Co(CH₄O)⁺ + CO and Co(C₂H₂O)⁺ + H₂O. An energetic threshold is observed in the energy dependence of the unimolecular microcanonical rate constants, k(E). This is interpreted as the reacting population surmounting a rate-limiting Eyring barrier in the reaction's potential energy surface. Measurements of the heavier isotopologue's reaction kinetics supports this interpretation. Kinetic signatures measured at energies below the Eyring barrier are attributed to H/D QMT. The below-the-barrier tunneling kinetics presents an unusually linear energy dependence and a staggeringly small tunneling KIE of ∼1.4 over a wide energy range. We explain this surprising observation in terms of a narrow tunneling barrier, wherein the electronic structure of the Co⁺ metal plays a pivotal role in enhanced reactivity by promoting efficient tunneling. These results suggest that hydrogen tunneling could play important functions in transition metal chemistry, such as that found in enzymatic mechanisms, even if small KIE values are measured.

Deep eutectic solvents (DESs) are considered as designer solvents that serve as alternatives to traditional solvents. Numerous favourable properties and advantageous characteristics promote their utility in bio-catalysis. Therefore, they have emerged as attractive sustainable media for different biomacromolecules. In the present work, we have synthesized cholinium-based DESs having a hydrogen bond acceptor (HBA) : hydrogen bond donor (HBD) molar ratio of 1 : 2 by varying the cationic ratio in the HBA, which led to the formation of the DESs such as monocholinium citrate ([Chn][Cit]), dicholinium citrate ([Chn]₂[Cit]) and tricholinium citrate ([Chn]₃[Cit]), keeping the HBD ethylene glycol (EG) constant to study their suitability for α-chymotrypsin (α-CT). Herein, we have systematically evaluated the influence of DES-1 ([Chn][Cit]–[EG]), DES-2 ([Chn]₂[Cit]–[EG]) and DES-3 ([Chn]₃[Cit]–[EG]) on the structural and thermal stability, thermodynamic profile, colloidal stability and enzymatic activity of α-CT using different spectroscopic techniques. The spectroscopic results explicitly show enhanced structural stability and activity of the enzyme as the cationic ratio in the HBA increases. Fascinatingly, temperature-dependent studies through both fluorescence and activity measurements showed that DES-2 and DES-3 have highly beneficial effects on α-CT stability. The transition temperature (T_m) of α-CT was augmented by 12.0 °C in DES-2, 10.0 °C in DES-3 and 9.1 °C in DES-1 when compared to the enzyme in buffer. Furthermore, transmission electron microscopy (TEM) analysis revealed that the morphology of α-CT in DES-2 and DES-3 closely mirrored the structure of α-CT, while DES-1 exhibited only minor structural deviations. These findings were corroborated by hydrodynamic size (d_H) measurements and average decay time analysis, which confirmed the observed morphological similarities and perturbations. The long-term preservation ability and kinetics of DES-3 were eventually confirmed by Michaelis–Menten kinetics. Ultimately, these outcomes demonstrate that increasing the molar ratio of the cholinium cation in the HBA can enhance the ability of DESs to stabilize the α-CT structure. Our results also suggest that the effect imparted by DESs was due to DESs themselves rather than their constituent elements. Altogether, the present investigation provides a new insight into the dependence of protein's stability and conformational alterations on DES composition. Also, the biocompatibility of DESs towards enzymes can be varied by changing the molar ratios of the constituent components of DESs to facilitate the expansion of applicability of DESs in biocatalysis.

An aromatic boron-containing organic compound, C₂B₂H₂, with an unusual CC bond was experimentally synthesized in 2017. Here we investigate the structure and bonding nature of C₂B₂H₂ and its derivatives C₂B₂R₂ using DFT and VB theory. Although the CC bond in C₂B₂R₂ consists of a π bond and a charge-shift (CS) bond, C₂B₂F₂ has the lowest LUMO energy and its LUMO is similar to that of ethylene, suggesting that C₂B₂F₂ can be an ideal dienophile for the Diels–Alder reaction. Subsequently, the mechanism and stereoselectivity of the Diels–Alder reaction of C₂B₂F₂ with 5-substituted cyclopentadienes are studied. Computations demonstrate that these Diels–Alder reactions are feasible thermodynamically and kinetically. The stereoselectivity and distortion angles of C₂B₂R₂ exhibit linear correlations with the electronegativity difference between the two substituents bonded to the C(sp³) of cyclopentadiene, suggesting that the stereoselectivity of related Diels–Alder reaction products can be modulated by the substitution of cyclopentadiene. Considering the current interest in boron neutron capture therapy (BNCT), we design six BNCT drugs through the Diels–Alder reaction of C₂B₂F₂ with dienes containing peptide fragments. Thus, we demonstrate a new method for designing three-in-one BNCT drugs via the facile Diels–Alder reaction.

Crystal materials can exhibit novel properties under high pressure, which are completely different from properties under ambient conditions. Water ice has an exceptionally rich phase diagram with at least 20 known crystalline ice phases from experiments, where the high-pressure ice X and ice XVIII behave as an ionic state and a superionic state, respectively. Thus, the ice structures stabilized under high pressure are very likely to possess other novel properties. Herein, we constructed a sequence of hypothetical high-pressure ices whose structures were generated according to the topological frameworks of common metal oxides. Based on density functional theory calculations, the pressure-induced phase transition sequence is in order that the known Ag₂O-Pnm structure (ice X) transformed into a previously undiscovered TiO₂_brookite-Pbca structure at a pressure of 300 GPa, followed by a transition to a new NaO₂-Pa3 structure at a pressure of 2120 GPa. Hitherto unreported NaO₂-Pa3 ice with a cubic structure is in the ionic state, where the oxygen atoms in NaO₂-Pa3 have a face-centered cubic (fcc) sublattice, and the coordination number of H atoms increases to 3. These two structures are dynamically stable according to phonon spectrum analysis and remain stable at temperature of 100 K based on ab initio molecular dynamics (AIMD) simulations. More importantly, the NaO₂-Pa3 ice exhibits novel metallic properties with a closing band gap above a pressure of 2600 GPa, owing to the electron orbital coupling of oxygen atoms in close proximity induced by pressure.

Matter-wave interferometry with molecules is intriguing both because it demonstrates a fundamental quantum phenomenon and because it opens avenues to quantum-enhanced measurements in physical chemistry. One great challenge in such experiments is to establish matter-wave beam splitting mechanisms that are efficient and applicable to a wide range of particles. In the past, continuous standing light waves in the visible spectral range were used predominantly as phase gratings, while pulsed vacuum ultraviolet light found applications in photoionization gratings. Here, we explore the regime of continuous, intense deep-ultraviolet (> 1 MW cm⁻², 266 nm) light masks, where a rich variety of photo-physical and photo-chemical phenomena and relaxation pathways must be considered. The improved understanding of the mechanisms in this interaction opens new potential pathways to protein interferometry and to matter-wave enhanced sensing of molecular properties.

Today, there is a huge need for highly efficient and sustainable energy resources to tackle environmental degradation and energy crisis. We have analyzed the electronic, mechanical and thermoelectric (TE) characteristics of two-dimensional (2D) BiSbTeX₂ (X = S, Se and Te) and Janus BiSbTeXY (X/Y = S, Se and Te) monolayers by implementing first principles simulations. These monolayers' dynamic stability and thermal stability have been demonstrated through phonon dispersion spectra and ab initio molecular dynamics (AIMD) simulations, respectively. The band structure of these monolayers can be tuned by applying uniaxial and biaxial strains. The investigated lattice thermal conductivity (κ_l) for these monolayers lies between 0.23 and 0.37 W m⁻¹ K⁻¹ at 300 K. For a more precise calculation of the scattering rate, we implemented electron–phonon coupling (EPC) and spin–orbit coupling effects to calculate the transport properties. For p(n)-type carriers, the power factor of these monolayers is predicted to be as high as 2.08 × 10⁻³ W m⁻¹ K⁻² and (0.47 × 10⁻³ W m⁻¹ K⁻²) at 300 K. The higher thermoelectric figure of merit (ZT) of p-type carriers at 300 K is obtained because of their very low value of κ_l and high power factor. Our theoretical investigation predicts that these monolayers can be potential candidates for fabricating highly efficient thermoelectric power generators.

Dynamic nuclear polarization (DNP) has proven to be a powerful technique to enhance nuclear spin polarization by transferring the much higher electron spin polarization to nuclear spins prior to detection. While major attention has been devoted to high-field applications with continuous microwave irradiation, the introduction of fast arbitrary waveform generators is gradually increasing opportunities for the realization of pulsed DNP. Here, we describe how static-powder DNP pulse sequences may systematically be designed using single-spin vector effective Hamiltonian theory. Particular attention is devoted to the intricate interplay between two important parts of the effective first-order Hamiltonian, namely, linear field (single-spin) terms and Fourier coefficients determining scaling of the bilinear coupling terms mediating polarization transfer. We address two cases. The first case operates in the regime, where the microwave field amplitude is lower than the nuclear Larmor frequency. Here, we illustrate the predictive strength of a single-spin vector model by comparing analytical calculations with experimental DNP results at 9.8 GHz/15 MHz on trityl radicals at 80 K. The second case operates in the high-power regime, where we combine the underlying single-spin vector design principles with numerical non-linear optimization to optimize the balance between the linear terms and the bilinear Fourier coefficients in a figure of merit function. We demonstrate, numerically and experimentally, a broadband DNP pulse sequence PLATO (PoLarizAtion Transfer via non-linear Optimization) with a bandwidth of 80 MHz and optimized for a microwave field with a maximum (peak) amplitude of 32 MHz.

With topological spin texture, magnetic domain walls have soliton-like dynamic behaviors in magnetic nanowires, which can be used in information transmission and storage technology. Therefore, precisely controlling the dynamic behavior of the magnetic domain wall and its pinning behavior is one of the important technical challenges in realizing domain-wall-based spintronic devices. In this work, a geometrically defect-free scheme for domain wall pinning/depinning is proposed using micromagnetic simulations based on a tie-shaped asymmetric nanowire, which can precisely control the position of the magnetic domain wall in an external magnetic field. The results show that the domain wall in tie-shaped nanowires exhibits excellent linear response and ultrafast time response to external magnetic fields, which endow them with potential applications for high-frequency weak-magnetic-field detection. We further propose a scheme for constructing a magnetic-field sensor using the tie-structured nanowire, and we study its feasibility.

Chemical, physical, and biological decay may partially or totally hide the historical and technological information carried by waterlogged wood. Investigation of the above-mentioned decay processes is essential to assess the wood preservation state, and it is important to find new methods for the consolidation and safeguarding of wooden archaeological heritage. A conventional method for assessing the wood preservation state is light microscopy. However, the method requires sample slicing, which is destructive and challenging when dealing with fragile and spongy submerged remains of heritage wood. To this end, a promising alternative non-destructive technique is proton nuclear magnetic resonance (¹H-NMR) which considers wood as a porous system. This work aimed to perform a comprehensive analysis of structures, porosity, water distribution, decay, and possible structural inclusions of three archaeological waterlogged wood fragments of the Roman age using NMR relaxometry, micro-imaging (μ-MRI), NMR diffusometry, and NMR cryoporometry. The results were compared with a similar analysis of the three contemporary wood samples of the same species. The multimodal approach presented in this study allowed us to cover all the dimensional scales of wood, from nanometers to sub-millimeters, and reconstruct the alteration of the entire archaeological wood fragment caused by degradation.