Confinement Effects of Two-Dimensional Surfaces on Water Adsorption and Dissociation over Pt(111)

It has been established that the confined space created by stacking a two dimensional (2D) surface atop a metal catalyst serves as a nano-reactor. According to recent research, when a graphene (Gr) overlayer encloses a catalyst from above, the activation barrier for the water dissociation reaction, a process with major industrial significance, decreases. In order to investigate how the effect of confinement varies among different two-dimensional (2D) materials, we study the adsorption and dissociation barriers of water molecule on (111) under graphene, hexagonal boron nitride (h-BN), and heptazine-based graphitic carbon nitride (g-C3N4) layers using density functional theory calculations. Our findings reveal that the strength of adsorption does not decrease consistently with a reduction in the height of the 2D overlayer. Furthermore, a smaller barrier is not always the consequence of poorer adsorption of the reactant. We also examine the effect of confinement on the shape of the reaction path, on the frequencies of vibrational modes, and on the rate constants derived using the harmonic transition state theory. Overall, all three 2D surfaces cause a decrease in barrier height and a weakening of adsorption, though to differing degrees due to a mix of mechanical, geometric and electronic variables.

Temperature effects on the branching dynamics in the model ambimodal (6 + 4)/(4 + 2) intramolecular cycloaddition reaction

C14H20 (tetradecapentaene) is a simple model system exhibiting post transition-state behavior, wherein both the (6 + 4) and (4 + 2) cycloaddition products are formed from one ambimocal transition state structure. We studied the bifurcation dynamics starting from the two ambimodal transition state structures, the chair-form and boat-form, using the quasi-classical trajectory, classical molecular dynamics, and ring-polymer molecular dynamics methods on the parameter-optimized semiempirical GFN2-xTB potential energy surface. It was found that the calculated branching fractions differ between the chair-form and boat-form due to the different nature in the IRC pathways. We also investigated the effects of temperature on bifurcation dynamics and found that, at higher temperatures, trajectories stay longer in the intermediate region of the potential energy surface.

Full-dimensional coupled-channel statistical approach to atom-triatom systems and applications to H/D + O3 reaction

The statistical quantum model (SQM), which assumes that the reactivity is controlled by entrance/exit channel quantum capture probabilities, is well suited for chemical reactions with a long-lived intermediate complex. In this work, a time-independent coupled-channel implementation of the SQM approach is developed for atom-triatom systems in full dimensionality. As SQM treats the capture dynamics quantum mechanically, it is capable of handling quantum effects such as tunneling. A detailed study of the H/D + O3 capture dynamics was performed by applying the newly developed SQM method on an accurate global potential energy surface. Agreement with previous ring polymer molecular dynamics (RPMD) results on the same potential energy surface is excellent except for very low temperatures. The SQM results are also in reasonably good agreement with available experimental rate coefficients. The strong H/D kinetic isotope effect underscores the dominant role of quantum tunneling under an entrance channel barrier at low temperatures.

Time-resolved Coulomb explosion imaging of vibrational wave packets in alkali dimers on helium nanodroplets

Vibrational wave packets are created in the lowest triplet state 13Σu+ of K2 and Rb2 residing on the surface of helium nanodroplets, through non-resonant stimulated impulsive Raman scattering induced by a moderately intense near-infrared laser pulse. A delayed, intense 50-fs laser pulse doubly ionizes the alkali dimers via multiphoton absorption and thereby causes them to Coulomb explode into a pair of alkali ions Ak+. From the kinetic energy distribution P(Ekin) of the Ak+ fragment ions, measured at a large number of delays, we determine the time-dependent internuclear distribution P(R, t), which represents the modulus square of the wave packet within the accuracy of the experiment. For both K2 and Rb2, P(R, t) exhibits a periodic oscillatory structure throughout the respective 300 and 100 ps observation times. The oscillatory structure is reflected in the time-dependent mean value of R, ⟨R⟩(t). The Fourier transformation of ⟨R⟩(t) shows that the wave packets are composed mainly of the vibrational ground state and the first excited vibrational state, in agreement with numerical simulations. In the case of K2, the oscillations are observed for 300 ps, corresponding to more than 180 vibrational periods with an amplitude that decreases gradually from 0.035 to 0.020 Å. Using time-resolved spectral analysis, we find that the decay time of the amplitude is ∼260 ps. The decrease is ascribed to the weak coupling between the vibrating dimers and the droplet.

Characterisation of the electronic ground states of BaH+ and BaD+ by high-resolution photoelectron spectroscopy

The rovibrational energy-level structures of BaH+ and BaD+ in their X+ 1Σ+ electronic ground state have been characterised by pulsed-field-ionisation zero-kinetic-energy photoelectron spectroscopy following resonance-enhanced (1 + 1') two-photon excitation from the BaH/BaD X 2Σ+ ground state via the E 2Π1/2 (v' = 0, 1) intermediate levels. A full set of rovibrational molecular constants for the BaH+ and BaD+ ground states has been derived for the first time and the adiabatic ionisation energies of BaH and BaD were determined to be 38 679.96(20) and 38 652.69(20) cm-1, respectively. Photoelectron spectra recorded via E-state levels of selected rovibronic parity exhibit pronounced intensity alternations of transitions to rotational states of the cations with even- and odd-valued rotational-angular-momentum quantum number N+. This observation is interpreted by invoking dominant contributions of even- photoelectron partial waves in the photoionisation of the E 2Π1/2 (v' = 0, 1) intermediate states of barium hydride. The lowest pure-rotational transition frequencies of BaH+ and BaD+ are derived from the photoelectron spectra which may help the detection of BaH+ in the microwave and millimetre-wave ranges.

Photodissociation Spectroscopy and Photofragment Imaging of the Mg+(Benzene) Complex

Tunable laser photodissociation spectroscopy and photofragment imaging experiments are employed to investigate the spectroscopy and dissociation dynamics of the Mg+(benzene) ion-molecule complex. When excited with ultraviolet radiation, Mg+(benzene) photodissociates efficiently, producing both Mg+ and benzene+ fragments, with branching ratios depending on the wavelength. The wavelength dependence of these processes are similar, with intense resonances at 330 and 241 nm and weaker features at 290 and 258 nm. Comparisons of the experimental spectra to those predicted by computational chemistry at the TD-DFT level allow assignment of these to metal ion-based (330 and 241 nm), charge-transfer (290 nm), and benzene-based (258 nm) transitions. However, the observation of the benzene cation fragment at all wavelengths, which can only result from charge-transfer, indicates unanticipated excited state dynamics. Spectroscopy experiments are complemented by photofragment imaging to investigate these dynamics. The high kinetic energy release indicates that multiphoton absorption based on the intense atomic resonances is responsible at least in part for the dissociation processes.

Dynamics Calculations of the Flexibility and Vibrational Spectrum of the Linear Alkane C14H30, Based on Machine-Learned Potentials

Hydrocarbons are the central feedstock of fuels, solvents, lubricants, and the starting materials for many synthetic materials, and thus the physical properties of hydrocarbons have received intense study. Among these, the molecular flexibility and the power and infrared spectroscopies are the focus of this paper. These are examined for the linear alkane C14H30 using molecular dynamics (MD) calculations and recent machine-learned potentials. All MD calculations are microcanonical and start at the global linear minimum. The radius of gyration, the number of gauche bond conformations and the distributions of all C-C distances are reported as a function of the total internal energy and as a function of time. These are compared to the power spectra and to the double harmonic spectra of stationary points. Spectral features of the double harmonic spectra smoothly track structural differences, measured by the number of gauche conformations in the molecule. Preliminary calculations using the quantum local mode model for the CH-stretch are presented and satisfactorily capture anharmonic effects.

Benchmark Ab Initio Mapping of the F- + CH2ClI SN2 and Proton-Abstraction Reactions

The experimental and theoretical studies of gas-phase SN2 reactions have significantly broadened our understanding of the mechanisms governing even the simplest chemical processes. These investigations have not only advanced our knowledge of reaction pathways but also provided critical insights into the fundamental dynamics of chemical systems. Nevertheless, in the case of the prototypical X- + CH3Y → Y- + CH3X [X, Y = F, Cl, Br, and I] SN2 reactions, the effect of the additional halogenation of CH3Y has not been thoroughly explored. Thus, here, we perform the first high-level ab initio characterization of the F- + CH2ClI SN2 and proton-abstraction reactions utilizing the explicitly-correlated CCSD(T)-F12b method. Two possible SN2 channels leading to the Cl- + CH2FI and I- + CH2FCl products are distinguished, in which we investigate four different pathways of back-side attack Walden inversion, front-side attack, double inversion, and halogen-bonded complex formation. In order to obtain the benchmark energies of the geometries of the stationary points, determined at the CCSD(T)-F12b/aug-cc-pVTZ level of theory, additional computations are carried out considering the basis set effects, post-CCSD(T) correlations, and core corrections. Using the benchmark data, we assess the accuracy of the MP2, DF-MP2, MP2-F12, and DF-MP2-F12 methods as well. By comparing the present F- + CH2ClI system with the corresponding F- + CH3Y [Y = Cl and I] reactions, this study demonstrates that further halogenation of CH3Y significantly promotes the corresponding proton-abstraction and SN2 retention channels as well as the halogen-bonded complex formation, and as a consequence, the traditional back-side attack Walden-inversion mechanism becomes less pronounced.

Local Exchange Integrand: Looking into Quantum Contributions to Chemical Bonds

In this work, we present maps of the integrand of the exchange energy K(r1,r2) in terms of atomic contributions. This quantity helps to improve the understanding of this quantum correction in chemical bonding. With a set of covalent and noncovalent diatomic molecules, we show that it is possible to develop useful vocabularies for identifying regions where the exchange correction removes or adds electrons predicted by the classical Coulomb term. Applying the results to four halogen-bonded complexes, we prove that it is possible to gain a further understanding of the characteristics of the interaction between them and to offer a complete description of the anisotropy of the σ hole. The results are confirmed by molecular orbital, NBO, and SAPT(DFT) analyses and bode well for the use of K(r1,r2) in chemical bond analysis.

Accurate Structure and Spectroscopic Properties of Azulene and Its Derivatives by Means of Pisa Composite Schemes and Vibrational Perturbation Theory to Second Order

The structural and spectroscopic properties in the gas phase of azulene and some of its N-bearing derivatives have been analyzed by a general computational strategy based on the recent Pisa composite schemes (PCSs). First of all, an accurate semiexperimental equilibrium structure has been derived for azulene and employed to validate the geometrical parameters delivered by different quantum chemical methods. Next, different isomerization energies (azulene to naphthalene, 1-aza-azulene to quinoline and to other isomers) have been computed by an explicitly correlated PCS version employing frozen natural orbitals. Accurate geometries have been obtained by a cheaper PCS variant based on a double-hybrid functional improved by one-parameter bond corrections, with the same functional providing also remarkable harmonic frequencies. The corresponding equilibrium rotational constants show average deviations within 0.1% from experimental results when taking into account anharmonic vibrational corrections obtained by a global hybrid functional. Therefore, reliable computational estimates have been produced for the rotational constants of several nitrogen derivatives (isomeric aza-azulenes and guaiazulene), whose non-negligible dipole moments could allow experimental microwave characterizations. An analogous approach delivers infrared spectra in remarkable agreement with their experimental counterparts for naphthalene, quinoline, and azulene, together with reliable predictions for the still-unknown spectrum of 1-aza-azulene. In addition to their intrinsic interest, the results of this paper further confirm that a very accurate yet robust and user-friendly tool is now available for aiding high-resolution spectroscopic studies of quite large systems of current technological and/or biological interest.

The C-H···S-S hydrogen bonding in diethyl disulfide⋯difluoromethane: a combined microwave spectroscopic and computational study

The C-H⋯S weak interaction is crucial for comprehending the stability in biological macromolecules and their interactions with smaller molecules. Despite its prevalence, an in-depth understanding and recognition of such interaction remain elusive. Herein, the rotational spectra of a binary complex formed by diethyl disulfide and difluoromethane were investigated using Fourier transform microwave spectroscopy combined with theoretical calculations to examine the C-H⋯S-S interaction. The most stable conformation observed experimentally is stabilized by one C-H⋯S-S hydrogen bond and two weaker C-H⋯F hydrogen bonds. Non-covalent interaction, natural bond orbital, and symmetry-adapted perturbation theory methods were employed to describe the intermolecular interactions within the adduct. Experiments indicated H⋯F and H⋯S distances of 2.68(7) Å and 2.64(1) Å, respectively, with bonding angles of 121.0(4)° for C-H⋯F and 135.3(6)° for C-H⋯S hydrogen bonds. The geometric characteristics and theoretical analyses suggest that the C-H⋯S-S hydrogen bond is the predominant interaction, contributing an energy of 7.6 kJ mol-1. Additionally, the C-H⋯F hydrogen bond also contributes to the stability of the complex, contributing approximately 2.6 kJ mol-1. London dispersion is a primary factor in the stability of complexes, contributing 53% to the total attractive interaction. The results indicate that non-traditional hydrogen bond participants, such as C-H groups and S-S linkages, can form hydrogen bonds and fluorination enhances the interactions.

The role of responsive MRI probes in the past and the future of molecular imaging

Magnetic resonance imaging (MRI) has become an indispensable tool in biomedical research and clinical radiology today. It enables the tracking of physiological changes noninvasively and allows imaging of specific biological processes at the molecular or cellular level. To this end, bioresponsive MRI probes can greatly contribute to improving the specificity of MRI, as well as significantly expanding the scope of its application. A large number of these sensor probes has been reported in the past two decades. Importantly, their development was done hand in hand with the ongoing advances in MRI, including emerging methodologies such as chemical exchange saturation transfer (CEST) or hyperpolarised MRI. Consequently, several approaches on successfully using these probes in functional imaging studies have been reported recently, giving new momentum to the field of molecular imaging, also the chemistry of MRI probes. This Perspective summarizes the major strategies in the development of bioresponsive MRI probes, highlights the major research directions within an individual group of probes (T 1- and T 2-weighted, CEST, fluorinated, hyperpolarised) and discusses the practical aspects that should be considered in designing the MRI sensors, up to their intended application in vivo.

On the Performance of Second-Order Polarization Propagator Methods in the Calculation of 1JFC and nJFH NMR Spin-Spin Coupling Constants

This study evaluates the performance of doubles-corrected random phase approximation (RPA) and higher random phase approximation (HRPA) approaches in predicting nuclear magnetic resonance (NMR) coupling constants involving fluorine. Their performance is benchmarked against experimental data and compared with that of higher-level theoretical methods, specifically second-order polarization propagator (SOPPA) and SOPPA(CCSD). Additionally, we discuss their performance relative to density functional theory (DFT). We find that RPA(D) is severely constrained by (near) triplet instabilities, while HRPA(D) demonstrates markedly improved stability. Statistical analysis reveals stronger patterns for carbon-fluorine couplings across the methods and systems investigated compared with fluorine-hydrogen couplings. While SOPPA-based methodologies prove to be superior in accuracy, HRPA(D) shows promising performance in reducing the computational burden of these calculations, albeit with a tendency to underestimate the coupling strength. These findings highlight the potential of HRPA(D) as a practical alternative to SOPPA methods, even for such difficult properties as NMR spin-spin coupling constants involving fluorine, emphasizing its role in improving predictive accuracy and efficiency across diverse chemical environments.

High Viscosity and Two Phases Observed over a Range of Relative Humidities in Biomass Burning Organic Aerosol from Canadian Wildfires

Biomass burning organic aerosol (BBOA) is a major contributor to organic aerosol in the atmosphere. The impacts of BBOA on climate and health depend strongly upon their physicochemical properties, including viscosity and phase behavior (number and types of phases); these properties are not yet fully characterized. We collected BBOA field samples during the 2021 British Columbia wildfire season to constrain the viscosity and phase behavior at a range of relative humidities and compared them to previous studies on BBOA. Particles from all samples exhibited two-phased behavior with a polar hydrophilic phase and a nonpolar hydrophobic phase. We used the poke-flow viscosity technique to estimate the viscosity of the particles. Both phases of the BBOA had viscosities of >108 Pa s at relative humidities up to 50%. Such high viscosities correspond to mixing times within 200 nm BBOA particles of >5 h. Two phases and high viscosity have implications for how BBOA should be treated in atmospheric models.

Stereoselective Ring-Opening Reaction of α-Fluorinated Oxetanes: A Practical and Theoretical Investigation

The ring-opening reaction of fluorinated oxetanes by halides, including alkylidene oxetanes and spirocyclic oxetanes, was highly stereoselective and directed by the presence of a fluorine atom. This reaction allowed a stereoselective preparation of tetrasubstituted alkenes and substituted pyrrolidines containing all-carbon quaternary centers. Theoretical calculations were performed to shed light on experimentally observed regioselectivity in the opening of oxetane derivatives. Transition state calculations were carried out to compare the energy of transition states responsible for forming different diastereoisomers. These calculations were performed using several DFT functionals and benchmarked to DLPNO-CCSD(T)/def2-TZVP calculations. Intrinsic reaction coordinated (IRC) calculations were run to confirm if the found transition states connect reactants and products. The IRC paths were then decomposed into the electrostatic, steric hyperconjugative contributions to reaction barriers by using the natural bond orbital (NBO) analysis. The destabilizing Fδ-···Br- electrostatic interaction directs the reaction pathway.

Exploring and Regulating Heteroatom-Electronegativity-Associated with Ring Aromaticity and Excited State Intramolecular Proton Transfer Mechanism for Benzothiazole-Based Fluorophore

A novel fluorophore 2-(2'-hydroxy-5'-benzaldehyde phenyl) benzothiazole (HBBT) with excited state intramolecular proton transfer (ESIPT) characteristics, showing good selectivity for Cu+/Cu2+ ions had been synthesized experimentally (Molecules 2022, 27, 7678). However, its ESIPT mechanism and fluorescent performance related to atomic substituents have not been investigated systematically. In this work, two HBBT derivatives, 2-(2'-hydroxy-5'-benzaldehyde phenyl)benzoimidazole (HBBI) and 2-(2'-hydroxy-5'-benzaldehyde phenyl)benzopyrrole (HBBP), were obtained by respectively using -NH and -CH2 groups in place of the sulfur atom in the thiazole ring. The absorption/emission spectra and ring aromaticity as well as ESIPT processes of HBBT and its derivatives were studied using density functional theory (DFT) and time-dependent DFT (TD-DFT). The simulated absorption and fluorescence wavelengths of HBBT agreed well with the corresponding values obtained in the experiment. According to the analyses of geometry structures, electron densities, and infrared vibration frequencies, the intramolecular hydrogen bond becomes stronger upon light excitation. The frontier molecular orbitals were analyzed via establishing potential energy curves, and the ESIPT behavior was described deeply. Obviously, the NH-substitution makes ring 4 more aromatic, while the CH2-substitution changes ring 4 from aromatic to antiaromatic. The ESIPT process helps to alleviate the excited state antiaromaticity. The greater the antiaromaticity of the S1 state normal form, the higher the barrier of ESIPT.

Design of High-Performance Infrared Nonlinear Optical PAs3S3 with Perfectly Aligned Polar Molecular Cage via a Bipolar-Axis-Symmetry Coupling Strategy

Strong polar molecular cages have recently emerged as novel functional building units for high-performance infrared nonlinear optical (IR NLO) crystals. However, these highly polar molecular cages often arrange themselves in a way that cancels out their polarity, leading to a more energetically stable state. As a result, most cage crystal formations tend to crystallize in centrosymmetric space groups, which conflicts with the primary requirement for NLO crystals. Herein, we address the challenge of polar molecular cage arrangement through bipolar-axis-symmetry coupling strategy, utilizing classical NLO parent compounds. By substituting the C3v symmetric [B3O6] groups with polar C3v symmetric [PAs3S3] cages within the β-BBO polar aixs lattice, we successfully synthesized a new compound, PAs3S3 (PAS), which exhibits a consistent arrangement of polar molecular cages - crucial for maximizing NLO performance. Additionally, due to the non-covalent interactions among [PAs3S3] polar molecular cages, PAS demonstrates an unexpectedly strong second harmonic generation (SHG) about 8 times that of AgGaS2, along with a significant band gap of 2.75 eV. Furthermore, PAS exhibits remarkable stability against air and moisture. These findings validate our design strategy and position PAS as a promising candidate for applications in IR NLO crystals.

Nonlinear optics of graphitic carbon allotropes: from 0D to 3D

The dimensionality of materials fundamentally influences their electronic and optical properties, presenting a complex interplay with nonlinear optical (NLO) characteristics that remains largely unexplored. In this review, we focus on the influence of dimensionality on the NLO properties of graphitic allotropes, ranging from 0D fullerenes, 1D carbon nanotubes, and 2D graphene, to 3D graphite, all of which share a consistent sp2 hybridized chemical bonding structure. We examine the distinct physical and NLO properties across these dimensions, underscoring the profound impact of dimensionality. Notably, dimension-specific physical phenomena, such as Luttinger liquid in 1D and Landau quantization in 2D, play a significant role in shaping NLO phenomena. Finally, we explore the promising potential of NLO properties in systems with mixed dimensionalities, setting the stage for future breakthroughs and innovative applications.

Synergistic Effect in Hybrid Plasmonic Conjugates for Photothermal Applications

Photothermal conversion efficiency (η) plays a crucial role in selecting suitable gold nanoparticles for photothermal therapeutic applications. The photothermal efficiency depends on the material used for the nanoparticles as well as their various parameters, such as size and shape. By maximizing the light-to-heat conversion efficiency (η), one can reduce the concentration of nanoparticle drugs for photothermal cancer treatment and apply lower laser power to irradiate the tumor. In our study, we explored a new hybrid plasmonic conjugate for theranostic (therapy + diagnostic) applications. We conjugated PEG-functionalized 20 nm gold nanospheres with cyanine IR dyes via a PEG linker. The resulting conjugates exhibited significantly enhanced photothermal properties compared with bare nanoparticles. We experimentally showed that a proposed new hybrid plasmonic conjugate can achieve almost four times larger conversion efficiency (47.7%) than 20 nm gold nanospheres (12%). The enhanced photothermal properties of these gold conjugates can provide the required temperature for the photothermal treatment of cancer cells with lower concentrations of gold nanoparticles injected in the body as well as with lower applied incident laser power density. Moreover, the improved photothermal properties of the conjugates can be explained by a synergistic effect that has not been observed in the past. This effect results from the coupling between the metal nanosphere and the organic dye.

An overview on plasmon-enhanced photoluminescence via metallic nanoantennas

In the realm of nanotechnology, the integration of quantum emitters with plasmonic nanostructures has emerged as an innovative pathway for applications in quantum technologies, sensing, and imaging. This research paper provides a comprehensive exploration of the photoluminescence enhancement induced by the interaction between quantum emitters and tailored nanostructure configurations. Four canonical nanoantennas (spheres, rods, disks, and crescents) are systematically investigated theoretically in three distinct configurations (single, gap, and nanoparticle-on-mirror nanoantennas), as a representative selection of the most fundamental and commonly studied structures and arrangements. A detailed analysis reveals that the rod gap nanoantenna configuration achieves the largest photoluminescence enhancement factor, of up to three orders of magnitude. The study presented here provides insights for the strategic design of plasmonic nanoantennas in the visible and near-IR spectral range, offering a roadmap for these structures to meet specific requirements in plasmon-enhanced fluorescence. Key properties such as the excitation rate, the quantum yield, the enhanced emitted power, or the directionality of the emission are thoroughly reviewed. The results of this overview contribute not only to the fundamental understanding of plasmon-enhanced emission of quantum emitters but also set the basis for the development of advanced nanophotonic devices with enhanced functionalities.

Bornite (Cu5FeS4) nanocrystals as an ultrasmall biocompatible NIR-II contrast agent for photoacoustic imaging

In this study, we demonstrate the potential of the bornite crystal structure (Cu5FeS4) of copper iron sulfide as a second near infrared (NIR-II) photoacoustic (PA) contrast agent. Bornite exhibits comparable dose-dependent biocompatibility to copper sulfide nanoparticles in a cell viability study with HepG2 cells, while exhibiting a 10-fold increase in PA amplitude. In comparison to other benchmark contrast agents at similar mass concentrations, bornite demonstrated a 10× increase in PA amplitude compared to indocyanine green (ICG) and a 5× increase compared to gold nanorods (AuNRs). PA signal was detectable with a light pathlength greater than 5 cm in porcine tissue phantoms at bornite concentrations where in vitro cell viability was maintained. In vivo imaging of mice vasculature resulted in a 2× increase in PA amplitude compared to AuNRs. In summary, bornite is a promising NIR-II contrast agent for deep tissue PA imaging.

SbO(OH)2(IO3): The First Polar Sb5+-Iodate with a Strong Second-Harmonic Generation Response and a Wide Bandgap

Widening the bandgaps while maintaining a strong second harmonic generation response has always been a research hotspot in the field of nonlinear optical iodate materials. A strategy involving covalent bonding is proposed that leverages the high valent later main group cation to construct iodates with predominantly covalent interactions. By using BiO(IO3) as a template, the first Sb5+-containing polar iodate, SbO(OH)2(IO3) is successfully isolated. The introduction of the two hydroxide anions led to the reduction of layered BiO(IO3) into 1D SbO(OH)2(IO3) in which two corner-sharing SbO4(OH)2 octahedra are further bridged by an iodate group. The covalently bonded [SbO(OH)2]+ chains and the optimal packing fashion of the asymmetric IO3 - groups generate a very strong second harmonic generation signal of 14 times that of KH2PO4. Furthermore, SbO(OH)2(IO3) exhibits a wide bandgap of 4.14 eV and a high laser induced damage threshold [27.9 × AgGaS2, 0.2 × KH2PO4 (10 ns, 10 Hz)].

Self-contacting Cys, Ser, and Thr residues in high-resolution protein crystal structures: Tertiary constraints or hydrogen bonds?

Functional groups in the side-chains of at least 10 amino acids are mainly involved in tertiary interactions. However, structural and functional significance of intra-residue interactions has not been fully recognized. In this study, we have analyzed ~5800 non-redundant high-resolution protein structures and identified 1166 self-contacts between the side-chain S-H/O-H and backbone C=O groups in Cys, Ser, and Thr residues that satisfied the geometric criteria to form hydrogen bonds. Quantum chemical calculations using model compounds were used to evaluate single point energy for 45 representative examples from different allowed regions of Ramachandran map. Relative energy profiles obtained by varying the side-chain dihedral angle χ1 revealed that the energy difference between the crystal structure and the minimum energy conformations is between 0 and 3 kcal/mol. Natural bond orbital analysis (NBO) of self-contacting Cys residues revealed no charge transfer between Cys side-chain S-H and the backbone C=O groups. However, side-chain hydroxyl and the backbone C=O groups of 90%-95% of all self-contacting Ser and Thr residues are involved in charge transfer and the second order perturbation energy of majority of them is above 1 kcal/mol. Interaction energies calculated for model compounds along with NBO and NCIPLOT analyses demonstrate that the self-contacts observed in Ser and Thr residues can be described as hydrogen bonds. These interactions may provide stability to the loop/coil conformations. Self-contacting Cys residues are buried and the self-contacts appear to be mostly due to tertiary constraints. Dispersion between the self-contacting groups is one way to explain the close approach in Cys residues. Mutation studies will further validate and reveal the structural and functional significance of these self-contacting residues.

Fulgide Derivatives as Photo-Switchable Coatings for Cathodes of Lithium Ion Batteries - A DFT Study

Photo-switchable coatings for lithium ion batteries (LIB) can offer the possibility to control the diffusion processes from the electrode materials to the electrolyte and thus, for example, reducing the energy loss in the fully charged state. Fulgide derivatives, as known photo-switches, are investigated concerning their use as coating for vanadium pentoxide, a potential cathode material for LIB. With the help of Density Functional Theory calculations, two fulgide derivatives are characterized with respect to their photophysics, their aggregation behaviour on the cathode material and the ability to form self-assembled monolayers (SAM). Furthermore, the two states of the photo-switchable coating are tested with respect to lithium diffusion from the cathode material, passing the SAM and entering the electrolyte. We found a difference for the energy barriers depending on the state of the photo-switch, preferring its closed form. This behaviour can be used to prevent the loss of charge in batteries of portable devices.

Electroactive Nanomaterials for the Prevention and Treatment of Heart Failure: From Materials and Mechanisms to Applications

Heart failure (HF) represents a cardiovascular disease that significantly threatens global well-being and quality of life. Electroactive nanomaterials, characterized by their distinctive physical and chemical properties, emerge as promising candidates for HF prevention and management. This review comprehensively examines electroactive nanomaterials and their applications in HF intervention. It presents the definition, classification, and intrinsic characteristics of conductive, piezoelectric, and triboelectric nanomaterials, emphasizing their mechanical robustness, electrical conductivity, and piezoelectric coefficients. The review elucidates their applications and mechanisms: 1) early detection and diagnosis, employing nanomaterial-based sensors for real-time cardiac health monitoring; 2) cardiac tissue repair and regeneration, providing mechanical, chemical, and electrical stimuli for tissue restoration; 3) localized administration of bioactive biomolecules, genes, or pharmacotherapeutic agents, using nanomaterials as advanced drug delivery systems; and 4) electrical stimulation therapies, leveraging their properties for innovative pacemaker and neurostimulation technologies. Challenges in clinical translation, such as biocompatibility, stability, and scalability, are discussed, along with future prospects and potential innovations, including multifunctional and stimuli-responsive nanomaterials for precise HF therapies. This review encapsulates current research and future directions concerning the use of electroactive nanomaterials in HF prevention and management, highlighting their potential to innovating in cardiovascular medicine.

Discovery of new molecular hybrid derivatives with coumarin scaffold bearing pyrazole/oxadiazole moieties: Molecular docking, POM analyses, in silico pharmacokinetics and in vitro antimicrobial evaluation with identification of potent antitumor pharmacophore sites

This synthetic organic methodology involves the creation of novel coumarin-based hybrids of series (1-4) with pyrazole ring and (5-8) with oxadiazole moiety. The targeted compounds were tested for In vitro Antimicrobial efficacy against Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, and Candida albicans pathogenic microbes using disc diffusion and broth microdilution with ciprofloxacin and fluconazole as reference standards. Density functional theory (DFT) studies were used to study atomic structure and reactivity, including absolute electronegativity (χ), electrophilicity (ω), electron acceptor (ω+), donor capabilities (ω-), electron affinity (EA), energy gap (ΔE), global hardness (η), global softness (S), and ionisation potential (IP) and FMOs, NBOs, MEP, and Mulliken Charge analysis. The POM tests found three integrated pharmacophore sites with antibacterial, antiviral, and anticancer activities. Molecular docking studies are also used to determine the S. aureus nucleoside diphosphate kinase receptor's affinity and mode of action for the synthesized drugs. In silico analysis of thermodynamic and therapeutic effectiveness properties, including Lipinski's 'rule of five', Veber's rule, and ADME properties, predicted toxicity-free, non-carcinogenic, and risk-free oral administration of the synthesized complexes.

Shedding new light on quadrupolar 1,4-dihydropyrrolo[3,2-b]pyrroles: impact of electron-deficient scaffolds over emission

In this work, we disclose a series of seven quadrupolar centrosymmetric 1,4-dihydropyrrolo[3,2-b]pyrroles (DHPPs) linked to the two peripheral, strongly electron-accepting heterocycles such as benzoxadiazole, benzothiadiazole and benzoselenadiazole. This represents the first study probing the influence of electron-deficient heterocycles, rather that small electron-withdrawing substituents, on photophysics of DHPPs. These new acceptor-donor-acceptor hybrid dyes exhibit an appreciable combination of photophysical properties including absorption maxima in the range of 470-620 nm, and emission in the range of 500-720 nm with fluorescence quantum yields reaching 0.88. We discovered that the presence of two 7-nitro-benzoxadiazolyl substituents at positions 2 and 5 of DHPP core, evokes a strong fluorescence in non-polar solvents shifted to 639 nm. This is the most bathochromically shifted emission for quadrupolar, centrosymmetric chromophore bearing exclusively biaryl linkages. Interestingly, 1,2,4,5-tetraaryl-1,4-dihydropyrrolo[3,2-b]pyrrole (TAPP) possessing 4-benzothiadiazolyl groups is strongly emitting in the crystalline state (fluorescence quantum yield = 0.43). The combined photophysical and crystallographic studies point towards existence of intermolecular hydrogen bonds which modify the dihedral angles between the donor and acceptor moieties as a primary reason for this strong emission. Small structural alteration via the replacement of two 2,1,3-benzoxadiazole scaffolds with 2,1,3-benzoxadiazole-2-oxide moieties causes >103 decrease in the fluorescence intensity. Computational studies point out to strong charge transfer originating from exceptionally large dihedral angles as the pivotal reason of this phenomenon. Although internal conversion originating from the charge-transfer state is the prevailing non-radiative deactivation mechanism, intersystem crossing also plays a role. The rational design of DHPPs that enables modulation of emission will advance their applicability.

Coupling Trapped Ions to a Nanomechanical Oscillator

Cold ions in traps are well-established, highly controllable systems with a wide variety of applications in quantum science, precision spectroscopy, clocks, and chemistry. Nanomechanical oscillators are used in advanced sensing applications and for exploring the border between classical and quantum physics. Here, we report on the implementation of a hybrid system combining a metallic nanowire with laser-cooled ions in a miniaturized ion trap. Operating the experiment in the classical regime, we demonstrate resonant and off-resonant coupling of the two systems and the coherent motional excitation of the ion by the mechanical drive of the nanowire. The present results prove the feasibility of mechanically coupling ions to nanooscillators and open up avenues for mechanically manipulating the motion of trapped ions as well as for the development of ion-mechanical hybrid quantum systems.

Effects of Methyl Substitution on the Ultrafast Internal Conversion of Benzene

The effects of methyl substitution on the ultrafast internal conversion from the S2(1B1u, ππ*) state to the S0 state of benzene were studied using ultrafast extreme-ultraviolet photoelectron spectroscopy and electronic structure calculations. The quantum yield of the internal conversion to the S0 state reached ∼0.69 in benzene, while lower values of 0.35 and 0.12 were obtained for toluene and o-xylene, respectively. These results indicate that methyl substitution makes the conical intersections less accessible to the nuclear wave packet.

Borophene nanoclusters: Energetics and structures from analytical potentials

Boron shows a variety of properties, determining a chemistry rich and complementary to that of carbon, the neighbor atom in the Periodic Table. In this work, we investigated the strength and nature of the interaction involving B12 or B36 monomer, which represent molecular prototypes of borophene, the two-dimensional allotrope of elemental boron. For the representation of the intermolecular interaction, we developed new potential energy surfaces (PESs) that are based on accurate ab initio or density functional theory data. It is shown that borophene molecules are bound by weak intermolecular interactions of van der Waals nature, perturbed by antiaromatic effects. Moreover, the proposed PESs are given in an analytical form proper to investigate the structures and energetics of (B12)n and (B36)n clusters (with n = 2-10) by applying a global geometry optimization procedure. It is found that the most stable structures of (B12)n favor close contacts between the edges of the monomers, leading to cage-like clusters as n increases, and conversely, (B36)n clusters are mainly composed of stacked or herringbone structures. These results suggest the possibility to produce a novel class of two-dimensional borophene materials, exhibiting different features compared to graphene like structures, which could be of interest for the nanotechnology.

Selective determination of metal chlorocomplexes in saline waters by magnetic ionic liquid-based dispersive liquid-liquid microextraction

In this work, we explore a new dispersive liquid-liquid microextraction (DLLME) method to selectively separate chemical species of Cd and Zn in saline waters. It is based on the use of the magnetic ionic liquid (MIL) methyltrioctylammonium tetrachloroferrate ([N1,8,8,8+][FeCl4-]), which allows an efficient and environmentally friendly extraction of the target species. In addition, the paramagnetic component in the MIL simplifies the separation step required in DLLME, allowing for fast separation and recovery of the extracted species with a magnet, without a centrifugation step. The optimum conditions for the separation by MIL-DLLME were 3.3 mg mL-1 MIL, sample pH = 8, and an extraction time of 30 min. Under these conditions, metal chlorocomplexes (99.7% and 81.0% of total metal concentration for Cd and Zn, respectively) were quantitatively separated, remaining the free cations in the aqueous samples. In a second step, the extracted metal species were back-extracted with 1 mol L-1 HNO3 and a re-extraction time of 15 min. For cadmium, this acidic solution separated the neutral complex CdCl2 (60.5%), while CdCl+ (21.5%) and CdCl3- (18.1%) remained in the organic phase. For Zn, the anionic complex ZnCl3- (17.3%) was retained by the organic reagent, while ZnCl2 (45.7%) and ZnCl+ (37.0%) were re-extracted by the nitric acid solution. The separation of the chemical species of metals along the three liquid phases used allowed their quantification in several samples of real seawater and a certified reference material.

Computational Investigation of Meso-Substituted, Heavy Atom-Free BODIPY Derivatives as Photosensitizers: Insights From TDDFT and Dynamics Studies

This study investigates the structural and electronic properties of BODIPY (BDP) derivatives featuring meso-substituted donors arranged orthogonally, leveraging Time-Dependent Density Functional Theory (TD-DFT). These deriva-tives, selected based on experimental evidence of their quantum yield towards singlet oxygen generation, exhibit intricate excited-state dynamics, transitioning from fluorescence to intersystem crossing (ISC), thereby presenting a promising avenue for applications in photodynamic therapy. Emphasizing heavy-atom-free organic triplet photosensitizers, with BDP dyes highlighted for their exceptional adaptability in photophysical characteristics, our analysis contributes to a deeper understanding of the fundamental design principles governing such photosensitizers. Through a combined approach of static and dynamic calculations, we elucidate the mechanisms underlying the population transfer from singlet to triplet states, thereby providing valuable insights for the development of efficient photodynamic therapy agents.

Development of a Platform for High-Resolution Ion Mobility Separations Coupled with Messenger Tagging Infrared Spectroscopy for High-Precision Structural Characterizations

The ability to uniquely identify a compound requires highly precise and orthogonal measurements. Here we describe a newly developed analytical platform that integrates high resolution ion mobility and cryogenic vibrational ion spectroscopy for high-precision structural characterizations. This platform allows for the temporal separation of isomeric/isobaric ions and provides a highly sensitive description of the ion's adopted geometry in the gas phase. The combination of these orthogonal structural measurements yields precise descriptors that can be used to resolve between and confidently identify highly similar ions. The unique benefits of our instrument, which integrates a structures for lossless ion manipulations ion mobility (SLIM IM) device with messenger tagging infrared spectroscopy, include the ability to perform high-resolution ion mobility separations and to record the IR spectra of all ions simultaneously. The SLIM IM device, with its 13 m separation path length, allows for multipass experiments to be performed for increased resolution as needed. It is integrated with an Agilent qTOF MS where the collision cell was replaced with a cryogenically held (30 K) TW-SLIM module. The cryo-SLIM is operated in a novel manner that allows ions to be streamed through the device and collisionally cooled to a temperature where they can form noncovalently bound N2 complexes that are maintained as they exit the device and are detected by the TOF mass analyzer. The instrument can be operated in two modes: IMS+IR where the IR spectra for mobility-selected ions can be recorded and IR-only mode where the IR spectra for all mass-resolved ions can be recorded. In IR-only mode, IR spectra (400 cm-1 spectral range) can be recorded in as short as 2 s for high throughput measurements. This work details the construction of the instrument and modes of operation. It provides initial benchmarking of CCS and IR measurements to demonstrate the utility of this instrument for targeted and untargeted approaches.

Tutorial on Molecular Latent Space Simulators (LSSs): Spatially and Temporally Continuous Data-Driven Surrogate Dynamical Models of Molecular Systems

The inherently serial nature and requirement for short integration time steps in the numerical integration of molecular dynamics (MD) calculations place strong limitations on the accessible simulation time scales and statistical uncertainties in sampling slowly relaxing dynamical modes and rare events. Molecular latent space simulators (LSSs) are a data-driven approach to learning a surrogate dynamical model of the molecular system from modest MD training trajectories that can generate synthetic trajectories at a fraction of the computational cost. The training data may comprise single long trajectories or multiple short, discontinuous trajectories collected over, for example, distributed computing resources. Provided the training data provide sufficient sampling of the relevant thermodynamic states and dynamical transitions to robustly learn the underlying microscopic propagator, an LSS furnishes a global model of the dynamics capable of producing temporally and spatially continuous molecular trajectories. Trained LSS models have produced simulation trajectories at up to 6 orders of magnitude lower cost than standard MD to enable dense sampling of molecular phase space and large reduction of the statistical errors in structural, thermodynamic, and kinetic observables. The LSS employs three deep learning architectures to solve three independent learning problems over the training data: (i) an encoding of the high-dimensional MD into a low-dimensional slow latent space using state-free reversible VAMPnets (SRVs), (ii) a propagator of the microscopic dynamics within the low-dimensional latent space using mixture density networks (MDNs), and (iii) a generative decoding of the low-dimensional latent coordinates back to the original high-dimensional molecular configuration space using conditional Wasserstein generative adversarial networks (cWGANs) or denoising diffusion probability models (DDPMs). In this software tutorial, we introduce the mathematical and numerical background and theory of LSS and present example applications of a user-friendly Python package software implementation to alanine dipeptide and a 28-residue beta-beta-alpha (BBA) protein within simple Google Colab notebooks.

Advancing Non-Atom-Centered Basis Methods for More Accurate Interaction Energies: Benchmarks and Large-Scale Applications

Recent advances in local electron correlation approaches have enabled the relatively routine access to CCSD(T) [that is, coupled cluster (CC) with single, double, and perturbative triple excitations] computations for molecules of a hundred or more atoms. Here, approaching their complete basis set (CBS) limit becomes more challenging due to extensive basis set superposition errors, often necessitating the use of large atomic orbital (AO) basis sets with diffuse functions. Here, we study a potential remedy in the form of non-atom-centered or floating orbitals (FOs). FOs are still rarely employed even for small molecules due to the practical complication of defining their position, number, exponents, etc. The most frequently used FO method thus simply places a single FO center with a large number of FOs toward the middle of noncovalent dimers; however, a single FO center for larger complexes can soon become insufficient. A recent alternative uses a grid of FO centers around the monomers with a single s function per center, which is currently applicable only for H, C, N, and O atoms. Here, we build on the above advantages and mitigate some drawbacks of previous FO approaches by using a layer of FO centers and 4-9 FOs/center for each monomer. Thus, a double layer of FOs is placed between the interacting subsystems. When extending the double-ζ AO basis with this double layer of FOs, the quality of conventional augmented double-ζ or conventional triple-ζ AO bases can be reached or surpassed with less orbitals, leading to few tenths of a kcal/mol basis set errors for medium-sized dimers. This good performance extends to larger molecules (shown here up to 72 atoms), as efficient local natural orbital (LNO) CCSD(T) computations with only double-ζ AO and 4 FOs/center FO bases match our LNO-CCSD(T)/CBS reference within ca. 0.1 kcal/mol. These developments introduce FO methods to the accurate modeling of large molecular complexes without limitations to atom types by further accelerating efficient correlation calculations, like LNO-CCSD(T).

Evaluating the Fluorescence Quenching of Troxerutin for Commercial UV Sunscreen Filters

2-Phenylbenzimidazole-5-sulfonic acid (PBSA) and disodium phenyl dibenzimidazole tetrasulfonate (DPDT) are commercially available ultraviolet (UV) sunscreen filters that are known to undergo radiative relaxation following the absorption of UV light. The release of high-energy photons from this relaxation can be detrimental to human health; therefore, fluorescence quenchers need to be incorporated in commercial sunscreen formulations containing PBSA or DPDT. Troxerutin is a fluorescence quencher utilized for DPDT commercially. Here, its ability to quench the fluorescence of both PBSA and DPDT is evaluated using a dual-pronged approach by breaking down the multicomponent problem into its constituent parts. First, PBSA and DPDT's femtosecond to nanosecond photodynamics are uncovered in solution and on the surface of a human skin mimic to ascertain a benchmark. Second, these results are compared to their photodynamics in the presence of troxerutin. A significant reduction in the fluorescence lifetime is observed for both PBSA and DPDT on a human skin mimic with the addition of troxerutin, which is attributed to a Dexter energy transfer (DET) or Förster resonance energy transfer (FRET) quenching mechanism. This finding demonstrates the hitherto unseen fluorescence quenching mechanism of troxerutin on a human skin mimic and its role in quenching the fluorescence of commercial UV sunscreen filters through a DET or FRET mechanism.

Capturing ultrafast energy flow of a heme protein in crowded milieu

Energy flow in biomolecules is a dynamic process vital for understanding health, disease, and applications in biotechnology and medicine. In crowded environments, where biomolecular functions are modulated, comprehending energy flow becomes crucial for accurately understanding cellular processes like signaling and subsequent functions. This study employs ultrafast transient absorption spectroscopy to demonstrate energy funneling from the photoexcited heme of bovine heart cytochrome c to the protein exterior, in the presence of common synthetic (Dextran 40, Ficoll 70, PEG 8 and Dextran 70) and protein-based (BSA and β-LG) crowders. The through-space energy transfer mode for ferric and the methionine rebinding mode for ferrous cytochrome c show the strongest solvent coupling. The heterogeneous behaviour of crowders, influenced by crowder-protein interactions and caging effects at certain higher concentrations, reveal diverse trends. Notably, protein crowders perturb all transport routes of vibrational energy transfer, causing delays in energy transfer processes. These findings provide significant insights into the basic tenets of energy flow, one of the most fundamental processes, in crowded cellular environments.

Eureka Moments Shared by Chemists. Hints at Enhancing One's Own Creativity (and Even One's Joy)

Eureka moments can occur during all steps of discovery. Eighteen chemists and molecular scientists described their Eureka moments herein. Hints at fostering one's own Eureka moments are provided.

Indoor bioaerosols and asthma: Overview, implications, and mitigation strategies

Aerosolized particles with a biological origin are called bioaerosols. Bioaerosols from plants, animals, fungi, bacteria, and viruses are an important class of environmental exposures that are clinically relevant to asthma. However, there are important differences in the pathways by which various bioaerosols affect asthma. Additionally, differences in individual susceptibility to different bioaerosols affect exposure reduction and mitigation strategies. Strategies to reduce exposures to potential triggers of asthma are routinely considered as part of standard clinical care and asthma management guidelines. Ventilation standards in buildings may reduce bioaerosol exposure for everyone, but they are not necessarily designed specifically to protect patients with asthma. Direct measurement of a bioaerosol is not generally necessary for practical applications where the relevant source of the bioaerosol has been identified. Different types of bioaerosols can be controlled with similar strategies that prioritize source control (eg, reducing resuspension, integrated pest management, controlling moisture), and these can be supplemented by enhancing air filtration. The goal of this review is to summarize the latest information on bioaerosols, including allergens, fungi, bacteria, and viruses, that have been associated with adverse asthma outcomes and to discuss mitigation options.

How Vibrational Notations Can Spoil Infrared Spectroscopy: A Case Study on Isolated Methanol

Unraveling methanol's infrared spectrum has challenged spectroscopists for a century, with numerous loose ends still to be explored. We engage in this exploration based on experiments of isolating single methanol molecules in solid argon and neon matrices. We report infrared spectra of methanol in its natural isotopic composition and with partial and full deuteration. These experiments are accompanied by calculating wavenumbers involving anharmonicity and mode-coupling based on the vibrational configuration interaction approach. This allows for an unambiguous assignment of all fundamentals and resonances in the mid-infrared spectrum. An increasing degree of deuteration lifts resonances and aids in assigning bands uniquely. It also becomes evident that different notations typically used in chemistry or physics to describe molecular vibration from spectroscopy fail to describe the spectra appropriately. We highlight the shortcomings and suggest a more elaborate analysis using Sankey diagrams to unambiguously identify spectral features. Consequently, we demystify debated resonances occurring from various stretches and deformations of the methyl group.

Structural characterization of cage clusters assembled borophene and implication for cathode electrocatalysts in Li-O2 batteries

The successful fabrication of quasi-freestanding bilayer borophene in experiments, combined with its superior metallic character, has propelled it to a rising star among two-dimensional materials, making it highly promising for applications in micro-electronic devices. Using first-principles calculations, we comprehensively explore and characterize cage cluster assembled borophenes through various methods for experimental references. The simulated scanning tunneling microscope (STM) images under diverse bias voltages exhibit distinct morphologies and closely associated with the partial densities of states (PDOS) of the surface boron atoms. High-resolution and large-scale simulated transmission electron microscope (TEM) images are investigated to detect the internal crystal structures, facilitating better identification of non-monolayer borophenes. The partial densities of states, electronic localization functions and Mulliken bond populations have been calculated to analyze the differences of morphology in STM and TEM images. Furthermore, simulated X-ray diffraction (XRD), Raman, and infrared (IR) spectra are characterized to further assist in distinguishing the phases of borophene. In light of ultrahigh stability and excellent metallic character, cluster assembled borophene of P3¯m1 symmetry act as cathode materials of Li-O2 battery with lower overpotential in oxygen evolution reaction (OER) than oxygen reduction reaction (ORR) processes. The overpotential is closely related to the adsorption strength of LiO2 and Li2O2 intermediates on surface of boron sheets. These theoretical results offer crucial guidance for the experimental identification of borophenes and suggest that the new type of cluster assembled systems might be suitable for the cathode materials of future Li-O2 batteries.

Tuning Electronic Friction in Structural Superlubric Schottky Junctions

Friction at sliding interfaces, even in the atomistically smooth limit, can proceed through many energy dissipation channels, such as phononic and electronic excitation. These processes are often entangled and difficult to distinguish, eliminate, and control, especially in the presence of wear. Structural superlubricity (SSL) is a wear-free state with ultralow friction that closes most of the dissipation channels, except for electronic friction, which raises a critical concern of how to effectively eliminate and control such a channel. In this work, we construct a Schottky junction between a microscale graphite flake and a doped silicon substrate in the SSL state to address the issue and achieve wide-range (by 6×), continuous, and reversible electronic friction tuning by changing the bias voltage. No wear or oxidation at the sliding interfaces was observed, and the ultralow friction coefficient indicated that electronic friction dominated the friction tuning. The mechanism of electronic friction is elucidated by perturbative finite element analysis, which shows that migration of the space-charge region leads to drift and diffusion of charge carriers at Schottky junctions, resulting in energy dissipation.

Complex Dance of Molecules in the Sky: Choreography of Intermolecular Structure and Dynamics in the Cyclopentene-CO2-H2O Hetero Ternary Cluster

This study delves into driving forces behind the formation of a hetero ternary cluster consisting of volatile organic compounds from industrial or combustion sources, specifically cyclopentene, alongside greenhouse gases like carbon dioxide, and water vapor. While substantial progress has been made in understanding binary complexes, the structural intricacies of hetero ternary clusters remain largely uncharted. Our research characterized the cyclopentene-CO2-H2O hetero ternary cluster utilizing Fourier transform microwave spectroscopy. The observed isomer in the pulsed jet has CO2 and H2O aligning above the cyclopentene ring, with water undergoing an internal rotation approximately about its C2 symmetry axis. Theoretical analyses support these observations, identifying an O-H⋅⋅⋅π hydrogen bond and a secondary C⋅⋅⋅O tetrel bond within this cluster. This study marks a critical step towards comprehending the molecular dynamics and interactions of VOCs, greenhouse gases, and water in the atmosphere, paving the way for further investigations into their roles in climate dynamics and air quality.

Symmetry Breaking of Electronic Structure upon the π→π* Excitation in Anthranilic Acid Homodimer

The main purpose of this study is to characterize the nature of the low-energy singlet excited states of the anthranilic acid homodimer (AA2) and their changes (symmetry breaking) caused by deformation of the centrosymmetric, ground state structure of AA2 towards the geometry of the S1 state. We employ both the correlated ab initio methods (approximate Coupled Clusters Singles and Doubles-CC2 and CASSCF/NEVPT2) as well as the DFT/TDDFT calculations with two exchange-correlation functionals, i.e., B3LYP and CAM-B3LYP. The composition of the wavefunctions is investigated using the one-electron transition density matrix and difference density maps. We demonstrate that in the case of AA2, small asymmetric distortions of geometry bring about unproportionally large changes in the excited state wavefunctions. We further provide comprehensive characterization of the AA2 electronic structure, showing that the excitation is nearly completely localized on one of the monomers, which stands in agreement with the experimental evidence. The excitation increases the π-electronic coupling of the substituents and the aromatic ring, but only in the excited monomer, while the changes in the electronic structure of the unexcited monomer are negligible (after geometry relaxation). The increased electronic density strengthens both intra- and intermolecular hydrogen bonds formed by the carbonyl oxygen atom of the excited monomer, making them significantly stronger than in the ground state. Although the overall pattern of changes remains qualitatively consistent across all methods employed, CC2 predicts more pronounced excitation-induced modifications of the electronic structure compared to the more routinely used TDDFT approach. The most important deficiency of the B3LYP functional in the present context is locating two charge-transfer states at erroneously low energies, in close proximity of the S1 and S2 states. The range-corrected CAM-B3LYP exchange-correlation functional gives a considerably improved description of the CT states at the price of overshot excitation energies.

Cross Peaks on Two-Dimensional Optical Spectra Arising from Quantum Cross-Correlation Functions

Cross peaks on 2D optical spectra are indicative of interactions between molecular excitonic states. Currently, the two conventional assignments of cross peaks are direct coupling and population transfer between excitonic states. Here, we show that there is another possible source of cross peaks. We theoretically demonstrate that for a model comprising two nondirectly interacting excitons or two-level systems (TLSs), cross peaks can arise if there is a complex-valued or quantum frequency-gap cross-correlation function between the two TLSs. Considering only real-valued or classical cross-correlation functions will result in no cross peaks. We derive and validate the mathematical expressions describing such cross peaks. We then simulate the 2D electronic spectra of an example model system comprising nondirectly interacting TLSs whose quantum cross-correlation functions arise from coupling to a common overdamped Brownian oscillator mode. We show that there are clear observational differences between such quantum correlation cross peaks with conventional direct coupling and population transfer cross peaks.

Toward a Correct Description of Initial Electronic Coherence in Nonadiabatic Dynamics Simulations

The recent improvement in experimental capabilities for interrogating and controlling molecular systems with ultrafast coherent light sources calls for the development of theoretical approaches that can accurately and efficiently treat electronic coherence. However, the most popular and practical nonadiabatic molecular dynamics techniques, Tully's fewest-switches surface hopping and Ehrenfest mean-field dynamics, are unable to describe the dynamics proceeding from an initial electronic coherence. While such issues are not encountered with the analogous coupled-trajectory algorithms or numerically exact quantum dynamics methods, applying such techniques necessarily comes with a higher computational cost. Here we show that a correct description of initial electronic coherence can indeed be achieved using independent-trajectory methods derived from the semiclassical mapping formalism. The key is the introduction of an initial sampling over the electronic phase space and a means of incorporating phase interference between trajectories, both of which are naturally achieved when working within the semiclassical mapping framework.

Vibrational State-Resolved Differential Cross Sections of the F + CH4 → HF + CH3 Reaction at the Collision Energies of 3.1-13.8 kcal/mol

We report a high-resolution crossed molecular beam experiment investigating the reaction dynamics of the F + CH4 → HF + CH3 reaction across a broad range of collision energies (3.1-13.8 kcal/mol). By using time-sliced velocity map imaging within the crossed molecular beam apparatus, we obtain correlated angular distributions and branching ratios for various product pairs (CH3(ν), HF(ν')). The resolved reactive rainbow-like features that display a distinct bulge in the angular distribution in different channels reveal the formation of vibrationally ground and excited states of the methyl radical accompanied by different rovibrationally excited HF products. These findings suggest distinct reaction dynamics for channels leading to vibrationally excited methyl radicals compared to those forming ground-state methyl radicals.

Response of a 4-nitrothiophenol monolayer to rapid heating studied by vibrational sum frequency spectroscopy

A monolayer of 4-nitrothiophenol adsorbed on an Au substrate was heated by illuminating the substrate with a 19 ps laser pulse of 532 nm wavelength. Within 91 ps, the temperature of the sample increased from room temperature by 113 K. Vibrational sum frequency spectroscopy was used to characterize the adsorption geometry of the molecules in the ordered domains in the monolayer film. Upon heating, the initially ordered monolayer largely lost its structure. While the molecules are initially tilted by about 50° with respect to the surface normal, the analysis indicates that the mean tilt angle increased to 80° with a spread for individual molecules of up to a tilt angle of 40° upon heating. The evolution of this loss of order lagged about 100 ps behind the temperature rise of the substrate.

C3H8O2 Isomers: Insights into Potential Interstellar Species

2-Methoxyethanol, with a formula C3H8O2, was recently identified in the massive protocluster NGC 6334I. However, its structural isomers, 1,2-propanediol and 1,3-propanediol, remain undetected despite extensive searches in the Sgr B2 region. In this study, we explored the potential energy surface of the C3H8O2 system using CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ calculations, identifying 11 species, with the geminal diols 2,2-propanediol and 1,1-propanediol as the most stable forms. We examined the gas-phase decomposition barrier of these geminal diols and found that 1,1-propanediol is thermodynamically stable at low temperatures (10-150 K). C3H8O2 isomers with energies below 30 kcal/mol are relevant to the ISM, as they have been identified or tentatively detected in irradiation experiments of ice analogs of CO, H2O, and CH3OH.

What is the Exchange Repulsion Energy? Insight by Partitioning into Physically Meaningful Contributions

It is shown that the exchange repulsion energy, Exr, can be rationalized by partitioning the respective energy expression for two systems with Hartree-Fock orbitals into physically meaningful contributions. A division of Exr into a positive kinetic and a negative potential part is possible, but these contributions correlate only poorly with the actual exchange repulsion energy. A more meaningful partitioning is derived, where all kinetic energy contributions are collected in a term that vanishes for exact Hartree-Fock orbitals due to their stationarity conditions. The remaining terms can be distinguished into an exchange integral contribution as well as contributions to the repulsion energy with two, three and four orbital indices. The forms, relationships and absolute sizes of these terms suggest an intuitive partitioning of the exchange repulsion energy into Molecular Orbital Pair Contributions to the Exchange repulsion energy (MOPCE). Insight into the analytic form and quantitative size of these contributions is provided by considering the3 Σ u + ( 1 σ g 1 σ u ) ${^3 \Sigma _u^+ (1\sigma _g 1\sigma _u )}$ state of the H2 molecule, the water dimer, as well as an argon atom interacting with Cl2 and N2.