Journal of Chemical Physics最新文献

In this study, we synthesize strontium hexaferrite (SrFe12O19, SrM) with transition metal substitution cobalt (SrFe12-xCoxO19, SrCoM) with weight percentage (x = 3%, 5%, and 7%) through a chemical co-precipitation approach. The hexaferrite phase (P63/mmc) along with a small amount of secondary phase identified as hematite (α-Fe2O3) and halite (NaCl) were found from the x-ray diffraction analysis. A three-phase Rietveld refinement using FullProf software was used to evaluate the average bond lengths and angles, offering important information on superexchange interactions' impact inside the lattice. The magnetic ordering and crystal structure were also analyzed using neutron powder diffraction and the produced samples showed a ferrimagnetic arrangement at ambient temperature. The crystal structure, magnetic parameters, and maximal energy product (BH)max of the synthesized materials were found to be considerably affected by cobalt substitution: post-substitution, coercivity increases, while saturation magnetization and retentivity decrease. The present work emphasizes the potential of strontium hexaferrite and its cobalt-substituted derivatives as attractive magnetic materials for diverse magnetic applications.

We report the high-resolution photoelectron spectroscopy of transition metal carbide cluster anions TaCn- (n = 2-4) using a cryogenic ion trap combined with the slow electron velocity imaging (cryo-SEVI) technique. From the vibrationally resolved photoelectron spectra and associated ab initio calculations, the electron affinities of TaCn (n = 2-4) were determined with high precision: 1.818(2), 2.202(5), and 2.431(2) eV, respectively. The electronic and vibrational structures observed in the photoelectron spectra were interpreted using density-functional theory and coupled-cluster singles and doubles with perturbative triples calculations. Both the neutral TaCn clusters and their anions exhibit planar C2v structures, where the Ta atom bridges each C atom. Furthermore, we observed the spin-orbit splitting in the ground state of TaC2 (X̃4B1), with a measured splitting of 256(25) cm-1. This splitting is well explained by the calculated E1/2(±3/2)-E1/2(±1/2) splitting of 216 cm-1, obtained using the MRCI+SOC method.

Ultrafast time-resolved photoelectron spectra are reported for the vacuum-ultraviolet (VUV) photoionization of acetylene following excitation to the Ã1Au state via UV absorption at 200 nm. The excitation energy lies above the lowest dissociation threshold to C2H X̃2Σ+ + H, as well as above the threshold for adiabatic dissociation of the Ã1Au state to form C2H (Ã2Π) + H. The time-dependent mass spectra and photoelectron spectra provide insight into the intramolecular decay processes of the Ã1Au state. In addition, photoelectron spectra of the Ã1Au state with VUV light access both the X̃2Πu and Ã2Σg+ states of the ion, as well as the predicted, but previously unobserved, 1 2Πg state, which corresponds to a two-hole, one-particle configuration that lies in close proximity to the Ã2Σg+ state. The 1 2Πg state is split into 2A2 + 2B2 and 2Ag + 2Bg states in the cis and trans configurations, respectively. Electronic structure calculations, along with trajectory calculations, reproduce the principal features of the experimental data and confirm the assignment of the 1 2Πg state.

The Random First-Order Transition (RFOT) theory predicts that transport proceeds by the cooperative movement of particles in domains, whose sizes increase as a liquid is compressed above a characteristic volume fraction, ϕd. The rounded dynamical transition around ϕd, which signals a crossover to activated transport, is accompanied by a growing correlation length that is predicted to diverge at the thermodynamic glass transition density (>ϕd). Simulations and imaging experiments probed the single particle dynamics of mobile particles in response to pinning all the particles in a semi-infinite space or randomly pinning (RP) a fraction of particles in a liquid at equilibrium. The extracted dynamic length increases non-monotonically with a peak around ϕd, which not only depends on the pinning method but is also different from ϕd of the actual liquid. This finding is at variance with the results obtained using the small wavelength limit of a four-point structure factor for unpinned systems. To obtain a consistent picture of the growth of the dynamic length, one that is impervious to the use of RP, we introduce a multiparticle structure factor, Smpc(q,t), that probes collective dynamics. The collective dynamical length, calculated from the small wave vector limit of Smpc(q,t), increases monotonically as a function of the volume fraction in a glass-forming binary mixture of charged colloidal particles in both unpinned and pinned systems. This prediction, which also holds in the presence of added monovalent salt, may be validated using imaging experiments.

The wetting properties of a liquid in contact with a solid are commonly described by Young's equation, which defines the relationship between the angle made by a fluid droplet onto the solid surface and the interfacial properties of the different interfaces involved. When modeling such interfacial systems, several assumptions are usually made to determine this angle of contact, such as a completely rigid solid or the use of the tension at the interface instead of the surface free energy. In this work, we perform molecular dynamics simulations of a Lennard-Jones liquid in contact with a Lennard-Jones crystal and compare the contact angles measured from a droplet simulation with those calculated using Young's equation based on surface free energy or surface stress. We analyze cases where the solid atoms are kept frozen in their positions and where they are allowed to relax and simulate surfaces with different wettability and degrees of softness. Our results show that using either surface free energy or surface stress in Young's equation leads to similar contact angles but different interfacial properties. We find that the approximation of keeping the solid atoms frozen must be done carefully, especially if the liquid can efficiently pack at the interface. Finally, we show that to correctly reproduce the measured contact angles when the solid becomes soft, the quantity to be used in Young's equation is the surface free energy only and that the error committed in using the surface stress becomes larger as the softness of the solid increases.

We report the scanning tunneling microscopy/spectroscopy (STM/STS) studies on structural and electronic properties of picene films grown on the semimetallic Bi(111) substrate held at different temperatures. Under room-temperature deposition, the picene molecules form a crystalline (001) monolayer with the standing-up orientation, indicating the weak molecule-substrate interaction. When deposited on the Bi(111) substrate held at 150 K, picene molecules form a bulk-like (211̄ monolayer with building blocks of picene trimers. High-resolution STM images reveal that each trimer consists of two tilted molecules and one side-on molecule. Further reducing the deposition temperature to 90 K leads to the formation of nanostripe arrays, in which the side-on molecules adopt the π-π stacking. STS measurements demonstrate that the crystalline (001) monolayer of picene exhibits a larger gap compared with picene crystals, which can be attributed to the decoupling of the upright standing molecules from the semimetallic Bi(111) substrate.

We present results and discuss methods for computing the melting temperature of dense molecular hydrogen using a machine learned model trained on quantum Monte Carlo data. In this newly trained model, we emphasize the importance of accurate total energies in the training. We integrate a two phase method for estimating the melting temperature with estimates from the Clausius-Clapeyron relation to provide a more accurate melting curve from the model. We make detailed predictions of the melting temperature, solid and liquid volumes, latent heat, and internal energy from 50 to 180 GPa for both classical hydrogen and quantum hydrogen. At pressures of roughly 173 GPa and 1635 K, we observe molecular dissociation in the liquid phase. We compare with previous simulations and experimental measurements.

We analyzed the thermal, structural, and dynamic properties of maghemite using classical molecular dynamics, focusing on bulk and nanoparticle systems. We explored their behavior when heated to high temperatures (above the melting point) and during cooling, as well as under thermal cycles ending at intermediate temperatures. Our findings show that in the bulk system, both the tetrahedral and octahedral iron sub-lattices undergo a phase transition prior to melting. Cooling the system from above this transition, or from above the melting point, leads to the formation of different metastable maghemite structures. In contrast, this sub-lattice transition is absent in nanoparticles, where melting occurs through an interface-mediated process. At temperatures just above the transition, nanoparticles adopt an ellipsoidal shape, which is retained during cooling. In addition, the specific heat of both bulk and nanoparticle systems at temperatures above the Debye temperature is evaluated and compared with the available experimental data. Overall, our results highlight the complex thermal behavior of maghemite across a range of temperatures, which remains insufficiently explored experimentally. Further experimental investigations could also provide valuable feedback for model refinements.

The Effective Fragment Potential (EFP) method, a polarizable quantum mechanics-based force field for describing non-covalent interactions, is utilized to calculate protein-ligand interactions in seven inactive cyclin-dependent kinase 2-ligand complexes, employing structural data from molecular dynamics simulations to assess dynamic and solvent effects. Our results reveal high correlations between experimental binding affinities and EFP interaction energies across all the structural data considered. Using representative structures found by clustering analysis and excluding water molecules yields the highest correlation (R2 of 0.95). In addition, the EFP pairwise interaction energy decomposition analysis identifies critical interactions between the ligands and protein residues and provides insight into their nature. Overall, this study indicates the potential applications of the EFP method in structure-based drug design.

We have quantum chemically analyzed the trends in bond dissociation enthalpy (BDE) of H3C-XHn single bonds (XHn = CH3, NH2, OH, F, Cl, Br, I) along three different dissociation pathways at ZORA-BLYP-D3(BJ)/TZ2P: (i) homolytic dissociation into H3C∙ + ∙XHn, (ii) heterolytic dissociation into H3C+ + -XHn, and (iii) heterolytic dissociation into H3C- + +XHn. The associated BDEs for the three pathways differ not only quantitatively but, in some cases, also in terms of opposite trends along the C-X series. Based on activation strain analyses and quantitative molecular orbital theory, we explain how these differences are caused by the profoundly different electronic structures of, and thus bonding mechanisms between, the resulting fragments in the three different dissociation pathways. We demonstrate that the nature and strength of a chemical bond are only fully defined when considering both (i) the molecule in which the bond exists and (ii) the fragments from which it forms or into which it dissociates.