The Journal of Physical Chemistry Letters最新文献

Aromaticity is a century-old concept that is even introduced in high school textbooks. However, the determination of the order of aromaticity of molecules as simple as furan, thiophene, and selenophene is still challenging. This work describes how different theoretical and experimental methods posit different aromaticity orders. To benchmark the theoretical results and arrive at a conclusion, mass-selective electronic and vibrational spectroscopy of these five-membered heterocycles under isolated supersonic-jet-cooled conditions was necessary. Since the aromaticity order can be unveiled from the magnitude of the electron density in the ring, we used hydrogen bonding as a probe. The experimental results revealed that selenophene forms the strongest π-hydrogen bond, suggesting that selenophene is the most aromatic, followed by thiophene and furan. It is concluded that gauge-including magnetically induced currents (GIMIC) and relative ¹H and ¹³C NMR chemical shifts are better parameters to determine the aromaticity order in a similar class of molecules.

Thermodynamic integration (TI) offers a rigorous method for estimating free-energy differences by integrating over a sequence of interpolating conformational ensembles. However, TI calculations are computationally expensive and typically limited to coupling a small number of degrees of freedom due to the need to sample numerous intermediate ensembles with sufficient conformational-space overlap. In this work, we propose to perform TI along an alchemical pathway represented by a trainable neural network, which we term Neural TI. Critically, we parametrize a time-dependent Hamiltonian interpolating between the interacting and noninteracting systems and optimize its gradient using a score matching objective. The ability of the resulting energy-based diffusion model to sample all intermediate ensembles allows us to perform TI from a single reference calculation. We apply our method to Lennard-Jones fluids, where we report accurate calculations of the excess chemical potential, demonstrating that Neural TI reproduces the underlying changes in free energy without the need for simulations at interpolating Hamiltonians.

Cryogenic solid para-hydrogen (p-H₂) exhibits pronounced quantum effects, enabling unique experiments that are typically not possible in noble-gas matrices. The diminished cage effect facilitates the production of free radicals via in situ photolysis or photoinduced reactions. Electron bombardment during deposition readily produces protonated and hydrogenated species, such as polycyclic aromatic hydrocarbons, that are important in astrochemistry. In addition, quantum diffusion delocalizes hydrogen atoms in solid p-H₂, allowing efficient H atom reactions with astrochemical species and introducing new concepts in astrochemical models. Some H atom reactions display anomalous temperature behaviors, highlighting the rich chemistry in p-H₂. The investigation on quantum diffusion of heavier atoms and molecules is also important for our understanding of the chemistry in interstellar ice. Additionally, matrix shifts of electronic transitions of polycyclic aromatic hydrocarbons in p-H₂ are less divergent than those in solid Ne such that systematic measurements in p-H₂ might help in the assignment of diffuse interstellar bands.

The phenomenon of thermal quenching of luminescence can significantly compromise the efficiency of luminescent materials, a process accompanied by the generation of substantial phonon populations. While plenty of models for elucidating this behavior have been proposed, the crucial role of phonon transport has largely been neglected, particularly in the enigmatic incommensurate scheelite structure with good luminescence performance. In this study, we delve into the thermal quenching dynamics of the near-infrared emission in the incommensurately modulated CaGd₂(MoO₄)₄:Yb/Er system. Our comprehensive investigation reveals distinct evolutionary patterns in electrical conductivity, luminescence intensity, thermal conductivity, and Raman scattering at varying temperature regimes. Notably, we have determined that thermally induced ion migration, occurring above ∼300 °C, serves as a pivotal trigger for the activation of all phonons and the enhancement of phonon–defect scattering within this incommensurate framework. This phenomenon not only diminishes the thermal conductivity but also accelerates the multiphonon relaxation of the Er³⁺ emission levels, culminating in a marked thermal quenching of luminescence. This work illuminates the thermal quenching mechanism of luminescence by focusing on phonon scattering dynamics, providing critical insights for the design of thermally robust near-infrared luminescent materials, which are essential for the advancement of optical amplification systems.

Gas phase reactions have been a subject of research interest, enabling reliable strategies to explore the stability and reactivity of metal clusters as well as to probe novel superatoms that form the building blocks to assemble new materials with tailored properties. Coinage metal clusters have attracted great research attention due to their simple electronic shell structures and rich photochemical and catalytic properties at relatively low cost. This perspective focuses on the recent progress made in studying the gas phase reactions of undamaged and single-atom-doped Cu_n^±,0 and Ag_n^±,0 clusters with O₂, CO, and NO molecules. It covers various aspects, such as reaction mechanisms, relationships between structure and activity, control of reactivity by changing cluster size and composition, and the identification of novel superatoms (Cu₁₈^–, Ag₁₃^–, Ag₁₇^–, and Ag₁₅O⁺). Lastly, we provide a detailed account of the obstacles and prospective avenues for future research in order to establish a connection between these findings and nanocluster systems that have practical applications.

Aryl ketones are often used as photosensitizers and photoinitiators. Free radical intermediates have been suggested, but not confirmed, to be generated after photoirradiation. Here we found, unexpectedly, that a persistent radical was produced from di-2-pyridyl ketone after UV irradiation, which was detected by the direct ESR method. Interestingly, the persistent radical was very sensitive to oxygen and the pH of the reaction medium. A similar persistent radical was also observed from phenyl-2-pyridyl ketone, but not from 3-benzoylpyridine, 4-benzoylpyridine, and benzophenone, suggesting that the presence of a carbonyl group connected to the ortho-position of the pyridine ring is critical for such radical production. By complementary applications of ESR, HPLC, and ESI-Q-TOF-MS, the possible chemical structures of the persistent radical and final product were identified, and the possible underlying reaction mechanism was proposed. This represents the first report on UV-induced persistent radical generation from 2-pyridyl ketones, which should be of great significance for future studies.

A block-correlated coupled cluster (BCCC) method based on the triplet generalized valence bond (GVB) wave function (GVB-BCCC) has been implemented for the first time. By introducing several techniques, we have developed a practical and efficient GVB-BCCC code. The GVB-BCCC3 method (with up to three-pair correlation) can be used to deal with strongly correlated (SC) systems with triplet or singlet ground states, allowing singlet–triplet (S-T) energy gaps in the active space of SC systems computationally available. For selected SC systems, our calculations show that GVB-BCCC3 can always provide correct ground-state spin multiplicity as the complete active space configuration interaction (CASCI) or density matrix renormalization group (DMRG). Furthermore, we found that the S-T energy gaps from GVB-BCCC3 are quite consistent with CASCI or DMRG results. This work demonstrates that GVB-BCCC3 is a promising theoretical tool for describing S-T energy gaps within the active space of SC systems with large active spaces.

Fluid exudation in cartilage under normal loading can be counteracted by a sliding-induced rehydration phenomenon, which has a hydrodynamic origin related to a wedge effect at the contact inlet. Similar to cartilage, hydrogels also exhibit tribological rehydration properties, and we mimic this phenomenon to restore hydration lubrication and overcome creeping. It occurs within a specific velocity range and is mainly dependent on the applied load and hydrogel network structures. Crucially, a certain velocity in the mixed lubrication regime can produce a hydrodynamic pressure peak at the wedge and drive the rehydration inflow to overcome the extrusion. At lower sliding velocities in the boundary lubrication regime, inflows are insufficient to counteract fluid exudation, whereas at higher velocities in the hydrodynamic lubrication regime, the inlet wedge effect would diminish. These results suggest that tribological rehydration offers a novel approach to enhancing load-bearing capacity and maintaining lubrication in the hydrogels.

The interaction of molecules, in particular, water, with solid interfaces has been studied by a multitude of methods, among them nuclear magnetic resonance spin relaxation. The frequency dependence of the relaxation times follows patterns that have been interpreted in terms of the molecular orientation and dynamics. Several different model approaches could successfully explain limiting cases of ¹H relaxation dispersion in systems with rigid surfaces such as silica gel or glass, but none of them can reproduce the relaxation of both ¹H and ²H nuclei, which differ in their respective relaxation mechanisms, dipolar vs quadrupolar. From detailed studies of the dynamics of hydration of water in biological materials, the importance of hydrogen and molecular exchange to the longitudinal relaxation time of T₁ was demonstrated. In this work, exchange times of both H₂O and D₂O in hydrophilic silica gel are varied in a controlled fashion in a wide range using disodium hydrogen phosphate, and the effect of physical exchange on spin relaxation is quantified for the first time in such systems using the exchange-mediated reorientation model.

The entropy of mixing of a multicomponent system of particles is a simple expression of the molar fractions for the equilibrium state, but its intermediate values for transient (nonequilibrium) states can not be calculated directly from the particle coordinates so far. We propose a simple expression for the configurational entropy of mixing based solely on the set of instantaneous coordinates, which is suitable for the on-the-fly determination of the degree of mixing along a molecular dynamics trajectory. We illustrate the applicability of our scheme with the example of several molecular mixtures that exhibit fast and slow mixing and demixing processes within a molecular dynamics simulation.