The Journal of Physical Chemistry A最新文献

We employ velocity map imaging (VMI) to investigate the photodissociation of manganese pentacarbonyl bromide (MnBr(CO)₅)), both in the gas phase and on a surface. Pulsed 266 nm and/or 230 nm laser light was used to initiate the photodissociation cleaving the Mn–CO bonds. A single 230 or 266 nm photon has enough energy to lead to the dissociation of all five Mn–CO bonds. The CO is ionized using a (2+1) resonance-enhanced multiphoton ionization (REMPI) scheme via the B¹Σ⁺–X¹Σ⁺ transition with ∼230 nm photons. To deconvolute the contributions from the gas-phase and surface photodissociation, one- and two-color experiments are compared. In the one-color experiment, a focused 230 nm light above the sample surface was used to dissociate gas-phase MnBr(CO)₅ and to ionize CO via REMPI. The two-color experiments use grazing angle illumination with a 266 nm laser beam to induce the dissociation of MnBr(CO)₅ on the sample surface and probe with a focused 230 nm laser beam above the sample surface to ionize the product CO at a specific time delay. The CO from the one-color gas-phase photodissociation is seen as a background to the surface dissociation CO signal and was subtracted from both the VMI images and Q-branch REMPI spectra of CO. In the two-color time-resolved detection of CO above the surface after photodissociation at the surface, we observe a complex arrival time distribution. To explain this arrival time distribution, we hypothesize that there are four main pathways for the formation of CO: (1) hyperthermal CO ejected directly into the gas phase from the surface as a result of 266 nm photodissociation, (2) thermalized CO coming from the photodissociation of MnBr(CO)₅ that resides on the surface for sufficient time to come into thermal equilibrium with the surface before desorption, (3) CO from photodissociation of intact MnBr(CO)₅ ejected from the surface and dissociated by the 230 nm laser beam, and (4) CO from the gas-phase photodissociation of MnBr(CO)₅ with 266 nm laser light near the surface. Q-branch REMPI spectra of CO and VMI images were collected at various time delays and are consistent with our hypothesis.

Atmospheric ammonia, in both particulate and gaseous forms, has major ecological, health, and economic impacts, making it essential to understand its chemical processes. The reactions of ammonia and ammonia complexed with nitric acid with hydroxyl radical and the oxidation of nitric acid by amidogen radical and ammonia by nitrate radical, both taking into account the effect of water vapor, have been investigated using quantum mechanical (QCISD and CCSD(T)) calculations with the 6-311+G(2df,2p), aug-cc-pVTZ, and aug-cc-pVQZ and extrapolation to the CBS basis sets. From a mechanistic point of view, the reaction of NH₃ + OH follows a conventional hydrogen transfer mechanism, but for the rest of reactions considered, the proton coupled electron transfer mechanism plays a key role. For the reaction of ammonia with hydroxyl radical we have computed a rate constant of 1.24 × 10^–13 cm³·molecule^–1·s^–1 at 298 K, and the effect of water vapor is negligible. The calculated rate constant for the HNO₃···NH₃ + OH reaction is 6.50 × 10^–16 cm³·molecule^–1·s^–1 at 298 K. and our results show that both HNO₃ and NH₃ moieties can be oxidized. The effect of water vapor on the oxidation of nitric acid by an amidogen radical is significant. We have computed a rate constant of 1.98 × 10^–13 cm³·molecule^–1·s^–1, at 298 K and 100% of RH for the whole HNO₃ + NH₂ + H₂O reaction, which is an 11% greater than the calculated value for the naked reaction. For the oxidation of ammonia by a nitrate radical, the effect of water vapor is huge. The calculated rate constant at 298 K and 100% of RH is 16 × 10^–15 cm³·molecule^–1·s^–1 for the whole NH₃ + NO₃ + H₂O reaction, that is, 751% greater than the value of the naked reaction at 298 K.

Perovskite solar cells (PSCs) exhibit outstanding photovoltaic performance, but the discovery of new hybrid organic–inorganic perovskites (HOIPs) remains constrained by the scarcity of high-quality data. Given the strong structure–property correlations in crystalline materials, we adopt the Crystal Graph Convolutional Neural Network (CGCNN) to learn intrinsic structural representations. To overcome the limitations of small data sets, we propose an Adaptive Transfer Learning-based CGCNN (ATL-CGCNN), which integrates transfer learning and adaptive fine-tuning within the CGCNN framework. The model is first pretrained on a large-scale crystal data set, then transferred to the HOIP domain, and subsequently fine-tuned using high-confidence samples identified based on prediction errors. Using ATL-CGCNN, we predicted the HSE06 band gaps of 160 unreported HOIPs. Seven candidates with promising photovoltaic potential were identified and validated using Density Functional Theory (DFT), showing good agreement between predicted and calculated values. Additionally, machine learning analysis confirmed that elemental features had limited contribution to bandgap prediction. This approach demonstrates a data-efficient and accurate strategy for accelerating the discovery of novel HOIP materials.

A central challenge in molecular magnetism is achieving scalable magnetic moments as molecular systems increase in size. Phenalenyl, a stable radical with an S = 1/2 spin state, serves as an appealing building block for molecular magnetic materials. However, synthesizing high-spin-state stacked dimers has proven difficult due to strong tendencies toward antiferromagnetic pairing. Here, we combine density functional theory with convolutional neural networks to investigate how the stacking geometry governs spin alignment in phenalenyl dimers. Our machine-learning-assisted approach enables efficient mapping of the high-dimensional configurational space, revealing that lateral in-plane displacement can dramatically alter the spin state. Notably, even eclipsed α-carbon stackings, which typically favor low-spin states, can be converted into high-spin configurations through sliding. These results uncover a stacking-dependent mechanism of spin-state control and offer a practical strategy for designing high-spin radicals relevant to quantum information science and molecular spintronics.

Understanding the role of the medium on chemical reactions occurring in thin water films covering solid surfaces is an important issue in many areas of chemistry, particularly in atmospheric and environmental sciences. The hydrolysis of nitrogen dioxide NO₂ into nitric and nitrous acids has long been considered a prototypical example of this type of chemistry. Various experiments have shown that this process, which is not favorable in gas-phase, occurs easily in supported water films. In general, the reacting species has been assumed to be the (NO₂)₂ dimer, which is known to spontaneously form the contact ion pair NO₃^–···NO⁺ in the presence of water. Using advanced molecular dynamics simulations based on combined quantum-classical force-fields we have studied the properties of this dimer in water films on hydrophobic surfaces. We show that solvent effects on several reactivity indices of the contact ion pair in this medium are larger compared to solvent effects in bulk water. Analysis of this unexpected result reveals that there is a significant shift in the electrostatic potential generated by the reorganization of water molecules around the ion pair in the films compared to the bulk solution. This finding bears similarities to previous research on solvent effects at the air–water interface of microdroplets, which are well-known to enhance chemical reactivity. The results presented here will be useful for designing new experiments and calculations to further explore the potential catalytic effect of thin water films supported on surfaces.

Exploring the ground-state structures of cluster molecules formed by hydrogen-bonding interactions remains a significant challenge for nanoscience. We propose a novel method called the hydrogen atom ratio-based docking evolution algorithm (HARDEA) for exploring the ground-state structures of molecules formed by hydrogen-bond interactions. This method converges faster than traditional evolutionary algorithms and has a higher probability of discovering new ground-state structures. Eighteen new ground-state structures were discovered using the HARDEA method for sulfuric acid (SA)–dimethylamine (DMA) ((SA)_n(DMA)_m (n = 1–4, m = 1–4)) and sulfuric acid (SA)–3-methyl-1,2,3-butane-tricarboxylic acid (MBTCA) ((SA)_n(MBTCA)_m (n = 1–3, m = 1–3)) systems and account for 67% of the total structures. The lowest-energy structures are 5.69 and 4.97 kcal/mol lower than those reported in the literature for (SA)₄(DMA)₄ and (SA)₁(MBTCA)₂, respectively. Compared to the field and experimental measurements of new particle formation rates, the simulated formation rates based on these new ground-state structures show improved accuracy by 20 and 3.7% than literature values for the (SA)_n(DMA)_m (n = 1–4, m = 1–4) and (SA)_n(MBTCA)_m (n = 1–3, m = 1–3) systems, respectively. The HARDEA method is general and flexible and can be applied to different kinds of problems with hydrogen-bond interaction, such as molecular crystal structure prediction, new particle formation, and protein–drug interactions.

Singlet fission (SF) is attracting attention due to its potential applications in organic photovoltaics, quantum computing, and dynamic nuclear polarization. In this article, we have theoretically investigated the potential of 4,4′-diphenoquinone (1a) as a promising backbone for efficient SF chromophores. Single-reference ab initio correlated methods were employed to investigate the excited states of 1a. A strong photoabsorption is predicted for the S₀–S₃ transition, which is described by HOMO–LUMO excitation. Although the excitation energies of S₃ and T₁ states satisfy the exothermic energy matching condition of SF, internal conversion from the S₃ to a low-lying nπ* singlet excited state may suppress the efficient generation of ¹(T₁T₁) in the condensed phase. We have demonstrated that the obstacle of 1a can be overcome by introducing typical donors or halogen atoms and fusing aromatic six-membered ring(s) while retaining the advantages of the diphenoquinone backbone.

The composite of quantum dots (QDs) with graphene (GR) has significantly enhanced its potential for optoelectronic applications. Although existing research has experimentally confirmed the excellent optoelectronic properties of quantum dots-graphene (QD-GR) heterostructures, the underlying charge transfer mechanisms have not been fully elucidated. This study uses the Marcus theory to investigate the photoinduced charge transfer characteristics of QD-GR heterostructures under external electric field (F_ext) modulation. We first constructed a lead sulfide quantum dots-graphene (PbS QD-GR) heterojunction model using a minimal-sized lead sulfide quantum dots (PbS QDs) cluster and a single-layer flake graphene. Next, we systematically analyzed the electronic state distribution at the interface and elucidated the essential mechanism underlying the heterojunction’s structural stability. The excited state properties of the PbS QD-GR heterojunction under F_ext modulation were systematically investigated. Finally, based on Marcus theory, the reorganization energy (λ), Gibbs free energy (ΔG) and electron coupling matrix element (V_da) were quantitatively calculated to reasonably predict the charge transfer rate (K). The study revealed that the charge separation rate significantly exceeds the charge recombination rate (KCS ≫ KCR), demonstrating the heterostructure’s exceptional exciton dissociation capability. Our findings elucidate the trend of charge transfer parameters in specific PbS QD–graphene heterojunctions under external electric fields, thereby providing theoretical support for a deeper understanding of optoelectronic device performance.

A software implementation of analytic geometric derivatives of electron-repulsion integrals up to second order is presented for the modeling of vibrational spectroscopies at the level of first-principles Kohn–Sham density functional theory (DFT). In line with the general goals of the VeloxChem program, it targets efficient execution in high-performance computing environments with a hybrid MPI/OpenMP parallelization model and is based on the technique of automatic C++ code generation for high versatility. Gradient calculations scale identically with conventional Fock matrix constructions, and also with the prefactor taken into account, the computational cost of the gradient is significantly lower than that of the self-consistent field (SCF) optimization of the reference state. The Hessian calculation shows a scaling of N^3.5 with N being the number of contracted Gaussian basis functions. The computational bottleneck in the Hessian calculation is the solving of the coupled-perturbed Kohn–Sham equations that with VeloxChem can be offloaded to GPU-accelerated nodes. The large-scale virtues of the presented software module are demonstrated by the DFT/B3LYP calculation of the IR spectrum of the entire ubiquitin protein with 1,152 atoms in the quantum mechanical (QM) region and TIP3P water in the molecular mechanics (MM) region. The simulated amide I band shows to be in excellent agreement with experiment.

Carboxylic acids are ubiquitous in the atmosphere in both the gas and particle phase. One of their major degradation pathways is known to be their reaction with OH radicals. To enhance our understanding of carboxylic acid reactivity, we investigated the gas phase reactions of OH radicals with a series of these acids by performing quantum chemistry calculations at the ωB97X-V/def2-TZVPPD, ωB97M-V/def2-TZVPPD, CCSD(T)/6–311+G(2df,2p), and CCSD(T)/aug-cc-pVTZ level. Our calculations are mainly focused on the reactive decarboxylation pathway energetics that occur following the OH-driven abstraction of the carboxylic hydrogen of formic, acetic, malonic, fumaric, maleic, trifluoroacetic, and trichloro-acetic acids. All the reactions are exoergic, and in all cases, we find that a hydrogen-bonded complex with a 6-membered ring structure is formed between the acid group and the OH, followed by reactive decarboxylation with small or nonexistent barriers to both abstraction and decarboxylation steps. Finally, in the case of trifluoroacetic acid decarboxylation, we calculate the energetics of several pathways that may lead to stable oxidation product formation, and we compare our theoretical work to other experimental and theoretical studies.