The Journal of Physical Chemistry A最新文献

We report novel total electron scattering cross sections (TCS) from nitric oxide (NO) in the impact energy range from 1 to 15 eV by using a magnetically confined electron transmission apparatus. The accuracy of the data to within 5% and its consistency across the energy range investigated, shows significant discrepancies from previous works as to the major resonance features and magnitude of the TCS. Within the shape of the TCS, we have identified nine features which have been assigned to electron attachment resonances, most of them reported for the first time, while a comprehensive analysis of those peaking at 7.0, 7.8, and 8.8 eV has led to solve the controversy about dissociative electron attachment (DEA) cross-section that persisted for more than 50 years.

This study investigates the formation of negative ion fragments from gas-phase 2-propanol molecules after interaction with soft X-rays near the O K-edge. The experiment was performed by detecting negative and positive ions in coincidence with time-of-flight spectrometry. The analysis of two- and three-ion coincidence data revealed that nine different anions were produced: H^-, C^-, CH^-, CH₂^-, O^-, OH^-, C₂^-, C₂H^-, and C₃^-. For all anions, the most common three-ion events were those involving two protons. The results highlight the sensitivity of negative-ion/positive-ion coincidence spectroscopy and provide new insight into the fragmentation processes of organic molecules under soft X-ray excitation.

A theoretical study on the mechanism, regioselectivity, and enantioselectivity of NHC-catalyzed dearomatizing annulation of benzoxazoles with enals has been conducted using density functional theory calculations. Our calculated results indicate that the favored mechanism occurs through eight reaction steps: initial binding of the NHC to enals, followed by formation of the Breslow intermediate via proton transfer. Subsequent oxidation generates the α,β-unsaturated acylazolium intermediate, which can undergo Michael addition with benzoxazoles. Sequential protonation/deprotonation/cyclization produces the six-membered cyclic intermediate that undergoes catalyst elimination, leading to the final product. DABCO·H⁺ was found to play important roles in proton transfer and cyclization. Without DABCO·H⁺, the energy barrier up to 44.2 kcal/mol for step 2 is too high to be accessible. With DABCO·H⁺, the corresponding value is lowered to 18.6 kcal/mol. The energy barrier for cyclization can be lowered by 7.4 kcal/mol by using DABCO·H⁺. The Michael addition step determines both the enantioselectivity and the regioselectivity. According to NCI analysis, the enantioselectivity is controlled by the strong interactions (such as C-H···O, C-H···N, and π···π) between the α,β-unsaturated acylazolium intermediate and benzoxazoles. We also discuss the solvent and substituent effects on the enantioselectivity and the role of the NHC. The mechanistic insights obtained in the present study would help improving current reaction systems or designing new synthetic routes.

Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are powerful analytical tools with diverse applications in research and medicine. However, the inherently poor signal-to-noise ratios induce technical limitations, which suppress their widespread use. Hyperpolarization enhances the NMR signals by inducing highly nonequilibrated population distributions among the nuclear spin states. We demonstrated real-time amino acid hyperpolarization using signal amplification by reversible exchange (SABRE). We aimed to hydrolyze hyperpolarized methyl esters to induce amino acid hyperpolarization. We successfully hyperpolarized 19 methylated amino acids via SABRE. This groundwork enabled the development of a predictive model for the hyperpolarization enhancement factors of methylated amino acids. The model accurately predicted the hyperpolarization of three synthetic methylated amino acids, paving the way for advanced bio-NMR and MRI applications requiring the immediate hyperpolarization of other amino acids. This research underlines the potential of hyperpolarization in overcoming the current limitations of NMR spectroscopy and MRI.

Nitrogen substitutions have shown a great impact for the development of thermally activated delayed fluorescence (TADF)-based organic light-emitting diode (OLED) materials. In particular, much focus has been devoted to nitrogen-substituted polycyclic aromatic hydrocarbons (PAHs) for TADF emitters. In this context, we provide here a molecular design approach for symmetric nitrogen substitutions in fused benzene ring PAHs based on the dibenzo[a,c]picene (DBP) molecule. We designed possible donor-acceptor (D-A) compounds with dimethylcarbazole (DMCz) and dimethyldiphenylamine (DMDPA) donors and studied the structure and photophysics of the designed D-A compounds. The twisted and extended D-A-type PAH emitters demonstrate red and near-infrared (NIR) TADF emission. Nitrogen substitutions lead to significant LUMO stabilization and reduced HOMO-LUMO energy gaps as well. Additionally, we computed significantly smaller singlet-triplet energy splittings (ΔE_ST) in comparison to non-nitrogen-substituted compounds. The investigated ortho-linked D-A compounds show relatively large donor-acceptor twisting separation and small ΔE_ST compared to their para-linked counterparts. For higher number nitrogen (4N)-substituted emitters, we predict small adiabatic ΔE_ST (ΔE_ST^adia) in the range 0.01-0.13 eV, and with the tert-butylated donors, we even obtained ΔE_ST^adia values as small as 0.007 eV. Computed spin-orbit coupling (SOC) for the T₁ triplet state on the order of 0.12-2.28 cm^-1 suggests significant repopulation of singlet charge transfer (¹CT) excitons from the triplet CT and locally excited (³CT+LE) states. Importantly, the small ΔE_ST^adia and large SOC values induce a reverse intersystem crossing (RISC) rate as high as 1 × 10⁶ s^-1, which will cause red and NIR delayed fluorescence in the 4N-substituted D-A emitters. Notably, we predict red TADF emission for the para-linked compound B4 at 670 nm and the ortho-linked compound D4 at 713 nm and delayed NIR emission at 987 and 1217 nm for the ortho-linked compounds D3 and E3, respectively.

The redox potentials for U, Np, Pu, and Am for oxidation states +III up to +VIII in alkaline aqueous solutions were predicted using density functional theory (DFT) and small-core pseudopotentials and their basis sets, with a hybrid explicit/implicit solvent model using SHE = 4.28 V. For each oxidation state, various oxo/hydroxo complexes were evaluated, resulting in a variety of one-electron redox pathways. For An(VIII/VII) couples, the predicted redox potentials for the [An(VIII)O₅(OH)]^-3/[An(VII)O₄(OH)₂]^-3 or [An(VIII)O₄(OH)₂]^-2/[An(VII)O₄(OH)₂]^-3 couples are in good agreement with existing estimates. For An(VII/VI) redox couples, all couples, particularly [An(VII)O₄(OH)₂]^-3/[An(VI)O₂(OH)₄]^-2, were in agreement with experimental values for U, Np, and Pu, but the results for Am showed larger differences from the estimated potentials. The An(VI/V) couples were consistent with experiments for dioxo/tetrahydroxo couples, and the An(V/IV) couples showed acceptable agreement based on actinide-specific couples, with neutral hydroxides often favored in the +IV state. The An(IV/III) couples were consistent with the literature values when modeled as soluble neutral hydroxides. The use of our approach yielded calculated redox potentials that were within ±0.2 V of experimental or estimated values consistent with our prior calculations on redox potentials of actinides from Ac to Am in acidic aqueous solutions. This supports the robustness of our DFT-based methodology for predicting actinide redox potentials, offering valuable insights into actinide chemistry in aqueous solutions.

The mechanisms of iron-catalyzed [4 + 2] cycloadditions of unactivated dienes were investigated using density functional theory calculations. The calculation results show that the reaction involves sequential key steps of an initial ligand exchange followed by oxidative coupling, isomerization to form a seven-membered ferracycle intermediate, and C-C reductive elimination to form the cyclohexene product. The C-C reductive elimination step is shown to be the rate-determining step of the catalytic cycle. Moreover, energy profiles with three possible spin states (S_Fe = 0, 1, 2) have been considered. The results show that spin crossing occurs mainly through quintet intermediates and triplet transition states, which indicates that the reaction has a two-state reactivity. In addition, the origins of the chemical selectivities and enantioselectivities are analyzed in detail. It was found that the spatial effect between the catalyst ligand and the substrate leads to high [4 + 2] chemoselectivity, while the stabilizing attractive interaction between the ligand and the substrate leads to high enantioselectivity.

This study investigates the modification of carrier dynamics in nanoscale multiple quantum wells (MQWs) through B⁺ ion implantation, combining experimental and theoretical approaches to provide a comprehensive understanding of the impact on ultrafast optoelectronic responses. Using femtosecond time-resolved transient absorption (TA) spectroscopy, we examine the changes in carrier dynamics in both pristine and B⁺-implanted In_0.25Ga_0.75As/GaAs_0.9P_0.1 MQWs. Our results reveal significant modifications in the transient absorption spectra, with ion implantation reducing the excited-state absorption cross section (σ_ES) and leading to faster carrier recovery times. To further analyze these changes, we introduce a novel cascade rate equation model that incorporates two effective relaxation times, allowing for more accurate simulations of the experimental data. The model captures the complex interactions between various carrier states and provides a deeper understanding of the ion implantation effects on carrier trapping, recombination, and recovery processes. The comparison of experimental results and theoretical simulations demonstrates that ion implantation enhances ultrafast recovery times and modulates the carrier dynamics, offering a pathway for tailoring the optoelectronic properties of semiconductor materials. This work provides both a theoretical framework and experimental evidence for the design of next-generation ultrafast photonic devices with optimized carrier dynamics.

Furfural is a typical representative molecule of furan compounds and an important intermediate species in the oxidation of furan derivatives. The rate constant of furfural with OH is calculated for the first time using a high-level quantum chemistry method combined with the Rice–Ramsperger–Kassel–Marcus theory/master equation method. The M06-2X/jun-cc-pVTZ method was used to construct the potential energy surface of the reaction path. The preliminary reactions can occur through three different pathways: H-abstraction from the furan ring, H-abstraction from the side chain, and a preliminary OH-addition. The pathways via the OH-addition mechanism of the furfural + OH system were superior to H-abstraction in the temperature range of 298–400 K. When the temperature exceeds 400 K, the H-abstraction will be faster. Moreover, with the increase of pressure, the competition of the pathway via the OH-addition mechanism in the low-temperature region will gradually weaken. Under low-temperature conditions, INT1 and INT4 are the main intermediate species. The formation of bimolecular products, the 2-furanol (P7) + aldehyde group and the (3E)-4-hydroxybuta-1,3-diene-1-one (P8) + aldehyde group at C(2) and C(5) sites, are the main reaction pathways via the OH-addition mechanism. The formation of (2-furanyl)(oxy) methyl (P4) + H₂O (i.e., R4) always dominates for the four H-abstraction reactions. For the initial H-abstraction reaction, there is no pressure dependence, but for the preliminary OH-addition reaction, there is a significant positive pressure dependence. This work not only provides the necessary rate constants for modeling development but also provides theoretical guidance for the practical application of furan-based fuel.

In this work, we describe the development of a new algorithm for the computation of Coulomb-type matrices using the well-known resolution of the identity (RI) or density fitting (DF) approximation. The method is linear-scaling with respect to system size and computationally highly efficient. For small molecules, it performs almost as well as the Split-RI-J algorithm (which might be the most efficient RI-J implementation to date), while outperforming it for larger systems with about 300 or more atoms. The method achieves linear scaling through multipole approximations and a hierarchical treatment of multipoles. However, unlike in the fast multipole method (FMM), the algorithm does not use a hierarchical boxing algorithm. Rather, close-lying objects like auxiliary basis shells and basis set shell pairs are grouped together in spheres that enclose the set of objects completely, which includes a new definition of the shell-pair extent that defines a real-space radius outside of which a given shell pair can be safely assumed to be negligible. We refer to these spheres as "bubbles" and therefore refer to the algorithm as the "Bubblepole" (BUPO) algorithm, with the acronym being RI-BUPO-J. The bubbles are constructed in a way to contain a nearly constant number of objects such that a very even workload arises. The hierarchical bubble structure adapts itself to the molecular topology and geometry. For any target object (shell pair or auxiliary shell), one might envision that the bubbles "carve" out what might be referred to as a "far-field surface". Using the default settings determined in this work, we demonstrate that the algorithm reaches submicro-Eh and even nano-Eh accuracy in the total Coulomb energy for systems as large as 700 atoms and 7000 basis functions. The largest calculations performed (the crambin protein solvated by 500 explicit water molecules in a triple-ζ basis) featured more than 2000 atoms and more than 33,000 basis functions.