The Journal of Physical Chemistry A最新文献

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.

Dissociative photoionization of SF₆ in the photon energy range of 15.00-16.50 eV has been investigated using threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging. Both the kinetic energy release distribution (KERD) and the angular distribution of the unique fragment ion, SF₅⁺, resulting from dissociation from the SF₆⁺(X²T_1g) ions, were obtained from the TPEPICO time-sliced images. The F-loss potential energy curve and ab initio classical trajectory calculations not only unravel its dissociation mechanism but also declare that the ν₆⁺ deformation vibration of the SF₅⁺(D_3h, X¹A₁) fragment is predominantly excited. By fitting the total KERD curves derived from the images, we identified the fragment energy distributions. Surprisingly, the average total kinetic energy released in dissociation remains nearly constant within the range of the X²T_1g state. To explain this unusual behavior in such a fast bond-cleavage process, an intramolecular vibrational energy redistribution mechanism is proposed. This mechanism accounts for the rapid energy transfer among vibrational modes prior to complete dissociation. In addition, an adiabatic appearance potential of AP₀(SF₅⁺/SF₆) is accurately determined to be 14.145 ± 0.01 eV, which is in excellent agreement with the high-accuracy ab initio calculation results.

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.

AtomDB is a free and open-source Python library for accessing and manipulating neutral and charged atomic species and their promolecular properties. It serves as a computational toolset, operating on an accompanying "extended periodic table" database, with experimental and computational data covering atomic species with a wide range of charges and multiplicities. AtomDB includes facilities for computing promolecules: local promolecular properties, constructed from the corresponding atomic densities, and scalar promolecular properties, computed from the corresponding scalar atomic properties, both taking into account whether properties are extensive or intensive. AtomDB is designed to be easy to use, extend, and maintain: it follows best practices for modern software development, including comprehensive documentation, extensive testing, continuous integration/delivery protocols, and package management. This article is the official release note for the AtomDB library.

We consider different approximations to compute the heat capacity for Al(OH)₃ and compare the computed results to the experiment. We find that the calculation of anharmonic effects for this molecule is not currently practical, and scaled harmonics, using the scaling factor for H₂O(g), offer the best approach to including anharmonic effects. The Pitzer-Gwinn approach for hindered rotations is recommended.

Ruthenium(II) polypyridine compounds often have a relatively long-lived triplet metal-ligand charge transfer (³MLCT) state, making these complexes useful as chromophores for photoactivated electron transfer in photomolecular devices (PMDs). As different PMDs typically require different ligands and as the luminescence lifetime of the ³MLCT is sensitive to the structure of the ligand, it is important to understand this state and what types of photoprocesses can lead to its quenching. Recent work has increasingly emphasized that there are likely multiple competing pathways involved, which should be explored in order to fully comprehend the ³MLCT state. However, the lowest barrier that needs to be crossed to pass over to the nonluminescent triplet metal-centered (³MC) state has been repeatedly found to be a trans dissociation of the complex, at least in the simpler cases studied. This is the fourth in a series of articles investigating the possibility of an orbital-based luminescence index (LI3, because it was the most successful of three) for predicting luminescence lifetimes. In an earlier study of bidentate (N^∧N) ligands, we showed that the gas-phase ³MLCT → ³MC mechanism proceeded via an initial charge transfer to a single N^∧N ligand, which moves symmetrically away from the central ruthenium atom, followed by a bifurcation pathway to one of two ³MC enantiomers. The actual transition state barrier was quite small and independent, to within the limits of our calculations, of the choice of ligand studied. Here, we investigate the same reaction in acetonitrile, CH₃CN, solution and find that the mechanism differs from that in the gas phase in that the reaction passes directly via a trans mechanism. This has implications for the interpretation of LI3 via the Bell-Evans-Polanyi principle.

The interest in the electron impact-induced ligand release from MeCpPtMe₃ [trimethyl(methylcyclopentadienyl)platinum(IV)] is motivated by its widespread use as a precursor in focused electron and ion beam nanofabrication. By experimentally studying the electron impact dissociative ionization of MeCpPtMe₃ under single-collision conditions, we have found that the removal of two methyl radicals is energetically more favorable than the removal of one radical and even energetically comparable to the nondissociative ionization of MeCpPtMe₃. This observation is explained by the structural rearrangement of the MeCpPtMe₃⁺ ion prior to dissociation, resulting in the removal of ethane instead of two methyl groups. This fragmentation pathway is computationally confirmed and studied by irradiation-driven molecular dynamics (IDMD) simulations. The formation of complex molecules in irradiation-induced molecular dissociation is a general phenomenon that can occur in various molecular systems. This study explains the puzzling results of previous experiments with MeCpPtMe₃ molecules and highlights the use of the IDMD approach to describe radiation-induced chemical transformations in molecular systems.

Conformational sampling is nowadays a standard routine in computational chemistry. Within this work, we present a method to perform conformational sampling for systems exposed to elevated pressures within the CREST program, allowing us to model pressure-induced changes of molecular ensembles and structural parameters. For this purpose, we extend the molecular Hamiltonian with the PV (pressure times volume) term, using the solvent-accessible volume. The volume computation is performed within the new standalone library libpvol. A first application shows good agreement with experimental data and provides a reasonable explanation for severe pressure-induced structural and spectroscopic changes of the molecules dichloroethane and tetra(4-methoxyphenyl)ethylene.

Carboxylic acids are a common class of compounds found in atmospheric aerosols and cloud droplets. In this study, the oxidation kinetics of several carboxylic acids in the aqueous phase by the atmospherically relevant ^•OH radical were investigated to better understand the loss processes for this class of compounds. The rate constants for the reactions of the ^•OH radical were determined using the thiocyanate competition kinetics method for lactic acid, glyceric acid, and methylmalonic acid as a function of temperature and pH. The Arrhenius equations for oxidation by the ^•OH radical are as follows (unit in L mol^-1 s^-1): For lactic acid: k(T, HA) = (1.3 ± 0.1) × 10¹⁰ × exp[(-910 ± 160 K)/T] and k(T, A^-) = (1.3 ± 0.1) × 10¹⁰ × exp[(-800 ± 80 K)/T]; for glyceric acid: k(T, HA) = (6.0 ± 0.2) × 10¹⁰ × exp[(-1100 ± 170 K)/T] and k(T, HA^±) = (3.6 ± 0.1) × 10¹⁰ × exp[(-1500 ± 100 K)/T]; and for methylmalonic acid: k(T, H₂A) = (5.5 ± 0.1) × 10¹⁰ × exp[(-1760 ± 100 K)/T], k(T, HA^-) = (1.4 ± 0.1) × 10⁹ × exp[(-530 ± 80 K)/T] and k(T, A^2-) = (9.6 ± 0.4) × 10¹⁰ × exp[(-1530 ± 270 K)/T]. The general trend of the ^•OH rate constant was observed k_A^2- > k_HA^- > k_H2A. The energy barriers of the ^•OH radical reaction and thus the most probable site of H atom abstraction were calculated using density functional theory simulations in Gaussian with the M06-2X method and the 6-311++G(3df,2p) basis set. Kinetic data predicted from structure-activity relationships were compared to the measured ^•OH radical rate constants. ^•OH radical oxidation in the aqueous phase could serve as an important sink for carboxylic acids, and the pH- and T-dependent rate constants of ^•OH radical reactions provide a better description of the aqueous-phase sink processes. Hence, the atmospheric lifetime as well as the partitioning of the investigated carboxylic acids was calculated.

Infrared (IR) spectroscopy, a type of vibrational spectroscopy, provides extensive molecular structure details and is a highly effective technique for chemists to determine molecular structures. However, analyzing experimental spectra has always been challenging due to the specialized knowledge required and the variability of spectra under different experimental conditions. Here, we propose a transformer-based model with a patch-based self-attention spectrum embedding layer, designed to prevent the loss of spectral information while maintaining simplicity and effectiveness. To further enhance the model's understanding of IR spectra, we introduce a data augmentation approach, which selectively introduces vertical noise only at absorption peaks. Our approach not only achieves state-of-the-art performance on simulated data sets but also attains a top-1 accuracy of 55% on real experimental spectra, surpassing the previous state-of-the-art by approximately 10%. Additionally, our model demonstrates proficiency in analyzing intricate and variable fingerprint regions, effectively extracting critical structural information.