The Journal of Physical Chemistry A最新文献

This study investigates the pyrolysis behavior of methane (CH₄) and propane (C₃H₈) under high-temperature and high-density conditions using reactive molecular dynamics (ReaxFF-MD) simulations, with particular emphasis on the coproduction of hydrogen (H₂) and carbon-based byproducts. The results show that C₃H₈ decomposes more rapidly than CH₄ under similar conditions, primarily because its weaker C-C bonds have a lower activation energy for bond cleavage. In both systems, H₂ is primarily produced via hydrogen abstraction reactions involving H radicals formed during the early stages of the process. Acetylene (C₂H₂) arises through the stepwise dehydrogenation of C2 species. H₂ production progressively increases with pyrolysis time in both systems, driven by entropy effects. Notably, CH₄ yields more H₂ in the initial phase due to the early abundance of H radicals, whereas C₃H₈ exhibits a slower initial H₂ yield. Similarly, C₂H₂ formation in the CH₄ system requires more reaction steps, while C₃H₈ rapidly forms C₂H₅ intermediates that facilitate faster C₂H₂ generation, resulting in faster carbon condensation during C₃H₈ pyrolysis. The formation of carbon clusters proceeds through three distinct stages: feedstock fragmentation, carbon chain growth, and carbon cluster aromatization/graphitization. The final stage is characterized by the elimination of hydrogen and the formation of six-membered aromatic rings. In addition, the study provides a detailed analysis of the carbon nucleation process, suggesting that the Polycyclic Aromatic Hydrocarbon (PAH) model is likely more applicable at low densities and temperatures. In contrast, the polyyne model tends to dominate under high-density and high-temperature conditions. Overall, this study offers atomic-level insights into the pyrolysis of light hydrocarbons, highlighting the utility of ReaxFF-MD simulations in unraveling complex, coupled gas-phase, and condensation kinetics.

In this work, highly accurate ab initio calculations have been employed to predict the spectroscopic properties of protonated acrolein (both cis- and trans-isomers) and methylketene. These C₃H₅O⁺ systems are of astrochemical interest owing to their neutral analogues having already been detected in the interstellar medium. In total, we have characterized six species: syn and anti conformers of both O-protonated cis- and trans-acrolein, O-protonated gauche-methylketene and C-protonated methylketene. Specifically, the CCSD(T) method (coupled-cluster with all single and double substitutions and a quasiperturbative treatment of triple excitations) has been used in conjunction with extrapolation to the complete basis-set limit, and corrections for core-valence effects. To support experimental spectroscopy measurements aiming at guiding astronomical observations, we have focused on determining best theoretical estimates of rotational and vibrational spectroscopic parameters. The V₃ barriers to internal methyl rotation for both protonated methylketene species (O- and C-) have additionally been computed, and an estimate of the reduced rotation constant for internal rotation, F₀, has been deduced.

We develop a linear vibronic coupling (LVC) model for polyenes described by the extended Hubbard-Peierls Hamiltonian. This model is applied to trans-hexatriene to benchmark quantum-classical dynamics methods against fully quantum simulations. We find that surface-hopping methods describe short times more accurately than multitrajectory Ehrenfest. None of the quantum-classical methods studied obtain the long-time population oscillations found in fully quantum simulations. Varying the parameters of the LVC Hamiltonian, we find that surface hopping reproduces the correct trends in the long-time dynamics across a wide range of parameters, but generally overestimates the degree of internal conversion. On the other hand, multitrajectory Ehrenfest gives more accurate long-time populations in proximity to the hexatriene parameter set.

Data-driven design strategies have enabled the rapid generation of numerous energetic candidates, making efficient screening for synthesizable candidates critically important. Synthetic accessibility scoring models are widely applied in drug discovery, yet their applicability to energetic materials has not been systematically validated. Here, we present the energetic compound data set 100 (ECD100) based on the synthetic status-based labeling approach, a reproducible benchmark data set comprising 50 experimentally synthesized (easy to synthesize, ES) and 50 designed but unrealized (hard to synthesize, HS) energetic molecules. Eight representative SA models were benchmarked, among which SAscore achieved the best classification performance (ROC-AUC = 0.76). Further analysis identified fragmental structure distributions and molecular complexity as key contributors to model refinement. Furthermore, Pareto front analysis uncovered a trade-off between synthetic accessibility and detonation velocity, highlighting the need for multiobjective screening strategies. This work conducted an application evaluation of the synthetic accessibility scoring model in the field of energetic materials and provided new ideas for high-throughput screening of highly energetic molecules.

We discuss the computation of partial Auger decay widths with equation-of-motion ionization-potential coupled-cluster (EOMIP-CCSD) theory in the framework of non-Hermitian quantum mechanics (NHQM). In NHQM, the decaying character of metastable states is described with complex energies and the total decay width is obtained directly from the total energy. In contrast, the computation of partial decay widths, i.e., the contributions of different decay channels to the total width, requires further analysis. However, partial widths are important for Auger spectroscopy as they determine the probability with which different final states are formed and hence the shape of the Auger spectrum. Recently, we introduced Auger channel projectors (ACPs), which selectively remove decay channels from the EOMIP-CCSD excitation manifold. This method requires a separate EOMIP-CCSD calculation for each decay channel. Here, we suggest an alternative: We solve the EOMIP-CCSD equations for the core-ionized state in the full excitation manifold and decompose the imaginary part of the resulting energy. In this way, we obtain all partial decay widths at once. We compute Auger spectra for K-edge-ionized states of methane, ethane, and hydrogen sulfide, and a Coster–Kronig spectrum for L₁-edge-ionized hydrogen sulfide. The results obtained with our new approach differ only negligibly from ACP results. We also present the first Auger spectra for the cyanide anion, including vibrational broadening, and discuss the differences between the spectra of the carbon core hole and the nitrogen core hole.

Electronic-structure calculations using complex absorbing potentials (CAPs) to stabilize temporary anion states are very sensitive to the CAP configuration, including the coupling strength (η) and boundary geometry. We present high-resolution surveys of several two-dimensional parameter spaces (CAP spaces for short) for the CO^– (²Π) and N₂^– (²Π_g) resonances and propose an efficient optimization strategy for box-CAPs that relies on Gyamfi and Jagau’s ξ error function [J. Chem. Theory Comput. 2024, 20, 1096–1107]. In all CAP spaces probed, only narrow parameter ranges yield the highest-quality results (ξ ∼ 10^–4–10^–5) that minimize wave function reflections and perturbations. Such optimal conditions are unlikely to be found by ad hoc methods, necessitating a systematic optimization protocol. Ours begins with (η, r^o)_L optimization trajectories, where r^o is a geometric variable controlling the CAP boundary and L is a distinct parameter such as the box elongation. Unlike extensively studied η- and r^o-trajectories, the (η, r^o)_L trajectories are optimum-seeking paths in two-dimensional CAP spaces searching for ξ minima. After optimizing CAPs across multiple (η, r^o)_L spaces with varying L, CAP strength minimization along the L-trajectory defines the overall optimal configuration in the higher-dimensional (η, r^o, L) space. This hierarchical optimization, min ξ ≻ min η (where “≻” denotes lexicographic precedence), produces a low-error description of the resonance (min ξ) stabilized by the weakest (but sufficient) CAP possible, i.e., min η subject to the min ξ constraint.

The potential energy surface of C₂Li₄H₃^– was examined to identify stable structures containing planar hypercoordinate carbon centers. The lowest-energy form corresponds to a C_2v-symmetric arrangement in which a Li₄H₃ framework encloses a C≡C triple bond. Bonding analyses (NBO, AdNDP, EDA, and IQA) show that the system consists of a C₂^2– fragment interacting mainly through electrostatic attraction and weak covalent contributions with the Li₄H₃⁺ unit. Magnetic response calculations indicate localized electron density supported by the C≡C bond, contrasting with the delocalization mechanisms typically invoked for planar hypercoordinate carbons. Born–Oppenheimer molecular dynamics simulations confirm structural stability under thermal conditions. These results outline an alternative electronic route to achieve planar pentacoordination in carbon systems.

Aminoboranes are an emerging class of materials that exhibit versatile emission properties, such as delayed fluorescence (DF) and room temperature phosphorescence (RTP), yet their excited-state dynamics remain poorly understood. Here, we employ femtosecond transient absorption and stimulated Raman spectroscopy for the real-time tracking of ultrafast electronic relaxation and structural dynamics during charge transfer in carbazole-based aminoboranes. Following excitation of the S₁ state, transient absorption in polar solvents reveals the ultrafast onset of a red-shifted stimulated emission band, indicating evolution of a locally excited (LE) state into an intramolecular charge-transfer (ICT) configuration. Density functional theory calculations and numerical simulations using the response-function formalism and the multimode Brownian oscillator model suggest that swift evolution along the B–N stretching and torsional coordinates guides the development of ICT character, the time scale of which depends on the polar solvation time. Such excited-state structural evolution is accompanied by a blue-shifted B–C stretching frequency in transient Raman loss signals, reporting electron density localization around the boryl acceptor upon charge transfer and forming a distorted ICT state. The formation of this state can be structurally controlled and thus is important for controlling the balance between the RTP and delayed fluorescence efficiency. Our findings highlight the importance of excited-state structural control and engineering in designing aminoborane-based emitters with tailored luminescence properties.