The Journal of Physical Chemistry A最新文献

C₂H radicals and C_nH polyynes are among the most abundant interstellar polyatomic molecules, predominantly observed in molecular clouds and planetary nebulae. While the C₂H^– anion is considered a plausible component in the interstellar medium (ISM), it has not yet been detected. Given the extensive study of interstellar polyynes, MCCH/MCCH⁺ species are also expected to exist in the ISM. Via fundamental metal–carbon bonding, MCCH/MCCH⁺ molecules exhibit strong polarity, enhancing the intensity of their rotational transitions and improving the chances of astronomical observation. Theoretical spectroscopy enables the generation of molecular reference data to aid in detections. In this study, highly accurate single reference coupled cluster theory [CCSD(T)] is compared to multireference configuration interaction theory (MRCISD+Q) to survey the electronic structure of 3d metal mono acetylide cations (MCCH⁺, where M = Sc to Zn) which could catalyze gas phase reactions in cold astrochemical environments. CCSD(T) and MRCISD+Q theories gave equivalent predictions of ground electronic states for all cations excluding CoCCH⁺. The MRCISD+Q ground state for CoCCH⁺ (⁴Φ) is multireference and multiconfigurational, and single reference coupled cluster energies could not be converged. The investigated electronic states of MCCH⁺ arise from interactions between the ¹Σ⁺ ground state of the C₂H^– anion and the metal dication (M²⁺). A thorough analysis of multireference character of electronic states is reported. The ³Π excited state of NiCCH⁺ was the variationally accessible electronic state with the strongest reported multireference character (T₁ diagnostic = 0.15, D₁ diagnostic = 0.55, C₀² = 0.26). Harmonic vibrational frequencies, infrared intensities, and dipole moments are computed for the identified ground states. Theoretical characterization of the ground and excited electronic states of these specific ions will assist in their astronomical detection, constrain laser spectroscopy experiments of the neutral species, and guide characterization in synthetic laboratories.

Vibrational spectra, characterized by structurally sensitive features, offer critical insights into molecular structures, bonding, and dynamics. Yet, interpreting measured spectra and identifying corresponding structures require theoretical equivalents and quantitative analysis. Here, we introduce a new experimental database that includes broad-range ionization-detected stimulated Raman scattering signatures besides harmonic Raman frequencies calculated with widely used density functional methods/basis sets. By comparing experimental fundamental bands and computed data, we derive single global and multiple range- and mode-dependent scaling factors and analyze the resulting error distributions, showing that mode-dependent scaling provides the greatest accuracy. Additionally, we explore various methods for evaluating similarities between measured fundamental spectra of different compounds and calculated data sets of conformers. Our findings indicate that Euclidean and Manhattan distance metrics for frequencies and intensities uncover subtle structural variations, yielding spectral similarity rankings that facilitate structural identifications. This new database and methodology address key challenges in spectral assignment, and we anticipate that they will serve as benchmarks for future predictive models and foster the development of advanced strategies.

Halogen bonding (HaB) between bromine and nitrogen was investigated in a series of quinuclidine (QN) complexes with various bromo-substituted electrophiles and in bromine(I) species QN-Br-QN⁺ and Sac-Br-Sac^– (Sac = saccharin residue). X-ray crystallography and density functional theory (M0-2X/def2-TZVPP) calculations reveal that the Br···N interactions span a continuum from weak supramolecular contacts to covalent bonds, depending on the HaB donor strength. In complexes with the strongest HaB donors such as Br₂, the Br–N bond lengths are only 10–15% longer than fully developed covalent bonds. Electron densities of about 0.1 au and negative energy densities at these bond critical points, as well as values of electron localization function (ELF) and Mayer bond orders (MBOs) of ∼0.5–0.6 point to covalency of this bonding. Also, these strong Br–N bonds significantly weaken adjacent Br–X covalent bonds of HaB donors. Electron and energy densities at bond critical points, ELF and MBOs values of both Br–N and Br–X bonds in a series of complexes indicate that the transition toward partial covalency occurs at the Br–N distances about halfway between van der Waals contacts and covalent bonds. Energy decomposition analysis suggests that this transition is primarily driven by the increasing orbital (charge-transfer) contribution in the strong complexes. Overall, this study demonstrates that strong Br···N halogen bonds exhibit partial covalent character comparable to that in cationic and anionic bromine(I) complexes, providing insight into the continuum between supramolecular and covalent bonding.

The construction of diabatic representations is important for understanding nonadiabatic processes. Significant efforts have been devoted to developing robust and generalizable methodologies for diabatization, aiming to improve numerical accuracy, computational efficiency, and transferability across diverse systems. However, while substantial progress has been made on the methodological front, the fundamental chemical significance inherent to the diabatic representation itself has received less attention. In this study, we move beyond these technical performance metrics to decode the chemical meaning of diabatic representations in a series of nonlinear (bent) hydrogen atom transfer (HAT) systems. Using classical valence bond (VB) theory, we explicitly analyze the bonding nature (e.g., covalent, ionic) embodied by diabatic representation in model HAT systems (H₃, H₂F, NaH₂, NaHF, BeH₂⁺), revealing the specific electronic reorganization captured during the reaction. This mechanistic insight extends the traditional diabatic state representation and underscores the role of dynamic bonding pattern mixing.

Understanding the manner in which vibrational energy flows between molecular and lattice vibrations is of great interest in physical chemistry due to its central role in reactivity and energy dissipation in molecular materials. In this feature article, we highlight our recent efforts employing ultrafast broadband infrared spectroscopy toward understanding the interplay between molecular and lattice vibrations in energetic materials, motivated by the open questions surrounding the role of vibrational energy transfer (VET) in reaction initiation in these materials. Our work addresses the ongoing debate on the participation of doorway modes in VET. We further present new results from high-pressure ultrafast experiments on RDX, a hydrogen-bonded material, and BNFF, a hydrogen-free material, to explore how intermolecular interaction strength governs VET pathways and time scales. Collectively, our findings reveal that vibrational dynamics in these systems occurs across three distinct time regimes, with VET being incomplete out to hundreds of picoseconds, suggesting the importance of considering nonstatistical reactions in the modeling of these materials. These time scales vary as intermolecular interaction strength is indirectly modified by application of static pressure, indicating dramatic changes to the vibrational structure of these materials under shock-relevant conditions. Our results thus shed light on how intermolecular interactions shape vibrational energy redistribution in molecular materials, and highlight the need for further theoretical and experimental investigation.

Methyl methacrylate is the simplest unsaturated branched ester and can thus serve as a surrogate model for oxygenated and biodiesel fuels. It is also the monomer of poly(methyl methacrylate) (PMMA), a widely used polymer that is also used as a hybrid rocket fuel. PMMA rapidly depolymerizes upon heating, almost exclusively forming the monomer. Therefore, chemical kinetic models developed for the combustion and pyrolysis of MMA are typically also used for the polymer. Currently, models are, however, mostly limited to high-temperature conditions and are tuned to reproduce selected characteristics of MMA flames. Hence, many important mid- and low-temperature reaction channels are missing. To fill this gap, in a companion paper, we have identified and analyzed unimolecular concerted reactions that occur during the combustion and pyrolysis of MMA and provided a general classification of retroene-type reactions. Here, we use CBS-QB3 composite level calculations to explore the addition of the OH radical to the double bond of MMA and subsequent transformations. Further addition of molecular oxygen to the β-radical centers of the resulting adducts triggers a set of double chemical activation reactions. Remarkably, a low-energy, combustion-related pathway is revealed via a Waddington mechanism that leads to the formation of CH₂O and methyl pyruvate products, which have been observed as a dominant product set in experiments on the direct oxidation of MMA with OH radicals under atmospheric conditions. Kinetic parameters of the newly identified reactions were calculated and analyzed using Arkane as part of the Reaction Mechanism Generator (RMG) automated tool.

The structural evolution, electronic properties, and thermodynamic stability of ruthenium-doped silicon clusters (RuSi_n^–, n = 13–17) are systematically investigated through a combination of anion photoelectron spectroscopy and high-level theoretical calculations. Anion photoelectron spectra recorded at 266 nm exhibit size-dependent electronic transitions, with vertical detachment energies (VDEs) ranging from 2.87 to 3.28 eV─the computed VDE values show excellent agreement with the experimental data (deviation <0.11 eV), which validates the structural models. Global minimum structures are identified through a hybrid Crystal Structure AnaLYsis by Particle Swarm Optimization (CALYPSO) and ABCluster search strategy, refined using density functional calculations (DFT) at the B3LYP/def2-TZVPD level, and corroborated by DLPNO-CCSD(T) single-point energy calculations. Both neutral and anionic clusters adopt structural motifs based on a Ru-encapsulating distorted bicapped pentagonal prism, with successive Si atom capping governing geometric evolution and stability. Notable magic-number behavior emerges for anionic RuSi₁₇^– and neutral RuSi₁₄ (compact geometry and favorable electronic configuration). Natural population analysis confirms that Ru acts as an electron acceptor, with charge distributions closely correlating to VDE trends. Energetic descriptors─Ru binding energy (E_b (Ru)) and Si incremental binding energy (ΔE^I (Si))─reveal pronounced size- and charge-state-dependent stability: neutrals exhibit higher E_b (Ru) (9.14–10.15 eV) than anions (6.88–7.91 eV), while positive ΔE^I (Si) values (3.41–5.37 eV for neutrals; 3.87–4.74 eV for anions) confirm that Si incorporation enhances stability. These results underscore the intricate interplay of Ru–Si bonding, geometric configuration, and electronic structure in governing cluster stability, with the charge state serving as a key modulator.

Given the central importance of the protonated water dimer to the study of the hydrated proton, we report new fits to the previous CCSD(T) data set of Huang, Braams, and Bowman (HBB) that are more precise and, unlike HBB, provide fast gradients. The new fits, like the HBB one, are based on linear regression with permutationally invariant polynomials (PIPs). The fast gradients are provided via reverse differentiation. They cost roughly just three times the cost for an energy call and are roughly 20 times faster than the HBB numerical gradients. The two new PESs are fits to the original HBB data sets up to roughly 60,000 and to 110,000 cm^–1. Comparisons to the CCSD(T) benchmarks and to the HBB results for stationary points and Diffusion Monte Carlo ZPEs are reported and show good agreement.

The unimolecular dissociative ionization pathways of 4-ethylguaiacol and eugenol were explored using imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. Threshold photoelectron spectra (TPES) for both species were recorded and analyzed with Franck–Condon simulations. Experimental adiabatic ionization energies (IE) are reported for 4-ethylguaiacol (7.65 ± 0.05 eV) and eugenol (7.67 ± 0.05 eV), the latter of which agrees with previous measurements. The first excited state of the 4-ethylguaiacol radical cation and the first two excited states for the eugenol radical cation are also discussed in detail. Breakdown diagrams were analyzed using Rice–Ramsperger–Kassel–Marcus (RRKM) theory. 4-Ethylguaiacol dominantly loses a ^•CH₃ group at low energies, consistent with our prior mass-analyzed ion kinetic energy (MIKE) study, although traces of methanol loss are also seen. Nonetheless, the discrepancy between the RRKM-fitted methyl-loss E₀ of 1.88 eV and the previously proposed theoretical value (2.15 eV) led us to find a new reaction pathway involving sequential hydrogen shifts and structural rearrangements consistent with the experimental results. The eugenol radical cation was found to dissociate by the loss of ^•CH₃ and CH₃OH, in agreement with MIKE results. The energy barriers derived from the RRKM analysis (1.60 and 1.52 eV) were, again, significantly lower than previous computational reaction barriers of 2.63 and 3.21 eV, respectively. Alternative, lower-energy, isomerization–fragmentation mechanisms, analogous to those of 4-ethylguaiacol, were found to be active. Furthermore, an additional fragment ion was observed at m/z 104. This study highlights the critical role of experimental techniques in validating and refining computational models and demonstrates how quantitative spectroscopic data can uncover previously unidentified reaction mechanisms.