The Journal of Physical Chemistry A最新文献

Conjugated cyclic organic molecules are common across many fields such as pharmaceuticals, are naturally occurring in biological systems, and are used in synthetic materials. One particular area of interest from a photochemical point of view is the formation of highly strained cyclic organics. We investigate the photoinduced reaction of cyclopentadiene, a five-membered organic ring molecule, which can form strained three and four carbon rings after photoexcitation with UV light, with the gas-phase ultrafast electron diffraction instrument at the SLAC MeV-UED facility. Electron diffraction offers a direct probe sensitive to the nuclear geometry during the reaction, allowing for the determination of the distribution of products formed following photoexcitation. We observe the simultaneous formation of the highly strained ring-closed bicyclo[2.1.0]pentene and vibrationally hot cyclopentadiene within the temporal resolution of the experiment and determine the relative yield of all reaction products. The experimental results are in good agreement with the predictions of trajectory simulations.

Exploring low-energy conformers of tripeptides with different side chains using first-principles methods is important not only to interpret, validate, and predict experimental infrared (IR) spectra in the gas phase but also to understand how the relative stability of different conformations can be modulated by the interplay of basic molecular interactions. In this work, we identified low-energy conformers of 27 protonated tripeptides at M06–2X/6–311+G(d,p) by employing a deep-learning-based neural network potential (DL-NNP) to speed up the structure search. Our methodology also demonstrates a seamless transition from gas phase to implicit-solvent models using the polarizable continuum model (PCM), achieving a mean absolute error (MAE) of energies less than 1.1 and 2.1 kJ/mol, respectively. We found that the number of distinct minima below 25 kJ/mol for a given tripeptide ranges from 10 to 59 in the gas phase and 60–361 in PCM-water. Analysis of the structures of these low-energy minima reveals how methylation modulates molecular interactions through both electronic and steric effects. Finally, the low-energy conformers of methylated tripeptides identified in this study provide valuable insights to compare with available experimental data and to stimulate future experimental and theoretical investigations.

Indene is contemplated as the elementary moiety in molecular mass growth processes, leading to nonplanar polycyclic aromatic hydrocarbons (PAHs), such as buckminsterfullerene (C₆₀). Chemical reaction routes involving resonantly stabilized free radicals are recognized as fundamental and efficient pathways to form indene. Exploiting a chemical microreactor in combination with tunable synchrotron photoionization and molecular beam mass spectrometry techniques, the present experiment provides compelling evidence on the formation of indene in the presence of cyclopentadienyl, the simplest C₅-ring molecule, and the unsaturated hydrocarbon vinylacetylene. Theoretical calculations on the potential energy surfaces on C₉H₈ constants reveal that indene can be efficiently produced through two direct addition channels of cyclopentadienyl and vinylacetylene, along with multistep isomerization. RRKM-Master Equation simulations demonstrate that indene formation is kinetically favored at elevated temperatures and reduced pressures. Furthermore, H-assisted isomerization mechanisms on C₉H₈ intermediates are proposed to significantly enhance indene yields under thermally activated conditions. These findings establish a fundamental framework for PAH growth in hydrogen-abundant systems, including combustion flames and interstellar ones, thereby advancing our understanding of the knowledge of PAH propagation and even the formation of carbonaceous nanoparticles in interstellar and terrestrial environments.

The development of new electronic structure methods is a very time-consuming and error prone process when done by hand. SpinAdaptedSecondQuantization.jl is an open-source Julia package that we have developed for working with automated electronic structure theory development. The code focuses on being user-friendly and extensible, allowing for easy use of both user- and predefined Fermionic and/or bosonic operators, tensors, and orbital spaces. This allows the code to be used to efficiently investigate and prototype new electronic structure methods for many different types of systems. This includes both exotic systems with wave functions consisting of different kinds of particles at once as well as new parametrizations for traditional many electron systems. The code is spin-adapted, working directly with spin-adapted Fermionic operators, and can easily be used to derive common electronic structure theory equations and expressions, such as the coupled cluster energy, ground and excited state equations, one- and two-electron density matrices, etc. Additionally, the code can translate expressions into code, accelerating the process of going from ideas to implemented methods.

Excited-state intramolecular proton transfer (ESIPT) represents a fundamental process governing the photophysical behavior of hydrogen-bonded chromophores. In this study, we present a comprehensive theoretical investigation of two competitive ESIPT mechanisms in 4-amino-7-hydroxy-2-methylisoindoline-1,3-dione (AHMD), a newly synthesized fluorescent dye exhibiting high quantum yield and environmental stability. Using multiconfigurational electronic structure calculations (CASSCF/MS-CASPT2) combined with nonadiabatic surface-hopping dynamics simulations, we unravel the competitive proton transfer pathways and their coupling to nonradiative decay channels. Two distinct ESIPT routes are identified: a higher-barrier N2–H1 → O6 transfer (ESIPT-1, 7.26 kcal·mol^–1) and a near-barrierless O8–H7 → O12 transfer (ESIPT-2, 4.25 kcal·mol^–1), with the latter dominating ultrafast excited-state relaxation. The nonradiative deactivation predominantly occurs through a conical intersection (S₁S₀-C) associated with the ESIPT-2 channel, while the ESIPT-1 pathway is less favored both energetically and dynamically. Statistical analysis of surface-hopping trajectories shows that 33.8% of photoexcited molecules undergo nonradiative decay within 779 fs, whereas the majority persist in the excited state, rationalizing the high fluorescence efficiency observed experimentally. This study not only provides an atomistic resolution of proton transfer in a compact fluorophore but also offers guiding principles for the rational design of photostable ESIPT-active materials.

The reaction mechanisms for the synthesis of quinolinones and spiro compounds from N-arylpropynamides (1a, R = H; 1b, R = OMe; 1c, R = F) and diselenides, mediated by (dichloroiodo)benzene (PhICl₂), which have been investigated using density functional theory (DFT) at the M06–2X/6–311G(d,p) level of theory. The in situ-generated PhSeCl electrophilically adds to the alkyne moiety of the substrates, initiating a cascade involving addition-dechlorination, spirocyclization, and subsequent divergent pathways. For substrate 1a (R = H), the key azaspiro[4.5] intermediate undergoes 1,2-carbon migration and deprotonation to yield quinolin-2-one as the major product. In contrast, for 1b (R = OMe), the spiro intermediate favors a demethylation pathway, leading to an azaspiro[4.5]trienone. For the fluoro-substituted substrate 1c, the pathway involves nucleophilic addition of water to the spiro intermediate, followed by HF elimination to form the spiro product. The calculated energy barriers and thermodynamic profiles rationalize the experimentally observed product selectivity, demonstrating that the para-substituent effectively controls the reaction outcome by steering the fate of a common spirocyclic cation intermediate.

The HOMO of Be(CO)₃ resembles a Be lone pair that can donate charge in the context of a halogen bond. This propensity is tested via quantum chemical calculations wherein Be(CO)₃ is paired with XF, XCl, XCN, and XCF₃ (X = I, Br, Cl). In the case of the latter two molecules where X is attached to C, a fully noncovalent halogen bond is formed that varies in strength between 2.2 and 9.7 kcal/mol. The bonding causes a stretch of the internal X–C bond and a red shift of its stretching frequency. Interactions with XF and XCl are much stronger, 40 kcal/mol and higher, containing strong elements of covalency. The bridging X atom in these complexes shifts a good deal toward the Be center in what may be classified as partial transfer or halogen sharing. In contrast to the noncovalent halogen bonds involving XCN and XCF₃, where the interaction is strengthened as X grows larger, the opposite trend is observed for the halogen-shared complexes including XF and XCl.

4-Methylcyclohexanol (4MCHexOH) is widely utilized as a biofuel additive and chemical feedstock, yet its environmental fate and potential risks remain poorly understood. In this study, density functional theory combined with multistructural variational transition state theory was applied to investigate the reaction mechanism and kinetic properties of 4MCHexOH with ^•OH radicals. A total of 12 hydrogen abstraction pathways (R1–R12) were identified, among which R2 (−CH< adjacent to −CH₃) exhibited the lowest energy barrier. The calculated rate coefficient at 298 K was 1.84 × 10^–11 cm³ molecule^–1 s^–1, in good agreement with experimental measurements. The tunneling effect significantly influences the rate coefficient within 200–900 K, while the multistructural effect becomes notable between 900 and 2500 K. The atmospheric lifetime of 4MCHexOH was determined to range from 0.92 to 16.96 h under tropospheric temperatures (200–278 K), whereas it drops to approximately 14 s under combustion conditions at 2500 K. The risk assessment results show that coupling reactions involving HO₂^• or NO produce toxic and persistent substances, while hydrogen migration pathways and functional group elimination reactions result in less toxic and more environmentally benign products. This study advances the understanding of biofuel optimization and environmental risk management associated with multicarbon cyclic saturated alcohols.

Vibrational wave-packet dynamics on the electronic ground state of the neutral copper pentamer (Cu₅) are studied by femtosecond (fs) pump–probe spectroscopy using the ‘negative ion to neutral to positive ion’ excitation scheme (NeNePo). A vibrational wave packet is prepared on the electronic ground state (²A₁) of Cu₅ via photodetachment of a mass-selected, cryogenically cooled Cu₅^– anion using the first fs pump pulse. The temporal evolution of the vibrational wave packet is then probed by a second ultrafast probe pulse via resonant multiphoton ionization to Cu₅⁺. A frequency analysis of the femtosecond NeNePo transients for pump–probe delay times from 0.2 to 20.0 ps reveals two primary beating frequencies at 148 and 108 cm^–1 as well as weak and transient frequency features at 222, 216, 76, and 40 cm^–1. A comparison of experimentally obtained beating frequencies to the harmonic frequencies of normal modes obtained from quantum chemistry calculations confirms that Cu₅ in the gas phase adopts a planar trapezoidal geometry. NeNePo transients measured at ion-trap temperatures from 20 to 270 K probe the influence of the ion temperature on the wave-packet dynamics obtained. The inverse correlation between the oscillation lifetime τ_1/2 and the square root of the temperature indicates a vibrational decoherence channel originating from the anharmonicity of high-energy vibrational levels.

We report elastic and electronically inelastic cross sections for scattering of electrons with energies up to 30 eV by the tetrahydrofuran molecule in the gas phase. The calculations were performed by using the Schwinger multichannel method implemented with norm-conserving pseudopotentials. We analyzed five distinct scattering models to explore the impact of multichannel coupling effects on the description of elastic and electronically inelastic collisions, each incorporating a different channel coupling scheme. Our computed elastic cross sections exhibit strong agreement with existing experimental data. The present results exhibit three resonances of the π*─character and the respective positions are in excellent agreement with the literature. For electronically inelastic processes, we find that the number of coupled channels is the dominant factor in suppressing cross-section magnitudes, with exhaustive channel coupling providing the most rigorous benchmark for these previously uncharacterized individual excitation channels.