The Journal of Physical Chemistry A最新文献

We report a synchrotron radiation-based study of the simplest Polycyclic Aromatic Hydrocarbon (PAH) molecule, naphthalene, in the energy region 35,000–88,000 cm^–1 (4.4–10.9 eV). A complete spectral analysis of the VUV region is carried out for the first time, and several new bands are reported, while reassignments are made for many of the bands reported earlier. An extensive Rydberg series converging to the first seven ionization energies of naphthalene are observed, interspersed with several valence transitions. Rydberg series of ns, np, and nd types are assigned based on quantum defect analysis and correlated with theoretical calculations. Time-dependent density functional calculations performed using several functionals and basis set combinations helped in verifying and consolidating spectral assignments. From the absolute absorption cross-section data, the UV–VUV photolysis rates at different altitudes are estimated. It is found that the lower limit to the photolysis lifetime varies from ∼1 h at 20 km to ∼4 s at 50 km. Potential energy curves of the first few singlet and triplet excited states with respect to C–H bond length do not show any evidence of direct dissociation, thus implying that the H loss channel may not be very prominent in neutral naphthalene, in contrast to cationic naphthalene.

Halogen bonds (XBs) are a cornerstone of supramolecular chemistry, yet their response to external perturbations remains poorly investigated, particularly in systems with heavy elements where relativistic effects are significant. We benchmark two prototypical iodine-chloride X-bonded complexes, ClI···N(CH₃)₃ and ClI···NCH, under electric fields (EFs) using quantum chemical calculations up to CCSD and CCSD(T). Relativistic basis sets, including the all-electron jorge-TZP-DKH, are assessed against non-relativistic and pseudopotential-based alternatives (def2-TZVP, SDD, LANL2DZ) for their impact on XB geometries, binding energies, vibrational Stark shifts, and electron density redistribution. Explicit relativistic treatments substantially reduce the exaggerated field response otherwise observed. Benchmarking M06-2X and B3LYP with various basis sets against correlated methods confirms the accuracy of M06-2X, while showing that the relativistic effects included in the basis set influence the results more than the choice of functional itself. Symmetry-Adapted Perturbation Theory (SAPT) indicates that electrostatics dominate XB stabilization, with induction becoming more relevant under strong positive fields. Overall, XBs prove markedly more sensitive to external EFs than H-bonds across different field arrangements.

As a typical dielectric ceramic material, BaTiO₃ has attracted considerable interest owing to its high dielectric constant. Using a combination of high-pressure AC impedance spectroscopy, Raman spectroscopy, and theoretical calculations, this study investigated the structural and electrical properties of nanocrystalline BaTiO₃ at pressures of up to 30 GPa. The material underwent two phase transitions: from a mixed orthorhombic/tetragonal phase to a pure tetragonal phase and finally to a cubic phase. The superior dielectric constant of the tetragonal phase, compared to that of the other two phases, results from the rapid polarization switching of its 180° domains. The phase transition from the tetragonal phase to the cubic phase leads to a transformation from mixed ionic–electronic conduction to pure electronic conduction, as the high migration energy barrier in the cubic phase hinders ionic conduction. This work demonstrates that applying pressure is a feasible strategy to enhance the dielectric performance of BaTiO₃-type dielectrics.

Thermodynamic properties of real gases can be accurately described using realistic intermolecular potential energy surfaces. In this work, a first-order correction to the ideal gas equation of state is introduced through the computation of the classical second virial coefficient, B(T), derived from the configurational partition function, which explicitly depends on the intermolecular interaction potential. As a case study, the double many-body expansion (DMBE) potential energy surface for the ground electronic state of the N₂H₂ system was employed to derive pairwise interaction potentials for H₂···N₂ and NH···NH. These potentials were used to numerically evaluate the canonical partition function. Second virial coefficients, compressibility factors, and constant-volume heat capacities were computed in the temperature range 30–2000 K. The calculated B(T) values for H₂···N₂ are in good agreement with previous literature data, while the results for NH···NH lie within expected trends observed for similar systems.

Molecular-scale materials with bistable behavior and tunable properties are increasingly relevant for next-generation nanoscale electronic devices. Helical foldamers are promising candidates, but their structural and mechanical properties are highly sensitive to conformational stability and environmental conditions. A systematic methodology based on quantum-chemical calculations is proposed for assessing solvent-dependent mechanical behavior, combining analysis of π–π stacking interactions, conformational energetics, and environmental effects. Using this methodology we identified simple design principles for the rapid screening of new compounds, allowing evaluation of their conformational stability and effective mechanical rigidity. Applying these principles, we identify a modified helical aromatic foldamer that exhibits improved mechanical and stability characteristics compared to the initial reference compound.

In this study, we investigated the exceptional basicity of diazahelicenes through a comprehensive computational analysis using DFT calculations. Eighteen diazahelicene compounds were examined and compared with 1,8-bis(dimethylamino)naphthalene (DMAN), revealing that most exhibit stronger basicity than DMAN, qualifying them as proton sponges. The study employed the M06-2X functional with the def2tzvp basis set to analyze proton affinity, gas-phase basicity (GB), and structural changes upon protonation. We found that the basicity is significantly influenced by the interplanar angle, hydrogen bonding, conjugation, and aromaticity of the compounds. Most compounds demonstrated a decrease in interplanar angle upon protonation, with compounds containing nitrogen atoms in close proximity showing frustrated base behavior. Aromaticity analysis using HOMA (Harmonic Oscillator Model of Aromaticity) and Nucleus-independent Chemical Shift indices indicated enhanced electron delocalization after protonation. Further insights into electronic structure and bonding were obtained through AIM (atomic in molecules), MEP (molecule in electrostatic potential), NBO (Natural Bonding Orbital), ELF (electron localization function), and LOL (Localized Orbital Locator) analyses, providing a comprehensive understanding of the factors contributing to the exceptional basicity of these compounds.

Fully nonfused ring electron acceptors (NFREAs) have attracted growing attention as cost-effective alternatives to fused-ring electron acceptors (FREAs) in organic solar cells (OSCs). However, their molecular frameworks, linked entirely by single bonds, typically result in wider bandgaps, limited near-infrared (NIR) absorption, and poor electron mobility, leading to inferior performance compared to FREAs. In this work, we propose a molecular engineering strategy to design olefin-bridged NFREAs (OB-NFREAs) by introducing olefin bridges into the fully nonfused backbone of TBT-26. Theoretical calculations indicate that all OB-NFREAs, namely D-O1, D-O2, and D-O3, demonstrate a significant enhancement in optoelectronic performance. The olefin double bonds extend π-conjugation, reduce the bandgap, and improve molecular planarity. Consequently, the absorption edge of the absorption range of OB-NFREAs expands from 300 to 900 nm (TBT-26) to 300–1200 nm (D-O3), with a significant redshift of the absorption peak to 895 nm and 1.8-fold increase in integrated absorption intensity. Moreover, electron mobility is substantially enhanced, with D-O1 reaching 1.14 × 10^–3 cm² V^–1 s^–1, which is more than an order of magnitude higher than that of TBT-26. These results indicate that rational olefin bridge design can enhance π–π stacking, facilitate more efficient charge transfer, improve electron mobility, and redshift the absorption spectrum. Fine-tuning olefin bridges is thus a powerful strategy for constructing NFREAs with high mobility and strong NIR absorption, paving the way for next-generation organic photovoltaic materials.

A transparent, data-driven route to improve approximate density functionals by learning only the position dependence of an exact-exchange admixture in local hybrid functionals is discussed. Motivated by the scarcity of exact constraints for valence regions and by gauge ambiguity issues of exchange-energy densities, we replace hand-crafted inhomogeneity measures by neural-network local mixing functions (n-LMFs) evaluated on rung-3 or rung-4 descriptors while keeping the overall structure of the functional transparent and explainable. This limited use of machine learning (ML) has already provided a number of practical outcomes. The LH24n-B95 and LH24n functionals achieve broad main-group accuracy and, strikingly, suppress gauge artifacts without use of so-called calibration functions. The reasons for the latter observation can be visualized and analyzed directly in real space. Extending the idea to rung-5 functionals leads to the first local double hybrids in which a position-dependent exact-exchange admixture is paired with an SCS-PT2 correlation, delivering consistent gains over constant-mixing analogues. To escape the zero-sum game between delocalization errors and static-correlation errors, an n-LMF has subsequently been trained in the presence of an explicit strong-correlation factor. The resulting LH25nP functional combines the so far best rung-4 performance for main-group energetics with improved spin-restricted bond-dissociation curves, fractional-spin behavior, and reduction of spin-contamination artifacts, while remaining numerically robust. The limited ML approach preserves explainability and facilitates the transfer of insights back to rational designs.

Formamide (HCONH₂), the simplest organic molecule with a peptide bond, has been detected in various interstellar environments and is regarded as a potential precursor to essential biomolecules, such as amino acids and nucleic acids. Many of the proposed synthetic routes involve its ionized and protonated forms in the interstellar medium, where ion–molecule gas-phase reactions are predominant. However, spectroscopic data on cationic and protonated formamide is limited. In this work, we report the first bare-ion and Ne-tagged broadband vibrational spectra of isomeric forms of the formamide cation [HCONH₂]⁺ (m/z 45) and protonated formamide [HCONH₂]H⁺ (m/z 46). The vibrational spectra were obtained using two techniques, leak-out spectroscopy and Ne-tagging infrared predissociation spectroscopy, in a cryogenic 22-pole ion trap at the infrared free-electron laser facility HFML-FELIX. The measured spectra within the fingerprint region 650–1800 cm^–1 are compared to anharmonic vibrational frequency calculations at the B2PLYP-D3/aug-cc-pVTZ level of theory for structural identification and band assignments. For the formamide cation, spectral signatures of both the low-energy aminohydroxycarbene isomer (NH₂–C^•+–OH) and the higher-energy canonical cation (HCONH₂^•+) were detected, while protonated formamide was observed exclusively as the more stable O-protonated isomer with no indication of N-protonation. Additionally, a comparative analysis of the two spectroscopic techniques, supported by potential energy surface and vibrational frequency calculations, highlights how the weakly bound Ne tag subtly affects the vibrational signatures of the ions.

Optical emission spectroscopy was used to spatially resolve excited OH (OH*) emission as a function of height along the flame for nitromethane (NM) combusting in both inert and oxidative environments at 34 bar. Emission spectra from OH A²Σ⁺ → X²Π relaxation show that electronically excited OH* emission persists over the length of a nitromethane flame in air, but emission diminishes abruptly within several millimeters of nitromethane flames burning in inert atmospheres (N₂). These findings mark the first reported instances of spatially resolved spectral images of NM combustion at elevated pressures and provide important benchmarks for monopropellant combustion models, where number densities of products and intermediates are often reported as a function of distance from a nominal flame front. Vibrational and rotational temperatures calculated as a function of distance along the flame are compared to Cantera-simulated adiabatic flame temperatures. OH* rotational temperatures appear thermalized, although vibrational temperatures suggest that the radical possesses excess vibrational energy. Additionally, vibrational temperatures are used to infer the formation pathway─chemical vs thermal─of OH* as a function of height within the flame.