The Journal of Physical Chemistry A最新文献

Menadione is a multifunctional molecule involved in critical biological processes such as blood coagulation, redox regulation, and cellular metabolism. Understanding its electron attachment properties and capacity to form stable anions is essential for elucidating its function in biological environments. In this study, we investigated electron attachment to menadione using a crossed electron-molecular beam experiment, complemented by quantum chemical and electron scattering calculations. Upon electron attachment, the efficient formation of the parent molecular anion is observed. Its signal extends from 0 to 2.5 eV, with pronounced peaks at ∼0 and 0.7 eV, assigned to the formation of different precursor anion states. Two fragment anions, namely, C₂H₂^– and CH₃^–, were also detected. In contrast to the parent anion, their formation occurs with significantly lower efficiency and only at higher electron energies, above 4 eV, consistent with the higher energy thresholds required for dissociative electron attachment. Our findings show, on the one hand, that the metastable parent anion of menadione has a relatively long lifetime, which may be further extended in biological environments due to solvent effects, and, on the other hand, that it is structurally stable in the interaction with low-energy electrons.

The incorporation of ¹³C isotopes into saccharides introduces additional signal multiplicity into the ¹H NMR spectra caused by ¹³C–¹H spin-coupling to the highly abundant ¹³C spins. Large ¹J_CH values have been exploited to relieve non-first-order (strong coupling) effects in these spectra by spitting the signal of a ¹H spin that overlaps the signal arising from a mutually coupled ¹H spin. Using NMR spin simulation and complementary experimental studies, we show that this approach is valid only under specific spectral conditions. When the homonuclear J_HH value between the strongly coupled hydrogens exceeds ∼6.5 Hz, the underlying multiplet exposed by ¹J_CH splitting will experience a chemical shift offset due to strong coupling that complicates the measurement of J_CH values if this hydrogen is also spin-coupled to the ¹³C-labeled carbon. Since the magnitude of the offset scales with the magnitude of the ³J_HH between the strongly coupled hydrogens after a threshold of ∼6.5 Hz is reached, antiperiplanar hydrogens in saccharide pyranosyl rings are particularly vulnerable to this complication. The offset is not eliminated when spectral data are collected in two (and presumably higher) dimensions. This heretofore under-appreciated effect causes significant errors in the measurement of J_CH values, especially those having dynamic ranges of <4–5 Hz. This behavior will be more difficult to recognize and treat in larger oligosaccharides that contain ¹³C enrichment at one or more carbons when multidimensional spectra are collected to improve the spectral resolution.

Herein, we reinvestigate the photophysics of ovalene, a prototypical nanographene for which conflicting spectroscopic results have been reported. Owing to its structural similarity and its identical D_2h point-group symmetry, ovalene can essentially be viewed as a larger pyrene. We show that its optical transitions can be understood using the same model that is invoked to explain the excited states of pyrene. Absorption and (polarized)-emission measurements reveal that the S₁ ← S₀ (¹B_3u ← ¹A_g) transition is forbidden, whereas the first prominent absorption band can be assigned to the allowed S₂ ← S₀ (¹B_2u ← ¹A_g) transition, in contrast to recent reassignments. Temperature and time-dependent spectroscopic measurements show that the S₁ and S₂ states quickly establish a thermal pre-equilibrium, giving rise to thermally activated S₂ → S₀ emission at room-temperature. As a result, the fluorescence lifetime of ovalene decreases with increasing temperature while its fluorescence quantum yield increases. Contrary to the frequently cited small energy gap of ∼400 cm^–1, our measurements reveal a significantly larger S₂–S₁ gap of approximately 1200 cm^–1.

The interactions of noble gases (Ng = He, Ne, Ar, Kr) with highly electron-deficient main-group fragments are systematically investigated by using high-level CCSD and CCSD(T) calculations. A broad set of electrophilic acceptors is considered, spanning six-electron (O, S, F⁺, Cl⁺, Br⁺, OH⁺, SH⁺, NH₂⁺), four-electron (BF₂⁺, AlF₂⁺), and two-electron (BeF⁺, MgF⁺) centers. Optimized geometries, interaction energies, and electronic descriptors reveal a continuous evolution of Ng binding behavior across the series from weak polarization-dominated interactions for He and Ne, to donor–acceptor bonding for Ar, and to strongly covalent-like coordination in Kr complexes. The analysis, supported by natural bond orbital (NBO), quantum theory of atoms in molecules (QTAIM), symmetry-adapted perturbation theory (SAPT), and molecular electrostatic potential (MESP) descriptors, demonstrates that the strength and multiplicity of Ng binding are governed primarily by the electrophilicity and valence-electron deficiency of the acceptor fragment with noble-gas polarizability modulating the interaction strength. Within this context, a unified 2e–4e–6e valence-electron framework is employed as a descriptive tool to rationalize why six-electron centers preferentially bind one Ng atom, four-electron centers stabilize two Ng atoms, and highly electron-deficient two-electron centers accommodate three Ng atoms. Among the multi-noble-gas complexes examined, BeF⁺ and MgF⁺ are found to stabilize tri-noble-gas adducts across the He–Kr series, with Kr₃BeF⁺ exhibiting the strongest overall binding. Trihelium coordination to BeF⁺, with interaction energies of several kcal mol^–1 per He atom, highlights the remarkable stabilization that can arise in extreme electron-deficient environments. Overall, the results provide a unified and internally consistent framework for organizing mono-, di-, and tri-noble-gas binding motifs across the noble-gas series, clarifying the electronic factors that govern noble-gas coordination in highly electrophilic chemical regimes.

In the design of implantable electrodes, the choice of electrode materials and associated interface engineering directly determines the charge transport behavior. By focusing on silver, the best conductive metal, as the electrode, one can explore the design of the interface with better biocompatibility and conductivity. In recent years, atomically precise gold nanoclusters have been proposed as electrode interface modifiers, providing new possibilities for controlling the interfacial electronic structure and charge transport. Herein, the Ag-X-Ag (X = Au₂₅CH₃, Au₂₄CdCH₃, Au₂₄CuCH₃) model systems are constructed to study charge transport at the nanocluster-electrode interface with density functional theory-nonequilibrium Green's function calculation and neuroevolution potential-based molecular dynamics simulation. The structures of gold nanoclusters at the interface exhibit enhanced charge transport properties after relaxation, which is attributed to dopant-induced geometric rearrangement and enhanced interfacial coupling effects. The enhancement of the interfacial coupling effects is most prominent when the dopant is positioned near the left electrode (configuration 2). Cd doping yields the greatest enhancement, characterized by stronger resonant transmission, a higher density of states near the Fermi level, increased current, and reduced impedance. Furthermore, the transport properties show weak temperature dependence, which reflects the synergistic interaction between the metallic Ag electrode and the semiconducting-like nanocluster. Atomically precise doping in gold nanoclusters offers an effective approach to enhancing interfacial transport efficiency and provides a generalizable strategy for advanced electrode design in next-generation, high-performance transport devices.

Accurate determination of a molecule’s thermodynamic properties using quantum chemistry methods is crucial for developing kinetic models. A vital step in the quantum chemical workflow is calculating torsional energy profiles to apply the one-dimensional hindered-rotor approximation. However, these profiles are typically obtained by performing rotational scans using density functional theory (DFT) methods, which adds high computational cost to the workflow. Here, we assess the performance of the semiempirical GFN2-xTB method for rapid generation of these torsional profiles by calculating the contributions of GFN2-xTB and B3LYP profiles to the enthalpy, entropy, heat capacity, and Gibbs free energy of molecules. Several correction methods to improve the results are proposed and evaluated. It is found that correcting the energy profiles based on their second derivative at the optimized-geometry point yields the most accurate results. This optimal correction method results in a mean absolute error on the Gibbs free energy at 1000 K of 0.43 kJ mol^–1 for a hydrocarbons data set and 0.93 kJ mol^–1 for a data set of nitrogen-containing compounds. In conclusion, the GFN2-xTB method, when combined with a suitable correction method, can accelerate the generation of hindered rotor profiles by a factor of 700, with only a minor loss of accuracy.

A new method based on the solution of time-independent standard response equation has been developed for the calculation of spin–spin coupling density functions, entirely in the atomic orbital basis at both HF and DFT (GGA and hybrid GGA) level of theory. The study is not limited to the Fermi contact alone, but also includes all four Ramsey terms, which have sometimes been shown to be non-negligible. The current density induced by nuclear magnetic dipoles represents the leading motif followed in the development of the theory. A few molecules have been analyzed in detail. The mechanism of spin polarization can be visualized, and the distinction between through-space and through-bond interactions can now be understood in terms of all four Ramsey contributions.

Circular dichroism (CD) spectroscopy of gaseous molecules remains challenging because of intrinsically weak CD signals and low molecular number densities, which necessitate the use of pulsed molecular beams and high-power pulsed lasers. Pulse-to-pulse intensity fluctuations in both systems introduce variations in ionsignals, leading to fluctuation-induced artifacts in the measured CD values. To suppress these effects, we developed a CD measurement technique that employs randomly alternating left- and right-handed circularly polarized laser pulses (random-CP). Unlike conventional regularly alternating circularly polarized pulses (regular-CP), random-CP pulses decouple periodic experimental fluctuations from the CD signal, thereby minimizing systematic artifacts and improving precision. The advantages of this approach are demonstrated through resonant two-photon ionization CD and fluorescence-detected CD spectra of jet-cooled (R)-(+)-styrene oxide recorded near the origin band of the S₀ – S₁ transition. Compared with regular-CP pulses, random-CP pulses lead to more rapid convergence of CD values, more consistent CD band profiles, and effective suppression of offset biases. This random-CP pulse strategy provides a robust and broadly applicable method for enhancing the accuracy and reliability of gas-phase CD measurements.

The reaction force (RF) formalism has long been recognized as a useful method for analyzing energy changes occurring in a chemical reaction. In its original interpretation, the RF extrema are employed to divide the total energy along the reaction path into distinct regions associated with fundamental processes, such as changes in bonding and geometry reorganization. While effective, this decomposition of the total energy can lead to conceptual challenges and interpretations that lack intuitive clarity from a physical perspective. The central objective of this work is to further elucidate the distinct roles of electronic and nuclear repulsion energies in reaction barriers (i.e., forward and reverse directions) through a novel and alternative RF decomposition scheme. We achieve this by introducing an a priori separation of the total energy into these two fundamental contributions. In doing so, this strategy provides a uniquely transparent framework, enabling the reaction force to be partitioned into zones governed by electronic rearrangement and regions dominated by internuclear repulsion. Crucially, this approach reveals a key finding that confirms its physical insights; the nuclear–nuclear (V_nn) and electronic (E_e) energies exhibit contrasting and interpretable profiles (i.e., a double-maxima and a double-minima, respectively) along the reaction path. We support our conclusions by examining five simple reactions: (i) H_a^– + H_b–H_c → H_a–H_b + H_c^–, (ii) F_a^– + CH₃–F_b → F_a–CH₃ + F_b^–, (iii) F^– + CH₃–Cl → F–CH₃ + Cl^–, (iv) O=N–S–H → H–O–N=S, and (v) CO + HF → HCOF. By carefully scrutinizing these, we show that our force components behave in a general manner and confirm that the proposed method offers a more intuitive and reliable framework to distinguish between electronic reorganization and geometric changes throughout a reaction.

Aluminum clusters (Al_n) represent prototypical superatomic behaviors at n = 13, where their molecular electronic states mimic atomic shells. Specifically, Al₁₃ has been regarded as the “superhalogen archetype”, yet its fundamental properties remain controversial. Reported energies for the photodetachment of Al₁₃^– anions and the photoionization of Al₁₃ neutrals vary widely across density functional and correlated wave function methods, and even the underlying structural assignments remain disputed. Here, we employ diffusion Monte Carlo (DMC) with multideterminant trial wave functions to resolve these discrepancies. DMC reproduces the experimental adiabatic detachment energy of Al₁₃^– and reconciles the vertical detachment energy with experiment once vibronic effects are included. For the Al₁₃ neutrals, we show that the measured ionization energy of 6.42 eV corresponds not to the photoionization into the ground state cap cation but to a strongly distorted cation of an oblate isomer, consistent with experimental photoionization mass spectra. These results settle long-standing benchmark controversies, and demonstrate the power of quantum Monte Carlo as a reference method for superatomic Al₁₃ clusters.