The Journal of Physical Chemistry A最新文献

The redox potentials for U, Np, Pu, and Am for oxidation states +III up to +VIII in alkaline aqueous solutions were predicted using density functional theory (DFT) and small-core pseudopotentials and their basis sets, with a hybrid explicit/implicit solvent model using SHE = 4.28 V. For each oxidation state, various oxo/hydroxo complexes were evaluated, resulting in a variety of one-electron redox pathways. For An(VIII/VII) couples, the predicted redox potentials for the [An(VIII)O₅(OH)]^-3/[An(VII)O₄(OH)₂]^-3 or [An(VIII)O₄(OH)₂]^-2/[An(VII)O₄(OH)₂]^-3 couples are in good agreement with existing estimates. For An(VII/VI) redox couples, all couples, particularly [An(VII)O₄(OH)₂]^-3/[An(VI)O₂(OH)₄]^-2, were in agreement with experimental values for U, Np, and Pu, but the results for Am showed larger differences from the estimated potentials. The An(VI/V) couples were consistent with experiments for dioxo/tetrahydroxo couples, and the An(V/IV) couples showed acceptable agreement based on actinide-specific couples, with neutral hydroxides often favored in the +IV state. The An(IV/III) couples were consistent with the literature values when modeled as soluble neutral hydroxides. The use of our approach yielded calculated redox potentials that were within ±0.2 V of experimental or estimated values consistent with our prior calculations on redox potentials of actinides from Ac to Am in acidic aqueous solutions. This supports the robustness of our DFT-based methodology for predicting actinide redox potentials, offering valuable insights into actinide chemistry in aqueous solutions.

The reaction between NO₃ and HO₂ is one of the most important reaction in nighttime atmospheric chemistry. There are two pathways for this reaction: one leading to the formation of HNO₃, and the other resulting in the formation of the OH radical. Recent experimental and theoretical studies suggest that this reaction occurs through only the OH radical pathway. In this work, we have investigated the fate of this reaction in the presence of water using high-level quantum chemical and kinetics calculations over the temperature range of 213-400 K. Our investigation suggests that even in the presence of a water monomer, the reaction predominantly produces OH radicals.

Bithiophene has an electron-rich conjugated ring, enabling highly tunable photophysical properties for the design of novel organic light-emitting materials. Extensive research was focused on the functionalization of α-site-connected bithiophene, while recent work reported the synthesis of β-bithiophene, substantially enlarging the chemical space for bithiophene design. However, the design rule for modulating the physical properties of β-bithiophene has remained unexplored. We performed comprehensive quantum chemical calculations to investigate how functional groups and substituent sites control the absorption and emission wavelengths of β-bithiophene. Our results show that the functional groups lead to red-shifts of the wavelengths by extending the electron delocalization, while the substitution sites have fewer effects on the wavelengths. The absorption and emission calculation for trithiophene and tetrathiophene suggest that the photophysical properties of thiophene polymer are controlled by the short thiophene chains, underscoring the significance of the rational design of β-bithiophene derivatives.

Kinetics of the gas phase reaction between the stabilized Criegee intermediate formaldehyde oxide (CH₂OO) and nitrogen dioxide (NO₂) have been measured using laser flash photolysis of CH₂I₂/O₂/N₂/NO₂ mixtures coupled with time-resolved broadband ultraviolet absorption spectroscopy. Experiments were performed in N₂ under pseudo-first-order conditions at temperatures between 242 and 353 K and pressures in the range 25 to 300 Torr. The kinetics of CH₂OO + NO₂ are independent of pressure, with a mean rate coefficient of k₁ = (1.24 ± 0.22) × 10^-12 cm³ s^-1 at 298 K, where the uncertainty represents a combination of the 1σ statistical error and the systematic errors resulting from uncertainties in gas flow rates and in the concentration of NO₂. Measurements indicate upper limits of <5% for production of NO₃ and <5% for production of NO, and further studies of product yields are warranted. In contrast to expectations from theory, the kinetics of CH₂OO + NO₂ display a negative temperature dependence that can be described by k₁ = (1.07 ± 0.02) × 10^-12 × (T/298)^-(2.9±0.2) cm³ s^-1. Analysis using the Master Equation Solver for Multi-Energy well Reactions is able to reproduce a negative temperature dependence for the reaction if significant changes to barrier heights are made, but the overall agreement between the experiment and theory remains poor. This work highlights the challenges associated with calculations for systems with significant multi-reference character.

Since the size of an atmospheric particle determines many of its effects, we conducted experiments to better understand their rate of growth. Seed particles composed of sulfuric acid and water were exposed to photolytically generated H₂SO₄ molecules and their change in size was monitored with a mobility particle system. H₂SO₄ production rates were held steady while the seed particle diameter was varied from 3 to 25 nm to explore how growth is affected by size. The growth rate of 25 nm diameter particles was about 50% less than that for 3 nm diameter particles. Gas-kinetic hard-sphere growth rates decline only 18% over this size range, but a decrease of 35-to-50% in growth over this range is expected according to theories that include the effects of a van der Waals interaction between gaseous H₂SO₄ and the small particles. The size-dependence of the measured growth rates, which does not require knowledge of the H₂SO₄ gas concentration, suggests that the attractive force between hydrated H₂SO₄ and small sulfuric acid particles leads to a significant enhancement of the collision rate; this force depends strongly on particle size below 10 nm in diameter. Recent calculations based on a central field approximation for the van der Waals interaction are consistent with the measurements, although empirical enhancement factors better explain the data for some conditions. Nucleation experiments were also performed with H₂SO₄ detection, and simulations of these nucleation experiments required similar van der Waals enhancements to secure agreement between measured H₂SO₄ vapor and the size of the nucleated particles.

The separation of nuclear waste is one of the urgent problems to be solved in the nuclear industry; therefore, designing a new and promising extractant to deal with nuclear waste is a desirable but difficult task. Here, using quantum-mechanical calculations at the scalar-relativistic and spin-orbit coupling levels, we assessed the complexation of a series of actinyl moieties by phenanthroline-derived organophosphorus ligands. Specifically, we considered the [UO₂(L)]²⁺ and [AnO₂(BPP)]²⁺, An = U, Np, Pu, and L = DAP, BPP, and PIP species. First, we find that ligand BPP binds more strongly to the uranyl dication than DAP or PIP, and similarly, BPP binds more strongly to uranyl and neptunyl than plutonyl, as a result of the significant ionic bonding and covalent bonding in the equatorial plane. The results of this work generate a fundamental and qualitative understanding of bonding properties in actinyl complexes, suggest phosphine oxide-derived phenanthroline ligands as a potential extractant for uranyl, and provide some knowledge when designing efficient ligands for the extraction behaviors toward actinide elements.

The CaC₂ molecule, as an interstellar species that has already been detected, has attracted significant attention. To date, studies on the potential energy surface (PES) and the reaction dynamics of CaC₂ are largely lacking. In this work, ab initio energy values were obtained for 3877 configurations using the icMRCI+Q method, and these energies were subsequently fitted using a neural network approach. During parameter optimization, the trust region framework (TRF) method, which has superior performance compared to the previously used Levenberg-Marquardt (LM) method, was used. The root-mean-squared error (RMSE) for both the training and testing sets meets the requirement for chemical accuracy (error less than 1 kcal/mol). Using the neural network PES, we identified one stable structure and two metastable structures for the ground state (X̃¹A') of CaC₂. The stable structure is T-shaped, while the two metastable structures are linear. The potential well depths of the stable structure and the two metastable structures are -10.98, -9.75, and -4.70 eV, respectively. Based on the obtained full-dimensional neural network PES, we investigated the CaC(X³Σ^-) + C(³P_g) → Ca + C₂ (Σv) reaction dynamics under different initial conditions. Under the condition that all other parameters remain unchanged, the reaction cross section and rate constant were found to be largest when the initial condition was v = 0 and j = 0. These findings indicate that the reaction rate is fastest when the CaC molecule is in its ground state.

A novel molecular structure that bridges the fields of molecular optical cycling and molecular photoswitching is presented. It is based on a photoswitching molecule azobenzene functionalized with one and two CaO- groups, which can act as optical cycling centers (OCCs). This paper characterizes the electronic structure of the resulting model systems, focusing on three questions: (1) how the electronic states of the photoswitch are impacted by a functionalization with an OCC; (2) how the states of the OCC are impacted by the scaffold of the photoswitch; and (3) whether the OCC can serve as a spectroscopic probe of isomerization. The experimental feasibility of the proposed design and the advantages that organic synthesis can offer in the further functionalization of this molecular scaffold are also discussed. This work brings into the field of molecular optical cycling a new dimension of chemical complexity intrinsic to only polyatomic molecules.

The mechanism and consequently the magnitude of vibrational relaxation of molecules on surfaces differ significantly between insulators and metals, making the vibrational energy transfer at the NO/metal versus the NO/insulator interface a canonical example in the field. We report the influence of the surface temperature, the initial vibrational state, and the incident translational energy on the vibrational relaxation probability of vibrationally excited NO(v_I = 3 and v_I = 11) undergoing a direct scattering from thin films of vanadium dioxide (VO₂) across the Mott transition at 68 °C. At that temperature, thin-film VO₂ transforms from the insulating to the metallic phase, exhibiting ∼4 orders of magnitude drop in resistivity. As VO₂ undergoes the Mott transition, at T > 68 °C, we observe a surprisingly small, yet measurable enhancement in the relaxation probability of NO(v_I = 3 and v_I = 11) due to the metallic phase of VO₂. The magnitudes of vibrational relaxation for NO(v_I = 3)/VO₂ and NO(v_I = 11)/VO₂ are ∼2 and ∼20%, respectively─considerably lower than expected, based on the S-shaped dependence of vibrational relaxation probability on the asymptotic affinity level, observed for diatomic molecules on coinage metal surfaces. By analyzing the distinct dynamic features of NO scattering, including the dependence of vibrational relaxation on the initial vibrational state and on the incidence energy, as well as the relationship between rotational excitation and vibrational inelasticity, we explain the low magnitude of vibrational relaxation of NO on VO₂ using the electron transfer model.