The Journal of Physical Chemistry A最新文献

In both nature and industry, aerosol droplets contain complex mixtures of solutes, which in many cases include multiple inorganic components. Understanding the drying kinetics of these droplets and the impact on resultant particle morphology is essential for a variety of applications including improving inhalable drugs, mitigating disease transmission, and developing more accurate climate models. However, the previous literature has only focused on the relationship between drying kinetics and particle morphology for aerosol droplets containing a single nonvolatile component. Here we investigate the drying kinetics of NaCl-(NH₄)₂SO₄, NaCl-NH₄NO₃, and NaCl-CaCl₂ mixed salt aqueous aerosol droplets (25-35 μm radius) and the resulting morphology and composition of the dried microparticles. A comparative kinetics electrodynamic balance was used to measure evaporation profiles for each mixed salt aerosol at a range of relative humidities (RH) (0-50% RH); measurements of the evaporation kinetics are shown to be consistent with predictions from the "Single Aerosol Drying Kinetics and Trajectories" model. Populations of the mixed salt droplets were dried in a falling droplet column under different RH conditions and imaged using scanning electron microscopy to observe the impact of the drying kinetics on the morphology. Energy dispersive spectroscopy was used in tandem to obtain atomic maps and view the impact of drying kinetics on the composition of the resultant particles. It has been shown that the relationship between drying kinetics and dry particle morphology in mixed salt solution droplets is compositionally dependent and determined by the predominant salts that crystallize (i.e., (NH₄)₂SO₄, Na₂SO₄, or NaCl). The degree of homogeneity in composition throughout the particle microstructure is dependent on the drying rate.

The methoxy radical, CH₃O, has long been studied experimentally and theoretically by spectroscopists because it displays a weak Jahn-Teller effect in its electronic ground state, combined with a strong spin-orbit interaction. In this work, we report an extension of the measurement of the pure rotational spectrum of the radical in its vibrational ground state in the submillimeter-wave region (350-860 GHz). CH₃O was produced by H-abstraction from methanol using F atoms, and its spectrum was probed in absorption using an association of source-frequency modulation and Zeeman modulation spectroscopy. All the observed transitions together with available literature data in ν = 0 were combined and fit using an effective Hamiltonian allowing to reproduce the data at their experimental accuracy. The newly measured transitions involve significantly higher frequencies and rotational quantum numbers than those reported in the literature (f < 860 GHz and N ≤ 15 instead of 372 GHz and 7, respectively), which results in significant improvements in the spectroscopic parameters determination. The present model is well constrained and allows a reliable calculation of the rotational spectrum of the radical over the entire microwave to submillimeter-wave domain. It can be used with confidence for future searches of CH₃O in the laboratory and in the interstellar medium.

Quantum computing presents a promising avenue for solving complex problems, particularly in quantum chemistry, where it could accelerate the computation of molecular properties and excited states. This work focuses on computing excitation energies with hybrid quantum-classical algorithms for near-term quantum devices, combining the quantum linear response (qLR) method with a polarizable embedding (PE) environment. We employ the self-consistent operator manifold of quantum linear response (q-sc-LR) on top of a unitary coupled cluster (UCC) wave function in combination with a Davidson solver. The latter removes the need to construct the entire electronic Hessian, improving computational efficiency when going toward larger molecules. We introduce a new superposition-state-based technique to compute Hessian-vector products and show that this approach is more resilient toward noise than our earlier gradient-based approach. We demonstrate the performance of the PE-UCCSD model on systems such as butadiene and para-nitroaniline in water and find that PE-UCCSD delivers comparable accuracy to classical PE-CCSD methods on such simple closed-shell systems. We also explore the challenges posed by hardware noise and propose simple error mitigation techniques to maintain accurate results on noisy quantum computers.

We report a new NMR method for treating two-site chemical exchange involving half-integer quadrupolar nuclei in a solution. The new method was experimentally verified with extensive ²³Na (I = 3/2), ³⁹K (I = 3/2), and ⁸⁷Rb (I = 3/2) NMR results from alkali metal ions (Na⁺, K⁺, and Rb⁺) in a solution over a wide range of molecular tumbling conditions. In the fast-motion limit, all allowed single-quantum NMR transitions for a particular quadrupolar nucleus are degenerate giving rise to one Lorentzian signal. In the slow-motion regime, although the NMR signal from quadrupolar nuclei should in principle exhibit a multi-Lorentzian line shape, only the quadrupole central transition (QCT) is often detectable in practice. In all the cases studied in this work, we found that alkali metal ions undergo fast exchange between free and bound states. Using the new theoretical method, we were able to interpret the experimental transverse relaxation data (i.e., line widths) obtained for ²³Na, ³⁹K, and ⁸⁷Rb NMR signals including QCT signals over a large temperature range and extract information about ion-binding dynamics in different chemical environments. This work fills a gap in the literature where a unified approach for treating NMR transverse relaxation data for quadrupolar nuclei over the entire range of motion has been lacking. Our results suggest that the new approach is applicable in the study of alkali metal ion binding to biological macromolecules.

Searching for single-molecule magnets (SMM) with large effective blocking barriers, long relaxation times, and high magnetic blocking temperatures is vitally important not only for the fundamental research of magnetism at the molecular level but also for the realization of new-generation magnetic memory unit. Actinides (An) atoms possess extremely strong spin-orbit coupling (SOC) due to their 5f orbitals, and their ground multiplets are largely split into several sublevels because of the strong interplay between the SOC of An atoms and the crystal field (CF) formed by ligand atoms. Compared to TM-based SMMs, more dispersed energy level widths of An-based SMMs will give a larger total zero field splitting (ZFS) and thus provide a necessary condition to derive a higher U_eff. In combination of the density functional theory (DFT) as well as the CF model Hamiltonian and ab initio calculation, we have investigated the structural stability and electronic structures as well as the magnetodynamic behavior of [AnPc₂]^0/- (An = U, Cf) molecules. We find that An atoms can strongly interact with its ligand N atoms in forming An-N ionic bonds, and 5f electrons are more localized in the Cf atom than in the U atom, giving U⁴⁺(5f²) and Cf³⁺(5f⁹) valence states. Although the UPc₂ molecule has a modest value of U_eff = 514 cm^-1, it is not a good SMM due to the easy occurrence of quantum tunneling of magnetization (QTM). Based on the consistent results of CF Hamiltonian and ab initio calculations on the [CfPc₂]^- molecule, we propose that almost prohibited QTM within the Kramers doublets (KDs) as well as very low transition probabilities between different states via hindered spin-flip transitions would result in a high U_eff = 1401 cm^-1. The estimated high magnetic blocking temperature (T_B) of 58 K renders [CfPc₂]^- an excellent SMM candidate, implying that magnetic hysteresis could be observed in future experiments.

Least-squares tensor hypercontraction (LS-THC) has received some attention in recent years as an approach to reduce the significant computational costs of wave function-based methods in quantum chemistry. However, previous work has demonstrated that LS-THC factorization performs disproportionately worse in the description of wave function components (e.g., cluster amplitudes T̂₂) than Hamiltonian components (e.g., electron repulsion integrals (pq|rs)). This work develops novel theoretical methods to study the source of these errors in the context of the real-space T̂₂ kernel, and reports, for the first time, the existence of a "correlation feature" in the errors of the LS-THC representation of the "exchange-like" correlation energy E_X and T̂₂ that is remarkably consistent across ten molecular species, three correlated wave functions, and four basis sets. This correlation feature portends the existence of a "pair point kernel" missing in the usual LS-THC representation of the wave function, which critically depends upon pairs of grid points situated close to atoms and with interpair distances between one and two Bohr radii. These findings point the way for future LS-THC developments to address these shortcomings.

Chemical kinetics for second oxygen addition reactions (·QOOH + O₂) of long-chain alkanes are of great importance in low-temperature combustion technologies. However, kinetic data for key reactions of ·QOOH + O₂ systems are often difficult to obtain experimentally and are primarily estimated or calculated by using theoretical methods. In this work, barrier heights (BHs), reaction energies (ΔEs), and relative energies (REs) of stationary points for key reactions of two representative ·QOOH + O₂ systems in the low-temperature oxidation of n-butyl as well as pressure-dependent rate constants for the involved reactions are calculated with the high-level quantum chemical method CCSD(T)-F12b/CBS. These results can be employed in the development of low-temperature combustion mechanisms for n-butane and longer straight-chain alkanes. In addition, the performance of some quantum chemistry methods with a lower computational cost on BHs, ΔEs, and REs as well as rate constants is also investigated. Our results indicate that the maximum error on these energies with PNO-LCCSD(T)-F12a is within 1 kcal/mol, and rate constants with this method are in the best agreement with reference values, with a maximum relative error of about half the reference values. Due to its low computational cost and memory requirements, this method is strongly recommended for studying low-temperature combustion reactions involving larger hydrocarbon fuels.

The complete conversion of dinitrogen to ammonia mediated by a side-on N₂-bound carbene-beryllium complex, [NHC-Be(η²-N₂)] has been studied considering both the symmetric and unsymmetric pathways. N-heterocyclic carbenes complexed with Be(η²-N₂) moieties were considered substrates in our study. We found that two mechanistic pathways were possible for the reduction of dinitrogen to form ammonia. Our calculations revealed that the symmetric pathway is more favorable compared to the unsymmetric one. The interconversion of the complex from the symmetric product to the unsymmetric one involves a large activation energy barrier for the proton transfer pathway. Both of these pathways were associated with high exergonicity, and the N-N bond is observed to be elongated, which indicates that the NHC-Be(η²-N₂) complex is a promising candidate for dinitrogen activation and subsequent reduction, resulting in the formation of ammonia. The bonding scenario of the NHC-Be(η²-N₂) complex can be explained well by the famous Dewar-Chatt-Duncanson (DCD) model. Our calculations reveal that the symmetric pathway is found to be more suitable due to more negative values of change in Gibbs free energy. Solvent phase calculations have identified the viability of the NHC-Be(η²-N₂) complex, indicating that the complex is sustainable in low-polar organic solvents, such as toluene and diethyl ether.

The first-stage acid-base equilibrium of 5,5,6-trihydroxy-6-methyldihydropyrimidine-2,4(1H,3H)-dione was studied for the first time in aqueous solutions. Its constant (pK_a1 = 9.23 ± 0.03) and thermodynamic parameters (ΔG₂₉₈ = 52 ± 1 kJ·mol^-1, ΔH = 83 ± 1 kJ·mol^-1, and ΔS₂₉₈ = 103 ± 4 J·mol^-1·K^-1) were determined by potentiometric titration. Computational analysis, including molecular dynamics (MD) simulations and quantum chemical calculations, was conducted to evaluate solvation effects and proton dissociation sites. MD simulations identified distinct solvation shells and interactions with water molecules, while quantum chemical calculations highlighted the primary deprotonation site. Fuzzy bond order (FBO) analysis and energy calculations of anionic forms corroborated these findings, demonstrating a strong correlation between the ΔE and FBO values. The research established the dissociation sequence for conformational R- and S-isomers of the title compound and validated the FBO method as an efficient tool for assessing dissociation processes in polybasic acids.