The Journal of Physical Chemistry A最新文献

The analysis of the solution absorption spectrum of the plutonyl ion in an aqueous environment was given by Eisenstein and Pryce (E&P) in 1968. In 2011 a new spectrum was published of the (PuO₂)²⁺ ion in 1 M HClO₄. We have been provided with the original data of this spectrum and have found in the data a previously unreported low-lying transition at 7385 cm^-1 which we have assigned as a magnetic dipole transition. We have fit most of the near-infrared and optical transitions with Gaussian fits and tabulated a new energy level list up to 22,000 cm^-1 which mostly agrees with the data of E&P. We assumed a crystal field of D_∞h (only axial symmetry) and utilized the intensity calculations published for the isoelectronic (NpO₂)¹⁺ ion using a complete basis set for the 5f² problem including the Coulombic, spin-orbit as well as the crystal field Hamiltonian. Our results differ substantially from those of E&P. Subsequently, we used a truncated Hamiltonian to try to establish the effects of assuming the σ antibonding orbitals are at such high energies that we can ignore their contributions to the lower lying φ and δ orbitals.

The radical character of molecules exhibiting singlet fission is related to the energy level matching relationships that facilitate this process. Using a linear H₄ model molecule, we employ quantum chemical topology descriptors based on full configuration interaction calculations to rationalize singlet fission. In this context, the influence of the closed-shell to diradical and diradical to tetraradical character on the singlet fission energy matching conditions is analyzed. We find that in the diradical limit the singlet fission efficiency can be manipulated considering the active molecule coupled to an excited diradical, while in the diradical to tetraradical limit, the efficiency is dependent only on the gap between the lowest-lying excited singlet and triplet state. Furthermore, our results reveal possible design strategies for molecules with radical character exhibiting efficient singlet fission.

To understand the mechanism of self-assembly and to predict the evolutionary pattern of the fusion-fission system over a long period of time, studying the dynamics of these processes is of great significance. The trajectories from molecular dynamics (MD) simulations of self-assembly processes contain numerous latent fusion and fission events. To analyze the fusion and fission events from the simulated trajectory, in this article, a dynamic clustering approach was developed by comparing the changes of monomer composition within clusters over simulated time. The rates of fusion and fission events obtained from dynamic clustering analysis were further coupled with the population balance model (PBM), and the evolution of the molecular assemblies calculated is reasonably consistent with the corresponding MD simulation results. This approach provides a new idea to analyze the big data of molecular self-assembly dynamics obtained from MD simulations, and it offers a computationally achievable approach to connect discrete events at the molecular scale with continuum equations such as the PBM at the macroscale.

Experimental and theoretical studies on the compositional changes of new particle formation in the nucleation and initial growth stages of acid-base systems (2 and 5 nm) are extremely challenging. This study proposes a machine learning method for predicting the composition change of the sulfuric acid-dimethylamine system in the transformation from monomer to nanoparticle by learning the structure and composition information on small-sized sulfuric acid (SA)-dimethylamine (DMA) molecular clusters. Based on this method and changes in components, we found that the sulfuric acid-dimethylamine growth was mainly through the alternate adsorption of (SA)₁(DMA)₁, (SA)₁(DMA)₂, and (SA)₁ clusters at the early stage of nucleation, which accounted for about 70, 20, and 10%, respectively. This can explain the nature of possible changes in cluster acidity during the initial nucleation stage for the sulfuric acid-dimethylamine system. This method can also predict the base-stabilization mechanism of the sulfuric acid-dimethylamine system without relying on any experimental data, thereby yielding results that are consistent with those of previous experimental measurement.

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.

Theoretical and simulated analyses of selective homonuclear dipolar recoupling sequences serve as primary tools for understanding and determining the robustness of these sequences under various conditions. In this article, we investigate the recently proposed first-order dipolar recoupling sequence known as MODIST (Modest Offset Difference Internuclear Selective Transfer). We evaluate the MODIST transfer efficiency, assessing its dependence on rf-field strengths and the number of simulated spins, extending up to 10 spins. This helps to identify conditions that enhance polarization transfer among spins that are nearby in frequency, particularly among aliphatic protons. The exploration uncovers a novel effect for first-order selective recoupling sequences that we term "facilitated dipolar recoupling". This effect amplifies the recoupled dipolar interaction between distant spins due to the presence of additional strongly dipolar-coupled spins. Unlike the third spin-assisted recoupling mechanism, facilitated dipolar recoupling only requires a coupling to one of the two distant spins of interest. Experimental demonstration of MODIST, including at different rf-field strengths, was carried out with the membrane protein influenza A M2 in lipid bilayers using 55 kHz magic-angle spinning (MAS). Reducing MODIST rf-field strength by a factor of 2 unveils possibilities for detecting Hα-Hα and H^Meth-H^Meth correlations with a 3D (H)C(H)(H)CH experiment under fast MAS rates, all achievable without specific spin labeling.

Diffusion generative models, a class of machine learning techniques, have shown remarkable promise in materials science and chemistry by enabling the precise generation of complex molecular structures. In this article, we propose a novel application of diffusion generative models for stabilizing reactive molecular structures identified through quantum mechanical screening. Specifically, we focus on the design challenge presented by singlet fission (SF), a phenomenon crucial for advancing solar cell efficiency beyond theoretical limits. While theoretical chemistry has been successful in predicting intermolecular arrangements with enhanced SF coupling, the practical implementation of these configurations faces challenges due to discrepancies between favorable and stabilized structures. To address this gap, we introduce a three-step strategy combining quantum mechanical screening for identifying optimal molecular arrangements and diffusion generative models for predicting stabilizing linkers. Through a case study of cibalackrot dimers, a promising SF material, we demonstrate the efficacy of our approach in enhancing SF efficiency by stabilizing the desired molecular arrangements.

The phenoxazine class of dyes has found widespread applications in chemistry and biology for more than a century, particularly for lipid membrane studies. Here, we report a general phenomenon on the ensemble spectral stability of traditional phenoxazine class of dyes (nile red, cresyl violet, and nile blue) that exhibit hours-long microstructural transitions reflected through systematic changes of electronic spectra over an hour. Mechanistic investigations reveal that such spectral dynamics of the dyes can be mitigated by tuning microenvironments, where microsolvation plays an underlying role. These microsolvation-induced microstructural changes in a single dye species tend to follow zeroth-order kinetics. The half-life values of such processes systematically vary with solvent hydrogen bonding strength and ionic radius of the dyes' counteranions. In so doing, using a model lipid membrane, we demonstrate that the spectral response of a phenoxazine dye must be utilized appropriately for studying membrane properties. These findings of the phenoxazine class of dyes are of high significance for their careful applications in chemistry and biology.

In experimental studies, hydrazine hydrate is widely employed as a reducing agent for the conversion of graphene oxide to graphene. Herein, we conducted theoretical calculations using cluster models to investigate the adsorption behavior of hydrazine hydrate on the surface of graphene. The calculated adsorption energy reveals that hydrazine hydrate can physically bind to the graphene surface. Our findings indicate that two hydrogen bonds stabilize the hydrazine hydrate molecule, while its adsorption onto the graphene surface is primarily driven by van der Waals forces. By combining computational simulations and experimental measurements, we thoroughly examined the Raman spectra of both free and adsorbed hydrazine hydrates, which enabled us to gain detailed insights into their molecular vibrations. Notably, in the Raman spectra of free hydrazine hydrate, a strong peak at around 3300 cm^-1 corresponds to the NH₂ vibration. Similarly, peaks near 3300 cm^-1 were observed in the Raman spectra of graphene with adsorbed hydrazine hydrate molecules. The results are expected to provide valuable references for future experimental investigations involving hydrazine hydrate.

The absolute total cross section for electron collisions with acetic acid has been measured using an electrostatic electron spectrometer and linear transmission method for collision energies ranging from 0.4 to 300 eV. Elastic electron scattering from acetic acid within a low-energy range has also been studied theoretically using the Schwinger multichannel and R-matrix methods, in the static-exchange and static-exchange plus polarization levels of approximation for energies up to 15 eV. The absolute total and the integral elastic cross sections display a π* shape resonance at around 1.7 eV and a broad structure spanned between 4 and 10 eV, which can be associated with a superposition of overlapping σ* resonances. We compared the obtained results with data available in the literature regarding the interaction of electrons with acetic acid. The results of electron collisions with acetic acid, methyl formate, and formic acid are also compared and discussed.