The Journal of Physical Chemistry A最新文献

Recent work has documented conjugate polycyclic hydrocarbons presenting unusual properties: accepting full on-bond electron pairing, they could be considered as closed-shell architectures, but their ground-state wave function is actually a pure diradical singlet, free of any ionic component, in contrast to diradicaloids. These so-called entangled molecules also differ from disjoint diradicals, which do not accept on-bond electron pairing, in that their singly occupied molecular orbitals (SOMOs) are spatially entangled rather than disjoint. The present work first extends the study to a broad series of architectures exhibiting the same properties, namely: they present two degenerate SOMOs in the topological Hückel Hamiltonian, and their pure diradical wave functions lead to symmetry-keeping geometries. These solutions are always of lower energy than the closed-shell solutions that break symmetry and destroy aromaticity of some six-membered rings. A topological criterion ensuring that a given conjugate hydrocarbon will behave as an entangled pure diradical is then formulated. Next, a second set of molecules is proposed, still exhibiting two degenerate Hückel SOMOs, but with smaller contrast between the energies of open-shell and closed-shell solutions. Conservation of six-membered rings aromaticity appears as the driving factor ruling the stability of diradical solutions.

Dimethyl sulfide (CH₃SCH₃) is the largest natural source of atmospheric sulfur. Bis(trifluoromethyl) sulfides (CF₃SCF₃) are one of the perfluorinated thioethers with great interest as the new refrigerant fluid and dielectric replacement gas for the sake of environmental concern. In order to clarify the effect of fluorine substitution, degradation mechanisms and kinetics for the reactions of CH₃SCH₃ and CF₃SCF₃ with OH radicals in the atmosphere have been calculated comprehensively in a comparative manner using various high-level ab initio methods. It is revealed that the CH₃SCH₃ + OH reaction is predominated by addition/elimination and hydrogen abstraction mechanisms. A stable van der Waals complex exists via the long-range S···O interaction with a binding energy 9.1 kcal/mol, which decomposes straightforwardly by the S-C bond rupture. The collisional deactivation of the complex competes with two distinct hydrogen-abstraction paths. Theoretical rate coefficients are in good agreement with the available experimental data. In contrast, CF₃SCF₃ reacts with OH through the shallow wells (0.7 kcal/mol) to form the less stable tricoordinated S (III) covalent intermediates before the endothermic S-C bond fission. The room-temperature rate coefficient for the CF₃SCF₃ + OH reaction is 4 orders of magnitude lower than that for the CH₃SCH₃ + OH reaction. It is demonstrated that the atmospheric loss of CF₃SCF₃ has been retarded considerably with the lifetime around 300 years. The radiative efficiency is 0.463 W m^2- ppb^-1 and the global warming potential of CF₃SCF₃ is predicted to be approximately 14,000, indicative of a new super greenhouse gas. The present theoretical results will stimulate experimental studies of the dramatic impact on the reactivity of thioethers due to fluorination.

Single-molecule magnets (SMMs) with slow relaxation of magnetization and blocking temperatures above that of liquid nitrogen are essential for practical applications in high-density data storage devices and quantum computers. A rapid and accurate prediction of the effective magnetic relaxation barrier (U_eff) is needed to accelerate the discovery of high-performance SMMs. Using density functional theory and multireference calculations, we explored correlations between U_eff, partial atomic charges, and the anisotropic barrier for a series of sandwich-type lanthanide complexes containing cyclooctatetraene, substituted cyclopentadiene, phospholyl, boratabenzene, or borane ligands. Our results show a correlation between the electrostatic potential charge of the lanthanide ion in the complex and U_eff. Systematic ligand modifications show that reducing ligand nucleophilicity and incorporating soft bases enhance magnetic anisotropy and U_eff values. This work identifies a correlation to predict U_eff values and optimization of ligand coordination environments in lanthanide-based SMMs.

The fact that the photoabsorption spectrum of a material contains information about the atomic structure, commonly understood in terms of multiple scattering theory, is the basis of the popular extended X-ray absorption spectroscopy (EXAFS) technique. How much of the same structural information is present in other complementary spectroscopic signals is not obvious. Here we use a machine learning approach to demonstrate that within theoretical models that accurately predict the EXAFS signal, the extended near-edge region does indeed contain the EXAFS-accessible structural information. We do this by exhibiting deep operator neural networks (DeepONets) that have learned the relationship between the extended and near edge portions of the X-ray absorption spectrum to predict the former from the latter. We find that we can accurately predict the EXAFS spectrum between 6 and 14 Å^-1 from the first 6 Å^-1 (≈100 eV) of the absorption spectrum of Cu²⁺ substitutional defects in the Fe³⁺ mineral hematite (α-Fe₂O₃). This surprising finding implies that theoretical analyses of X-ray absorption spectra could be implemented that extract the same conclusions as high-quality EXAFS studies from spectra collected over a much smaller range of photon energies. This relaxes a host of experimental limitations related to the X-ray source and measurement sample, including collection time, minimum dopant concentration, source brilliance, and energy range. We describe the theoretical data sets and DeepONet construction and show that the resulting DeepONets produce EXAFS that recovers linear combination fits to experimental data with accuracy approaching the original ab initio calculations. We discuss the implications of our findings for minor constituent characterization and for understanding the information content of spectroscopic data more broadly, including how this approach might be applied to measured experimental spectra. To encourage similar efforts, the simulated X-ray spectra, machine learning, and fitting code are publicly available.

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.

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.

Organic room-temperature phosphorescence (RTP) emitters with long lifetimes, high exciton utilizations, and tunable emission properties show promising applications in organic light-emitting diodes (OLEDs) and biomedical fields. Their excited-state properties are highly related to single molecular structure, aggregation morphology, and external stimulus (such as hydrostatic pressure effect). To gain a deeper understanding and effectively regulate the key factors of luminescent efficiency and lifetime for RTP emitters, we employ the thermal vibration correlation function (TVCF) theory coupled with quantum mechanics/molecular mechanics (QM/MM) calculations to investigate the photophysical properties of three reported RTP crystals (Bp-OEt, Xan-OEt, and Xan-OMe) with elastic/plastic deformation. By analyzing the geometric structures and stacking modes of these crystals, we observe that the geometric structure variations influence the electronic structures, subsequently modifying the transition properties and the energy consumption processes. Specifically, the presence of strong π-π interactions and hydrogen bonds in the Xan-OEt crystal inhibits a nonradiative decay process, thereby realizing long-lived emission. Additionally, the hybridized local and charge-transfer (HLCT) excited-state feature with the largest charge transfer excitation contributions (57.74%) for Xan-OEt stabilizes the triplet excitons and facilitates the radiative decay process, ultimately achieving high efficiency and long lifetime emissions. Furthermore, by applying high hydrostatic pressure for the Bp-OEt crystal, the RTP emission efficiencies and lifetimes are enhanced and blue-shifted. All of these results demonstrate the crucial role of molecular structure and stacking modes as well as the hydrostatic pressure effect in regulating RTP properties. Thus, our findings reveal the structure-packing-property relationship and highlight the control of molecular packing and the related tunable approaches, which could provide prospective strategies for constructing stimuli-responsive RTP emitters in practical applications.

The viability of the P═Se bond to serve as a monitor of the strength of a noncovalent bond was tested in the context of the (CH₃)₃PSe molecule. Density functional theory (DFT) computations paired this base with a collection of Lewis acids that spanned hydrogen, halogen, chalcogen, pnicogen, and tetrel bonding interactions and covered a wide range of bond strengths. A very strong linear correlation was observed between the interaction energy and the nuclear magnetic resonance (NMR) ¹J(PSe) coupling constant, which could serve as an accurate indicator of bond strength. Also correlating very well with the interaction energy is the stretch of the P═Se bond caused by complexation and the red shift of its stretching frequency. Moderate correlations arise in the chemical shifts of the P and Se nuclei. The σ-hole depth on the Lewis acid is poorly correlated with the energetics, and the same is true for the full electrostatic contribution to the bond energy. Of the various components, it is the polarization energy that correlates most closely with the interaction energy.

The mineral schreibersite, e.g., Fe₃P, is commonly found in iron-rich meteorites and could have served as an abiotic phosphorus source for prebiotic chemistry. However, atomistic calculations of its degradation chemistry generally require quantum simulation approaches, which can be too computationally cumbersome to study sufficient time and length scales for this process. In this regard, we have created a computationally efficient semiempirical quantum density functional tight binding (DFTB) model for iron and phosphorus-containing materials by adopting an existing semiautomated workflow that represents many-body interactions by linear combinations of Chebyshev polynomials. We have utilized a relatively small training set to optimize a DFTB model that is accurate for schreibersite physical and chemical properties, including its bulk properties, surface energies, and water absorption. We then show that our model shows strong transferability to several iron phosphide solids as well as multiple allotropes of iron metal. Our resulting DFTB parametrization will allow us to interrogate schreibersite aqueous decomposition at longer time and length scales than standard quantum approaches, providing for more detailed investigations of its role in prebiotic chemistry on early Earth.

The structures and rotational constants of prototypical monocyclic terpenes and terpenoids have been analyzed by a general computational strategy based on recent Pisa composite schemes (PCS) and vibrational perturbation theory at second order (VPT2). Concerning equilibrium geometries, a one-parameter empirical correction is added to bond lengths obtained by the revDSD-PBEP86 double hybrid functional in conjunction with a slightly modified cc-pVTZ-F12 basis set. The same functional and basis set give accurate harmonic frequencies, whereas the cheaper B3LYP hybrid functional in conjunction with a double-ζ basis set is employed to compute the semidiagonal cubic force constants needed to obtain vibrational corrections to the rotational constants in the framework of the VPT2 model. The final results obtained in this way show in most cases average deviations with respect to the experiment close to 0.1%, which correspond to errors around 1 mÅ and 0.1° for bond lengths and valence angles, respectively. The accuracy of the results has produced reliable estimates for species not analyzed yet experimentally. In addition to the intrinsic interest of the studied molecules, this article confirms that high-resolution spectroscopic studies of quite large systems can now be aided by a very accurate yet robust and user-friendly computational tool.