The Journal of Physical Chemistry A最新文献

Singlet fission (SF) is a phenomenon that generates multiple excitons (triplets) on different chromophores from a single exciton (singlet) on one chromophore. Owing to the strong electronic correlation and a complicated excited state manifold of carotenoids (polyenes), the SF mechanism in carotenoids is different from acenes shown in J. Phys. Chem. Lett., 2022, 13, 6800-6805. However, the mechanism is expected to have significant effects of the geometry in the excited state and strong vibronic couplings between these low-lying excited states. Employing high-level state-of-the-art electronic structure methods, we show that the dark A_g states and charge transfer components play a major role in the SF process. The success of the process is strongly dependent on the relative orientation of the monomers. We have also shown that the high-frequency modes involving changes in bond length alternation are strongly coupled to the excited electronic states. These nuclear vibrational modes facilitate the SF process.

A theoretical study on the mechanism, regioselectivity, and enantioselectivity of NHC-catalyzed dearomatizing annulation of benzoxazoles with enals has been conducted using density functional theory calculations. Our calculated results indicate that the favored mechanism occurs through eight reaction steps: initial binding of the NHC to enals, followed by formation of the Breslow intermediate via proton transfer. Subsequent oxidation generates the α,β-unsaturated acylazolium intermediate, which can undergo Michael addition with benzoxazoles. Sequential protonation/deprotonation/cyclization produces the six-membered cyclic intermediate that undergoes catalyst elimination, leading to the final product. DABCO·H⁺ was found to play important roles in proton transfer and cyclization. Without DABCO·H⁺, the energy barrier up to 44.2 kcal/mol for step 2 is too high to be accessible. With DABCO·H⁺, the corresponding value is lowered to 18.6 kcal/mol. The energy barrier for cyclization can be lowered by 7.4 kcal/mol by using DABCO·H⁺. The Michael addition step determines both the enantioselectivity and the regioselectivity. According to NCI analysis, the enantioselectivity is controlled by the strong interactions (such as C-H···O, C-H···N, and π···π) between the α,β-unsaturated acylazolium intermediate and benzoxazoles. We also discuss the solvent and substituent effects on the enantioselectivity and the role of the NHC. The mechanistic insights obtained in the present study would help improving current reaction systems or designing new synthetic routes.

Linear vibronic coupling (LVC) models have proven to be effective in describing coupled excited-state potential energy surfaces of rigid molecules. However, obtaining the LVC parameters in molecules with many degrees of freedom and a large number of, possibly (near-)degenerate, electronic states can be challenging. In this paper, we discuss how the linear intra- and interstate couplings can be computed correctly using a numerical differentiation scheme, requiring a phase correction and sufficient numerical precision in the involved electronic structure calculations. The numerical scheme is applied to three test systems with symmetry-induced state degeneracies: SO₃, [PtBr₆]^2-, and [Ru(bpy)₃]²⁺. The first two systems are employed to validate the performance of the parametrization scheme. LVC potentials for SO₃ are shown to reproduce the trigonal symmetry of the potential energy surfaces. The integration of the LVC potentials for [PtBr₆]^2- with the surface-hopping trajectory method illustrates how spurious parameters lead to erroneous trajectory behavior. In the transition metal complex [Ru(bpy)₃]²⁺, extensive nonadiabatic simulations using LVC potentials are compared to those conducted with direct on-the-fly potentials. The simulations with LVC potentials demonstrate excellent agreement with the on-the-fly results while incurring costs that are 5 orders of magnitude lower. Further, the simulations evidence that intersystem crossing in [Ru(bpy)₃]²⁺ occurs at a slightly slower rate than luminescence decay, underscoring the importance of simulating the actual experimental observable when comparing computed time constants with experimental time constants. Lastly, the initial nuclear response to excitation involves a rapid, short-lived, and small elongation of the Ru-N bonds, with no charge localization occurring on a sub-ps time scale.

Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are powerful analytical tools with diverse applications in research and medicine. However, the inherently poor signal-to-noise ratios induce technical limitations, which suppress their widespread use. Hyperpolarization enhances the NMR signals by inducing highly nonequilibrated population distributions among the nuclear spin states. We demonstrated real-time amino acid hyperpolarization using signal amplification by reversible exchange (SABRE). We aimed to hydrolyze hyperpolarized methyl esters to induce amino acid hyperpolarization. We successfully hyperpolarized 19 methylated amino acids via SABRE. This groundwork enabled the development of a predictive model for the hyperpolarization enhancement factors of methylated amino acids. The model accurately predicted the hyperpolarization of three synthetic methylated amino acids, paving the way for advanced bio-NMR and MRI applications requiring the immediate hyperpolarization of other amino acids. This research underlines the potential of hyperpolarization in overcoming the current limitations of NMR spectroscopy and MRI.

This study investigates the modification of carrier dynamics in nanoscale multiple quantum wells (MQWs) through B⁺ ion implantation, combining experimental and theoretical approaches to provide a comprehensive understanding of the impact on ultrafast optoelectronic responses. Using femtosecond time-resolved transient absorption (TA) spectroscopy, we examine the changes in carrier dynamics in both pristine and B⁺-implanted In_0.25Ga_0.75As/GaAs_0.9P_0.1 MQWs. Our results reveal significant modifications in the transient absorption spectra, with ion implantation reducing the excited-state absorption cross section (σ_ES) and leading to faster carrier recovery times. To further analyze these changes, we introduce a novel cascade rate equation model that incorporates two effective relaxation times, allowing for more accurate simulations of the experimental data. The model captures the complex interactions between various carrier states and provides a deeper understanding of the ion implantation effects on carrier trapping, recombination, and recovery processes. The comparison of experimental results and theoretical simulations demonstrates that ion implantation enhances ultrafast recovery times and modulates the carrier dynamics, offering a pathway for tailoring the optoelectronic properties of semiconductor materials. This work provides both a theoretical framework and experimental evidence for the design of next-generation ultrafast photonic devices with optimized carrier dynamics.

Excited states of transition metal complexes are generally strongly correlated due to the near-degeneracy of the metal d orbitals. Consequently, electronic structure calculations of such species often necessitate multireference approaches. However, widespread use of multireference methods is hindered due to the active space selection problem, which has historically required system-specific chemical knowledge and a trial-and-error approach. Here, we address this issue with an automated method combining the approximate pair coefficient (APC) scheme for estimating orbital entropies with the discrete variational selection (DVS) approach for evaluating active space quality. We apply DVS-APC to the calculation of 67 vertical excitations in transition metal diatomics as well as to two larger complexes. We show DVS-APC generated active spaces yield NEVPT2 mean absolute errors of 0.18 eV, in line with previous accuracies obtained for organic systems, but larger than errors achieved with hand-selected active spaces (0.14 eV). If instead of using DVS we identify the best results from our trial wave functions, we find improved performance (mean absolute error of 0.1 eV) over the manually selected results. We highlight this deviation between DVS and hand selected active spaces as a possible measure of bias introduced when hand selecting active spaces. However, we find that multiconfiguration pair-density functional theory (MC-PDFT) using the tPBE and tPBE0 functionals is roughly 0.15 eV less accurate than NEVPT2 across this class of diatomic systems, potentially accounting for the decreased performance of DVS-APC, which uses MC-PDFT energies to select between active spaces. We also showcase an ability to "down-sample" the DVS-APC wave functions using natural orbital occupancies to achieve smaller minimal active spaces which retain the accuracy of the larger starting active spaces. Finally, DVS-APC and tPBE0 are proven to be effective when applied to modeling excited states in two larger transition metal complexes, suggesting that the transition metal diatomics may be a particular outstanding challenge for DVS-APC and MC-PDFT approaches.

In this work, we describe the development of a new algorithm for the computation of Coulomb-type matrices using the well-known resolution of the identity (RI) or density fitting (DF) approximation. The method is linear-scaling with respect to system size and computationally highly efficient. For small molecules, it performs almost as well as the Split-RI-J algorithm (which might be the most efficient RI-J implementation to date), while outperforming it for larger systems with about 300 or more atoms. The method achieves linear scaling through multipole approximations and a hierarchical treatment of multipoles. However, unlike in the fast multipole method (FMM), the algorithm does not use a hierarchical boxing algorithm. Rather, close-lying objects like auxiliary basis shells and basis set shell pairs are grouped together in spheres that enclose the set of objects completely, which includes a new definition of the shell-pair extent that defines a real-space radius outside of which a given shell pair can be safely assumed to be negligible. We refer to these spheres as "bubbles" and therefore refer to the algorithm as the "Bubblepole" (BUPO) algorithm, with the acronym being RI-BUPO-J. The bubbles are constructed in a way to contain a nearly constant number of objects such that a very even workload arises. The hierarchical bubble structure adapts itself to the molecular topology and geometry. For any target object (shell pair or auxiliary shell), one might envision that the bubbles "carve" out what might be referred to as a "far-field surface". Using the default settings determined in this work, we demonstrate that the algorithm reaches submicro-Eh and even nano-Eh accuracy in the total Coulomb energy for systems as large as 700 atoms and 7000 basis functions. The largest calculations performed (the crambin protein solvated by 500 explicit water molecules in a triple-ζ basis) featured more than 2000 atoms and more than 33,000 basis functions.

DFT calculations were done to investigate the kinetic mechanism of benzaldehyde transfer hydrogenation using [Cp*IrCl₂]₂ complexes in isopropyl alcohol in the presence of potassium tert-butoxide. Predicted energy barriers provide evidence that the inner-sphere (IS) mechanism (effective barrier 53.0 kJ/mol) is favored over the outer-sphere (OS) and Meerwein-Pondorf-Verley (MPV) mechanisms. Reaction kinetics was studied using both homogeneous and immobilized Cp*Ir complexes as catalysts. A mathematical model was developed to simulate the transfer hydrogenation of benzaldehyde on these catalysts, accounting for possible mass transfer limitations for the immobilized catalyst. A microkinetic model was constructed using both our density functional theory calculations and fitting of the kinetic parameters of catalyst activation and deactivation reactions. The simulation results predict that only about a quarter of Ir immobilized complexes are involved in the reaction, and this is the main reason for the observed higher activity of the homogeneous catalyst. The activity of the immobilized catalyst was found to be related to the hydride species concentration, which is a function of the base concentration. The results suggest that the amount of base has a drastic effect on the immobilized catalyst activity.

The mechanisms of iron-catalyzed [4 + 2] cycloadditions of unactivated dienes were investigated using density functional theory calculations. The calculation results show that the reaction involves sequential key steps of an initial ligand exchange followed by oxidative coupling, isomerization to form a seven-membered ferracycle intermediate, and C-C reductive elimination to form the cyclohexene product. The C-C reductive elimination step is shown to be the rate-determining step of the catalytic cycle. Moreover, energy profiles with three possible spin states (S_Fe = 0, 1, 2) have been considered. The results show that spin crossing occurs mainly through quintet intermediates and triplet transition states, which indicates that the reaction has a two-state reactivity. In addition, the origins of the chemical selectivities and enantioselectivities are analyzed in detail. It was found that the spatial effect between the catalyst ligand and the substrate leads to high [4 + 2] chemoselectivity, while the stabilizing attractive interaction between the ligand and the substrate leads to high enantioselectivity.

Dissociative photoionization of SF₆ in the photon energy range of 15.00-16.50 eV has been investigated using threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging. Both the kinetic energy release distribution (KERD) and the angular distribution of the unique fragment ion, SF₅⁺, resulting from dissociation from the SF₆⁺(X²T_1g) ions, were obtained from the TPEPICO time-sliced images. The F-loss potential energy curve and ab initio classical trajectory calculations not only unravel its dissociation mechanism but also declare that the ν₆⁺ deformation vibration of the SF₅⁺(D_3h, X¹A₁) fragment is predominantly excited. By fitting the total KERD curves derived from the images, we identified the fragment energy distributions. Surprisingly, the average total kinetic energy released in dissociation remains nearly constant within the range of the X²T_1g state. To explain this unusual behavior in such a fast bond-cleavage process, an intramolecular vibrational energy redistribution mechanism is proposed. This mechanism accounts for the rapid energy transfer among vibrational modes prior to complete dissociation. In addition, an adiabatic appearance potential of AP₀(SF₅⁺/SF₆) is accurately determined to be 14.145 ± 0.01 eV, which is in excellent agreement with the high-accuracy ab initio calculation results.