The Journal of Physical Chemistry A最新文献

The polarization behavior analysis within dielectric materials is crucial for electronics. Here, we reconsidered the Kelvin probe (KP) technique, a widely used method for determining the work function and surface potential of solid materials, for assessing the polarization properties of deformable materials. Unlike impedance spectroscopy (IS), the KP technique measures displacement current by modulating the electrode spacing, rather than electrode potential. By phase-separating KP signal into displacement current and its delayed component (actual current), the KP method is expected to selectively measure polarization properties within the bulk, as the potential drop in the bulk and interface remains constant. We achieved precise phase separation of the KP signal using an optical lever signal synchronized with the electrode vibration as the reference for the lock-in amplifier. The complex dielectric constants ε_r, KP^* and ε_r, IS^* of liquid samples were measured by KP and IS measurement, respectively. For nonpolar octane, ε_r, KP^* was almost equal to ε_r, IS^*. Alternatively, for polar 1-octanol and 2-octanol, ε_r, KP^* was smaller than ε_r, IS^*. We also estimated that the bulk potential drop in 1-octanol and 2-octanol is approximately one-tenth of the total potential drop. The proposed approach offers a novel method for evaluating energy diagrams and provides insights into the polarization mechanisms of deformable materials.

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.

Two-dimensional electronic spectroscopy (2DES) is one of the premier tools for investigating photoinduced condensed phase dynamics, combining high temporal and spectral resolution to probe ultrafast phenomena. We have coupled an ultrabroadband laser source generated with a hollow-core fiber, compressing pulses to have a pulse duration of 8 fs, with a boxcars 2DES interferometer constructed from only conventional optics. The resulting ultrabroad bandwidth and high temporal resolution allow for superior spectral coverage of the typically broad molecular line shapes in the near-IR/visible region in room temperature solutions, and the exploration of the excited state dynamics at the earliest time epoch in complex systems. The new spectrometer is characterized by examining the dynamics of the dye molecule Rhodamine 700 in methanol solution. These data exhibit rich vibrational wavepacket dynamics, with 2DES data unraveling key molecular vibronic couplings between multiple vibrational modes. For the first time in a degenerate broadband 2DES experiment, we demonstrate the implementation of full-wavelength reference detection to correct wavelength-dependent laser intensity fluctuations. The net result is a 4-5× increased signal-to-noise (S/N) ratio compared to data acquired without reference detection, yielding a typical S/N ratio = 28. The increased S/N ratio facilitates more rapid data acquisition and examination of samples at lower optical densities, and thus concentrations, than typically used in 2DES experiments. These advances will help to alleviate the typical high demands on precious samples in 2DES measurements.

Chiral tricyclic 6,5,5-fused rings exhibit structural diversity and possess important biological activities in the synthesis of natural products. However, predicting the possible mechanism and origin of stereoselectivity in these reactions remains a challenge. In this article, we conducted a theoretical investigation into the NHC-catalyzed intramolecular [3 + 2] annulations of ynals to generate tricyclic 6,5,5-fused rings. Our calculations revealed that NHC could nucleophilically attack the carbonyl group of the ynal reactant, leading to the formation of a Breslow intermediate via a 1,2-proton transfer. Subsequently, an intramolecular Michael addition takes place, resulting in a 6-5 bicyclic intermediate. We then compared the competitive processes involving proton transfer and the Mannich reaction. The more energetically favorable process involves an HOAc-assisted proton transfer process, followed by the Mannich reaction. To ascertain the origin of the diastereoselectivity, we performed noncovalent interaction (NCI) and atom-in-molecule (AIM) analyses. This work is useful for understanding the general principles and detailed mechanisms of the synthesis of chiral 6,5,5-fused tricyclic scaffolds with unique diastereoselectivity.

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.