Journal of Chemical Physics最新文献

In the present work, we elucidate the inherent loss of net magnetization (⟨Iz⟩) in parahydrogen-induced polarization (PHIP) experiments with magnetic field cycling (MFC) for spin systems containing magnetically equivalent protons. The effects are shown for propane and diethyl ether as representative examples of potential hyperpolarized MRI contrast agents, but the findings of this work are equally applicable to other multispin systems in the liquid or gas phase. These results are relevant to both adiabatic longitudinal transport after dissociation engenders net alignment (ALTADENA) experiments (where 1H nuclei are polarized) and MFC protocols used to transfer parahydrogen spin order to a heteronucleus such as 13C. The investigated effects should be incorporated for a correct evaluation of both the maximum possible NMR signal enhancement and the pairwise selectivity, which are useful in the context of mechanistic studies in the field of catalytic hydrogenation. Among signal enhancement damping factors in ALTADENA, such as T1 relaxation and insufficient adiabaticity of a field sweep, the inherent loss of net magnetization in spin systems containing magnetically equivalent protons (especially in PHIP systems commonly used for mechanistic studies such as propene or propane) has not been thoroughly considered and needs to be clarified. The maximum possible net magnetization in ALTADENA for diethyl ether and propane was shown to be ∑|⟨Iiz⟩| ≈ 0.56 for diethyl ether and ∑|⟨Iiz⟩| ≈ 0.45 for propane, respectively. The inherent loss of net heteronuclear magnetization of the same order of magnitude with an increase in the number of magnetically equivalent protons was also demonstrated for AmMnX-type spin systems.

We investigate the performance of coupled-trajectory methods for nonadiabatic molecular dynamics in simulating the photodynamics of 4-(dimethylamino)benzonitrile (DMABN) and fulvene, with electronic structure provided by linear vibrational coupling models. We focus on the coupled-trajectory mixed quantum-classical (CTMQC) algorithm and on the (combined) coupled-trajectory Tully surface hopping [(C)CTTSH] in comparison to independent-trajectory approaches, such as multi-trajectory Ehrenfest and Tully surface hopping. Our analysis includes not only electronic populations but also additional electronic and nuclear properties in position and momentum space. For both DMABN and fulvene, the recently developed CCTTSH algorithm successfully resolves the internal inconsistencies of coupled-trajectory Tully surface hopping. Instead, we find that DMABN highlights a significant weakness of CTMQC, which arises when the trajectories remain for a long time in the vicinity of a region of strong nonadiabaticity.

Donor-acceptor (D-A) molecules are key motifs in electron transfer processes. Recently, significant progress has been made in the development of organic synthetic reactions that utilize D-A molecules as photoredox catalysts. In these electron-transfer reactions, preventing undesired back-electron transfer and achieving efficient conversion is essential. In this Perspective, we introduce two examples in which the dynamic effects of excitons derived from catalyst molecules are controlled through precise molecular design.

The simulation of excited states at low computational cost remains an open challenge for electronic structure (ES) methods. While much attention has been given to orthogonal ES methods, relatively little work has been done to develop nonorthogonal ES methods for excited states, particularly those involving nonorthogonal orbital optimization. We present here a numerically stable formulation of the Resonating Hartree-Fock (ResHF) method that uses the matrix adjugate to remove numerical instabilities arising from nearly orthogonal orbitals, and as a result, we demonstrate improvements to ResHF wavefunction optimization. We then benchmark the performance of ResHF against complete active space self-consistent field in the avoided crossing of LiF, the torsional rotation of ethene, and the singlet-triplet energy gaps of a selection of small molecules. ResHF is a promising excited state method because it incorporates the orbital relaxation of state-specific methods, while retaining the correct state crossings of state-averaged approaches. Our open-source ResHF implementation, yucca, is available on GitLab.

The presence of spin and spatial symmetry breaking upon variational optimization of mean-field wavefunctions is known to be an indicator of nondynamical electron correlation. However, a single mean-field wavefunction may not have sufficient flexibility to flag the correlated orbital space where there are multiple correlation mechanisms present. In such situations, there are multiple nearly degenerate self-consistent field solutions that describe different correlation mechanisms, but it is often not possible to know a priori when such situations will occur or if sufficient solutions have been obtained. In this work, we examine the role of spin and spatial symmetries of nonorthogonal multiconfigurational self-consistent field (NOMCSCF) calculations in revealing correlation mechanisms. We provide details of the theory for optimization of NOMCSCF wavefunctions with desired symmetries, establish which types of symmetries recover the most correlation energy when the symmetry constraints are relaxed, and discuss how the different-orbitals for different-configuration wavefunctions reveal the different correlation mechanisms present.

Histone modifications play a crucial role in regulating chromatin architecture and gene expression. Here we develop a multiscale model for incorporating methylation in our nucleosome-resolution physics-based chromatin model to investigate the mechanisms by which H3K9 and H3K27 trimethylation (H3K9me3 and H3K27me3) influence chromatin structure and gene regulation. We apply three types of energy terms for this purpose: short-range potentials are derived from all-atom molecular dynamics simulations of wildtype and methylated chromatosomes, which revealed subtle local changes; medium-range potentials are derived by incorporating contacts between HP1 and nucleosomes modified by H3K9me3, to incorporate experimental results of enhanced contacts for short chromatin fibers (12 nucleosomes); for long-range interactions we identify H3K9me3- and H3K27me3-associated contacts based on Hi-C maps with a machine learning approach. These combined multiscale effects can model methylation as a first approximation in our mesoscale chromatin model, and applications to gene systems offer new insights into the epigenetic regulation of genomes mediated by H3K9me3 and H3K27me3.

The ability to control chemical reactions by coupling organic molecules to confined light in a cavity has recently attracted much attention. While most previous studies have focused on single-mode photonic or plasmonic cavities, here we investigate the effect of hybrid metallodielectric cavities on photoisomerization reactions. Hybrid cavities, which support both photonic and plasmonic modes, offer unique opportunities that arise from the interplay between these two distinct types of modes. In particular, we demonstrate that interference in the spectral density due to a narrow photonic mode and a broad plasmonic mode that are coupled to each other enables hybrid cavities to provide an energy-selective Purcell effect. This effect enhances electronic relaxation only to the desired molecular geometry, providing the ability to increase the yield of photoisomerization reactions. As a test case, we study the asymmetric proton transfer reaction in the electronically excited state of 3-aminoacrolein. Our results, which are robust for a range of realistic cavity parameters, highlight the advantages of hybrid cavities in cavity-induced photochemical processes.

We introduce a novel signal enhancement technique, termed steadyDFS, for quadrupolar solid-state nuclear magnetic resonance spectroscopy. It can substantially increase the performance of double frequency sweeps (DFSs) for all half-integer quadrupolar spins (I = 3/2, 5/2, 7/2, and 9/2). In steadyDFS, the DFS and readout pulse are repeated multiple times with a repetition time of TR,DFS to generate a steady state that provides substantial sensitivity enhancement. Using a series of simulations, we show that steadyDFS can outperform conventional DFS methods, and enhancements per unit time of ∼5 to 21 can be achieved depending on the value of I. The sensitivity of steadyDFS is robust toward changes in repetition times and quadrupolar relaxation rates of the system. Moreover, steadyDFS is highly modular and can be combined with quadrupolar Carr-Purcell-Meiboom-Gill (QCPMG) detection. Using 39K (I = 3/2), 17O (I = 5/2), and 49Ti (I = 7/2) as representative challenging nuclei, we show that enhancements up to 46× can be realized experimentally via steadyDFS-QCPMG, translating to a 20× enhancement per unit time. We applied steadyDFS-QCPMG to protonated and deprotonated samples, as well as to samples with a diverse range of transverse relaxation times (i.e., T2/T2*), where steadyDFS-QCPMG can provide an enhancement per unit time of at least 7. For samples that are not amendable to QCPMG, we explored the use of steady-state free precession (SSFP). Although SSFP is fundamentally incompatible with steadyDFS, we show that beneficial results can be obtained when DFSs are combined with SSFP in an interruptive manner.

Molecular simulations are important tools for predicting the thermophysical properties of liquids, and a rigorous validation of the model potentials is crucial to ensure their reliability for new applications. In the existing literature on empirical force fields, there is an obvious lack of data for shear and bulk viscosity. While experimental or model values for shear viscosity are widely available and represent an excellent benchmark, bulk viscosity is more challenging to measure, and experimental values are available for only a handful of liquids. Here, we present an analysis of both shear and bulk viscosity, calculated from molecular dynamics simulations via the Green-Kubo relations, for over 140 small molecular Newtonian liquids from the Virtual Chemistry database. Therefore, we provide a comprehensive reference for these transport properties for the popular optimized potential for liquid simulations (OPLS) force field and the generalized Amber force field (GAFF).

Random ternary polymerization is a strategy for tuning the energy levels and improving the batch-to-batch reproducibility of polymer semiconductors for the application in polymer solar cells (PSCs). However, the influence of third component incorporation on exciton properties and charge photogeneration processes in terpolymer-based solar cells is still unclear. In this work, time-resolved spectroscopies were employed to study exciton properties and charge photogeneration processes in PSCs based on a series of terpolymers, PM1 and PM2, which have 20% and 50% of thiophene-thiazolothiazole (TTz) building blocks on the PM6 backbone, respectively. For neat terpolymer films, we found that the small amount (20%) of TTz incorporation in PM6 slightly reduces the exciton diffusion coefficient, but the exciton lifetime is significantly increased, resulting in a significant increase in exciton diffusion length. However, further increasing the TTz component (50%) in the PM6 backbone decreases exciton lifetime, the diffusion coefficient, and consequently exciton diffusion length. We found that a small amount of acceptor (Y6) addition can efficiently dissociate terpolymer excitons due to the weak molecular stacking of terpolymers in the blend films. For terpolymer:Y6-based blend films, we find that the small amount of TTz incorporation (PM1) could reduce the phase size of the donor and suppress bimolecular carrier recombination in blend films. Furthermore, we find that the energy level offset plays a critical role in charge photogeneration processes, and a HOMO energy level offset of 0.06 eV can dissociate acceptor excitons in terpolymer-based organic solar cells effectively.