The Journal of Physical Chemistry A最新文献

Photoinduced excited-state processes have been platforms for understanding the molecular mechanisms of many chemical and biological reactions. To elucidate associated chemical kinetics, time-resolved spectroscopic experiments have been performed tracking how the populations of reactants and products change during the reactions while reaction conditions such as the concentration of a reactant, temperature, and solvent properties change. Here, we simulate the lifetimes of a reactant and a product, and construct their lifetime-associated spectra with the various combinations of rate constants based on the analytical solutions of differential rate equations. Depending on the combinations of the rate constants, the results diverge, which has often been overlooked in previous works. To demonstrate the validity of our approach, the results are compared with the experimental results on diffusion-controlled excited-state proton transfer. The presented global analysis simulation can generally be applied to other excited two-state reactions.

An automatic code generated C++/HIP/CUDA implementation of the (auxiliary) Fock, or Kohn-Sham, matrix construction for execution in GPU-accelerated hardware environments is presented. The module is developed as part of the quantum chemistry software package VeloxChem, employing localized Gaussian atomic orbitals. The performance and scaling characteristics are analyzed in view of the specific requirements for self-consistent field optimization and response theory calculations. As an example, the electronic circular dichroism spectrum of a G-quadruplex is calculated at the level of time-dependent density functional theory in conjunction with the range-separated CAM-B3LYP exchange-correlation functional. Computational issues due to the high-density of states following the adoption of large-scale model systems are here bypassed with use of the complex polarization propagator approach. The origin of the negative Cotton effect in the long-wavelength onset of the experimental spectrum is elucidated in large-scale modeling and shown to be associated with the TTA nucleobase linkers in the G-quadruplex.

Diaryl thieno-[3,4-b]thiophenes (TT) are photoswitchable compounds that operate through reversible photoinduced cyclization/cycloreversion and have been designed specifically for integration within π-conjugated polymers to switchably manipulate polymer electronic properties. Here we report on how cross conjugating the central TT moiety impacts photocyclization dynamics as interrogated using transient absorption spectroscopy (TAS) for a series of switches built with electron-rich substituents that have various electronic interaction strengths with the TT core. For cross-conjugated structures exhibiting a propensity to switch in steady-state photoconversion experiments, ultrafast TAS reveals signatures of rapid dynamics (occurring within <1-10 ps) similar to those observed for unsubstituted switches and that are consistent with photocyclization. In contrast, TAS reveals comparatively slower spectral dynamics (∼100 ps) that are not consistent with cyclization for switches that are cross-conjugated with substituents that have greater electronic interaction with the TT core and that exhibit no propensity to photoswitch in photoconversion experiments. Microsecond TAS confirms that photoinduced cyclization occurs for the former and that a metastable triplet state localized on the conjugated backbone is generated with the latter. We find that the balance of these two deactivation pathways is sensitive to the interaction strength of the conjugated substituents with the core, with select structures exhibiting signatures of both. These findings are consistent with prior work demonstrating that the LUMO character is delocalized over the switch backbone when there are strong interactions with cross-conjugating groups and reveal that the competition between deactivation pathways can be controlled structurally by weakening π conjugation across the backbone.

The computation of magnetizability tensors using gauge-including atomic orbitals is discussed in the context of Cholesky decomposition (CD) for the two-electron repulsion integrals with a focus on the involved doubly differentiated integrals. Three schemes for their handling are suggested: the first exploits the density fitting (DF) aspect of Cholesky decomposition, the second uses expressions obtained by differentiating the CD expression for the unperturbed two-electron integrals, while the third addresses the issue that the first two schemes are not able to represent the doubly differentiated integrals with arbitrary accuracy. This scheme uses a separate Cholesky decomposition for the cross terms in the doubly differentiated two-electron integrals. Test calculations reveal that all three schemes are able to represent the integrals with similar accuracy and yield indistinguishable results for the values of the computed magnetizability tensor elements. Thus, we recommend our first scheme which has the lowest computational cost for routine computations. The applicability of our CD schemes is further shown in large-scale Hartree-Fock calculations of the magnetizability tensor of coronene (C₂₄H₁₂) with a doubly polarized triple-ζ basis consisting of 684 basis functions.

Radical-radical reaction channels are important in the pyrolysis and oxidation chemistry of perfluoroalkyl substances (PFAS). In particular, unimolecular dissociation reactions within unbranched n-perfluoroalkyl chains, and their corresponding reverse barrierless association reactions, are expected to be significant contributors to the gas-phase thermal decomposition of families of species such as perfluorinated carboxylic acids and perfluorinated sulfonic acids. Unfortunately, experimental data for these reactions are scarce and uncertain. Furthermore, obtaining reliable theoretical predictions for such reactions is a laborious and computationally intensive task. In this work, the chemical kinetics of the various association/decomposition reactions producing/decomposing the C₂-C₄ series of unbranched n-perfluoroalkanes (C₂F₆, C₃F₈, and C₄F₁₀) are examined using state-of-the-art ab initio transition-state-theory-based master-equation calculations. The variable-reaction-coordinate transition-state theory (VRC-TST) formalism is employed in computing the microcanonical and canonical rates for the association reactions. Reaction thermochemistry is obtained via composite quantum chemistry calculations and the laddering of error-canceling reaction schemes via a connectivity-based hierarchy approach employing ANL1/ANL0-style reference energies. Lennard-Jones collision model parameters for the considered systems were estimated by a direct dynamics approach, and collisional energy transfer parameters were obtained from analogies to systems of similar size and heavy-atom connectivity. A one-dimensional master equation approach was used to convert the microcanonical rate coefficients from the VRC-TST analysis into temperature- and pressure-dependent rate constants for the association reactions and the reverse dissociation reactions. The data are reported in standardized formats for usage in comprehensive chemical kinetic models for PFAS thermal destruction.

In this study, low-temperature EPR spectroscopy and quantum-chemical techniques were employed to investigate multispin systems─1,5-diphenyl-3-(3-nitrenophenyl)-6-oxoverdazyl and 1,5-diphenyl-3-(4-nitrenophenyl)-6-oxoverdazyl─that contain a nitrene center at either a meta- or para-position, respectively. Ground states and magnetic zero-field splitting (ZFS) parameters of these multispin systems were determined by experimental and computational methods. The results indicated that the high-spin quartet state is a ground state, and the quartet-doublet energy gap is close to 10 kcal/mol for the para-position of the nitrene group, with ZFS parameters D = 0.292 cm^-1 and E/D = 0.002 cm^-1. In contrast, for the meta-position, the low-spin doublet state is favored with an energy gap of 1 kcal/mol. The observed difference in the ground states could be qualitatively explained by an analysis of the spin density distribution and by delocalization of the unpaired π-electron of the nitrene center. Overall, this study provides valuable insights into the electronic structures and magnetic parameters of such multispin systems.

One of the most important areas of application for equation-of-motion coupled-cluster (EOM-CC) theory is the prediction, simulation, and analysis of various types of electronic spectra. In this work, the EOM-CC method for ionized states, known as EOM-IP-CC, is applied to the closely lying and coupled pair of states of the ozone cation─X̃²A₁ and òB₂─using highly accurate treatments including up to the full single, double, triple, and quadruple excitations (EOM-IP-CCSDTQ). Combined with a venerable and powerful method for calculating vibronic spectra from the Hamiltonian produced by EOM-IP-CC calculations, the simulations yield a spectrum that is in good agreement with the photoelectron spectrum of ozone. Importantly, the calculations suggest that the adiabatic gap separating these two electronic states is somewhat smaller than currently thought; an assignment of the simulated spectrum together with more precise band positions of the experimental measurements suggests T₀₀ = 1,368 ± 65 cm^-1.

Photoelectron imaging of the doubly deprotonated ethylenediaminetetraacetic acid dianion (EDTA^2-) at variable wavelengths indicates two electron loss pathways: direct detachment and thermionic emission from monoanions. The structure of EDTA^2- is also investigated by electronic structure calculations, which indicate that EDTA^2- has two intramolecular hydrogen bonds linking a carboxylate and carboxylic acid group at either end of the molecular backbone. The direct detachment feature in the photoelectron spectrum is very broad and provides evidence for a dissociative photodetachment, where decarboxylation occurs rapidly after electron loss. Near 0 eV kinetic energy electrons are only observed in the photoelectron spectrum of EDTA^2- at hν = 3.49 eV (high laser fluence), providing evidence for secondary electron loss via a two-photon process, mediated by an excited state of the decarboxylated anion, and likely resulting in a cyclic neutral product.

Helical π-conjugated polymers are promising as optically active functional materials. The present paper reports the synthesis of novel bipyridine-containing π-conjugated polymers with optically active amino-alcohol-derived side chains, examination of their higher-order structures, chiral recognition, and metal coordination properties. The polymers adopt a folded helical conformation and aggregate in CHCl₃/MeOH depending on the solvent composition, as supported by density functional theory calculations and molecular dynamics simulations. The polymer films showed differences in contact angles with aqueous solutions of (R)- and (S)-alcohols. Addition of some metal chlorides and perchlorates changed the intensity and color of the photoluminescence of the polymers.

The packing fashion of an organic molecule in the crystal plays a critical role in the global nonlinear optical (NLO) responses under ambient conditions. To better understand how the crystal packing affects the first hyperpolarizability (β) and achieve efficient NLO material, herein, the three positional isomers (regioisomers) through changing the substituted position of 3-carbazole-pyrazine-based isomers were performed. The phenyl groups with different positions (ortho-, meta-, and para-) of pyrazine, named 23-B3C, 25-B3C, and 26-B3C, are theoretically studied in gas, solvent, and solid states by using the polarizable continuum model and the combined quantum mechanics and molecular mechanics method, respectively. These two regioisomers (23-B3C and 25-B3C) exhibit totally different aggregation behaviors. Through density functional theory (DFT) and time-dependent DFT (TDDFT) calculations, it has been concluded that the geometric changes of 23-B3C and 25-B3C are mainly contributed by the bond length and dihedral angle from gas to the solid phase. Herein, a lower HOMO-LUMO energy gap and a better intramolecular charge-transfer absorption are found for 25-B3C owing to a more sizable π-conjugation effect with smaller bond length alternation. Importantly, the β value of 25-B3C in the crystal with the π-π stacking with the same CT direction between the whole backbones (H-aggregate) and the end-groups (J-aggregate) commonly can substantially increase compared to that of the monomer. However, the obtained reduction of β value for 23-B3C is mainly because of V-aggregate (α > 90°) and reverse J-aggregate in crystal. Thus, our work showcases a profound understanding of the relationship between solid-state packing and macroscopic second-order NLO properties and provides a feasible molecule strategy for the development of high-efficiency NLO materials.