J Phys Chem A [JOURNAL]
Sun
200 papers
Machine Learning Prediction of Absorption and Emission: A Unified Approach for Metal-Organic and Organic Chromophores.
Ilin EA, Ilina VV, Ioffe IN … +2 more , Medvedev MG, Goryunkov AA
J Phys Chem A · 2026 Jun · PMID 42261895 · Publisher ↗
Rational design of transition metal complexes with desired optical properties is a major challenge due to high computational costs of quantum-chemical methods that can deliver quantitatively reliable results. We present...
Rational design of transition metal complexes with desired optical properties is a major challenge due to high computational costs of quantum-chemical methods that can deliver quantitatively reliable results. We present a machine learning framework for predicting absorption and emission maxima in both transition metal coordination compounds and organic chromophores using joint training on a combined experimental data set. Our featurization strategy integrates ligand environment fingerprints (Morgan), metal center features (Coulomb matrices), and topological descriptors from persistent homology analysis. The combined training data set comprises 19,733 absorption and 2675 emission measurements for 17,359 metal complexes (with focus on Ir, Rh, Pt, and Ru systems) and 17,294 absorption and 18,141 emission measurements for 7065 organic molecules across 365 solvents. Among several architectures evaluated, multilayer perceptrons provide the best absorption predictions (RMSE = 33.5 nm, = 0.83, Pearson = 0.92 for metal-organic compounds), while gated recurrent units are optimal for emission (RMSE = 41.7 nm, = 0.83, Pearson = 0.90). Models trained jointly on both data sets show good universal applicability with moderate accuracy trade-offs: RMSE increases by approximately 7-19 nm for organic compounds compared to specialized models, and for metal-organic compounds, RMSE increases by 1-2 nm. In contrast, models trained on organic data alone fail catastrophically when applied to metal complexes ( = 0.01). For a test set of 35 metal complexes including metal centers beyond the main training distribution (V, W, Cu, and Os in addition to Ir, Rh, Pt, and Ru), our best models achieve an RMSE of ∼28 nm for absorption maxima, comparable to TDDFT-O3LYP predictions but at substantially lower computational costs. SHAP analysis reveals that Coulomb matrix descriptors are most important for metal complex predictions, while Morgan fingerprints prevail for purely organic compounds. The presented approach enables efficient screening of candidate compounds for various photophysical applications orders of magnitude faster than TDDFT calculations.
Symmetry-Match Fraction for Active-Space Selection: A Simple Route to Accurate Reaction Energies in VQE/UCCSD.
Sarkar M, Roy L, Gutal AP … +2 more , Kumar A, Paranjothy M
J Phys Chem A · 2026 Jun · PMID 42261107 · Publisher ↗
Simulation of chemical reactions to compute reaction energies using variational algorithms remains challenging in achieving chemical accuracy relative to benchmark computational chemistry methods due to limitations such...
Simulation of chemical reactions to compute reaction energies using variational algorithms remains challenging in achieving chemical accuracy relative to benchmark computational chemistry methods due to limitations such as qubit number, circuit depth, and noise. To address this issue, we propose the definition of different active spaces for studying chemical reactions, incorporating irreducible representations of both ground and excited states by defining the maximum contribution of excitation terms in the ansatz, implemented here within a second-order Trotterized UCCSD-VQE framework. Our results demonstrate that this approach can achieve chemical accuracy for several representative reactions and yields close agreement with benchmark methods across a broader range of reactions. For all reactions studied, the difference in reaction energies between VQE and CCSD remains within 1 kcal/mol. At the same time, comparison with FCI shows chemical accuracy for several cases and close agreement for the remaining systems. Furthermore, our analysis simplifies the selection of active spaces and electrons for each reaction, reducing it to a single optimal combination that supports chemically accurate or near-chemical-accuracy predictions.
Photoelectron Imaging Spectroscopy and Density Functional Theory Calculations of Al-Doped Silver Cluster Anions: AgAl ( = 4-17).
Nomi R, Suzuki Y, Ushiki Y … +2 more , Terasaki A, Horio T
J Phys Chem A · 2026 Jun · PMID 42260984 · Publisher ↗
We report a joint study of photoelectron imaging spectroscopy and density functional theory (DFT) calculation for aluminum-doped silver cluster anions, AgAl ( = 4-17). To explore their electronic and geometric structures...
We report a joint study of photoelectron imaging spectroscopy and density functional theory (DFT) calculation for aluminum-doped silver cluster anions, AgAl ( = 4-17). To explore their electronic and geometric structures, we measured size-dependent photoelectron spectra at a photon energy of 3.87 eV and compared them with simulated spectra for DFT-optimized structures. It is found that the Al atom is exposed on the cluster surface at 4 ≤ ≤ 15, whereas it is fully encapsulated at = 16 and 17; a highly symmetric framework similar to the Frank-Kasper-type tetrahedron is obtained as the most stable structure for = 16. For = 15, DFT calculations suggest the coexistence of endohedral forms; however, no clear experimental signatures supporting this prediction were obtained. Also found is that the vertical detachment energies of AgAl with = odd are lower than those for the neighboring sizes. Since the total number of the valence electrons of AgAl is + 4, this observation indicates that the outermost orbitals of the odd-numbered clusters are singly occupied. The present DFT calculation confirms that their spin multiplicities are indeed doublet, whereas those of the even-numbered clusters are singlet. Photoelectron angular distribution from the outermost orbital for = 15 exhibits a large positive anisotropy with respect to the laser polarization, which manifests a 1S1P1D2S electronic configuration as observed in valence isoelectronic systems of Ag and AgSc. While all sizes exhibit electronic shell structures similar to those observed in undoped silver clusters, the energy ordering of the discrete levels in the endohedrally doped clusters, Al@Ag ( = 16 and 17), is not as predicted by the spherical jellium model; 2S-like orbitals are located in the manifold of 1D-like orbitals.
Structure and Spectroscopy of Free Base, Copper, and Zinc Tetrapentylporphyrin.
Muldowney BE, Ajayi ND, Chen WY … +3 more , Geier GR, Ziegler CJ, Nemykin VN
J Phys Chem A · 2026 Jun · PMID 42260700 · Full text
Synthetic porphyrins have been crucial as models for heme units and other biological porphyrins, as well as components of advanced materials and catalysts. Much of the work on synthetic porphyrins has focused on the meso...
Synthetic porphyrins have been crucial as models for heme units and other biological porphyrins, as well as components of advanced materials and catalysts. Much of the work on synthetic porphyrins has focused on the meso-substituted 5,10,15,20-tetraphenylporphyrin (TPP) or the pyrrole-substituted 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP). In this report, we present a comprehensive spectroscopic, electrochemical, and computational study of free-base, zinc(II), and copper(II) 5,10,15,20-tetrapentylporphyrin (TPeP). TPeP can be prepared via a two-step, one-flask reaction of pyrrole and hexanal mediated by Montmorillonite K10, followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in yields of around 40%. Similar to TPP, the TPeP systems have the highest occupied molecular orbital (HOMO) with symmetry and are, in general, easier to oxidize than their corresponding TPP or OEP analogues.
A Fresh Look at Signatures of -Wave Scattering: Symmetry and the Breakdown of the Born-Oppenheimer Approximation.
Moorby RE, Mukherjee M, Utku C … +2 more , Jagau TC, Krylov AI
J Phys Chem A · 2026 Jun · PMID 42257556 · Publisher ↗
The peculiar behavior of σ-type anionic states is analyzed in terms of the interaction between bound and continuum states. Within a simple Landau-Zener model, strong diabatic coupling leads to a broad avoided crossing an...
The peculiar behavior of σ-type anionic states is analyzed in terms of the interaction between bound and continuum states. Within a simple Landau-Zener model, strong diabatic coupling leads to a broad avoided crossing and strongly mixed adiabatic states, whereas weak diabatic coupling leads to a narrow avoided crossing and an abrupt change in the character of the adiabatic states. By using symmetry analysis and calculations, we show that σ-type anionic states such as those in HCl, methyl chloride, and pyrrole interact more strongly with the continuum than π*-type anionic states such as the N resonance. Strong interaction leads to the breakdown of the Born-Oppenheimer approximation, which results in a finite probability to decay into the detachment continuum even in regions where the anionic state is energetically below the neutral state. We show that calculations with a complex absorbing potential capture this special character of σ-type anions through removal of the detachment continuum, affording diabatization of the anionic state while describing the decay into the continuum via the imaginary energy.
Density Functional Theory Study on the Isomerism in Porphyrins: Structural, Tautomerism and Atropisomerism.
Urrutia-Fernández K, Jaque P
J Phys Chem A · 2026 Jun · PMID 42257478 · Publisher ↗
Density functional theory (DFT) and time-dependent DFT (TD-DFT) methods were systematically assessed for their ability to describe multiple forms of isomerism in porphyrin systems, including structural isomerism, N-H tau...
Density functional theory (DFT) and time-dependent DFT (TD-DFT) methods were systematically assessed for their ability to describe multiple forms of isomerism in porphyrin systems, including structural isomerism, N-H tautomerism, and atropisomerism, as well as their impact on electronic and optical properties. The structural isomers porphine (-) and porphycene (-) were examined using eight exchange-correlation functionals spanning Jacob's ladder, revealing that nonlocal hybrid and range-separated functionals provide the most accurate geometries and stability trends. Relative stability analyses, supported by conceptual DFT descriptors, consistently identify - as the hardest and least polarizable isomer. Calculated N-H tautomerization barriers for - agree well with experimental NMR data, particularly with the ωB97XD functional, while - exhibits a low-barrier - pathway consistent with rapid proton dynamics and a second, energetically inaccessible pathway. The challenging equilibrium among atropisomers of 5,10,15,20-tetra(-hydroxyphenyl)porphyrin is shown to involve energy differences below 1.0 kcal/mol, with dispersion-corrected and range-separated hybrid functionals best reproducing experimental trends in solution. TD-DFT calculations of substituted porphyrins demonstrate that ωB97XD offers, again, the most balanced description of and bands, enabling clear structure-property relationships between substitution patterns, oscillator strengths, and singlet oxygen generation. Overall, this work establishes reliable DFT protocols for describing complex isomerism and photophysical behavior in porphyrin-based organic systems, guidance for the rational design of functional porphyrinoids.
Atmospheric Oxidation of C-C -Alkylcyclohexanes Initiated by Cl Atoms and OH Radicals: Mechanistic Insights, Kinetics, and the Inhibiting Effect of Water.
Zhao H, Liu F, Tang Y … +1 more , Pan Y
J Phys Chem A · 2026 Jun · PMID 42257442 · Publisher ↗
Long-chain alkanes are key intermediate volatility organic compounds (IVOCs) in the atmosphere and recognized as significant precursors to the formation of secondary organic aerosol (SOA). C-C -alkylcyclohexanes are repr...
Long-chain alkanes are key intermediate volatility organic compounds (IVOCs) in the atmosphere and recognized as significant precursors to the formation of secondary organic aerosol (SOA). C-C -alkylcyclohexanes are representative IVOCs that undergo oxidation by both OH radicals and Cl atoms. In this work, the initial hydrogen abstraction and subsequent multigeneration oxidation mechanisms of C-C -alkylcyclohexanes initiated by Cl atoms and OH radicals were systematically investigated using quantum chemical methods. The results show that both Cl atoms and OH radicals preferentially abstract a hydrogen atom from the tertiary carbon of the cyclohexane ring, proceeding via low-energy barriers. The formed alkyl radicals undergo barrierless additions to O subsequently and reactions with NO, forming alkoxy radicals (CHO, CHO, and CHO) that are consistent with experimentally observed products. The calculated rate constants for the reactions with Cl atoms and OH radicals at 298 K are in the ranges of 1.59-2.17 × 10 and 1.55-3.75 × 10 cm molecule s, respectively, showing a slight increasing trend with carbon-chain length. Notably, the presence of a single water molecule was found to significantly increase the energy barriers for H-abstraction, indicating that water vapor exerts a negative catalytic effect on these atmospheric oxidation reactions. This finding suggests that under high-humidity conditions, the gas-phase oxidation of such IVOCs may be slower than currently predicted, with potential implications for SOA formation estimates. By elucidating the mechanistic details and kinetic parameters for this important class of IVOCs, this work provides a theoretical foundation for improving chemical mechanisms in atmospheric models.
A Gauss-Radau-Laguerre Discrete Variable Representation for Use in Continuum Electron Dynamics.
Yip FL, Lucchese RR, McCurdy CW
J Phys Chem A · 2026 Jun · PMID 42252678 · Publisher ↗
We detail an implementation, suitable for calculations on highly correlated ionizing systems, of a modified finite element discrete variable representation (FE-DVR) appended with a Gauss-Radau-Laguerre element. The appen...
We detail an implementation, suitable for calculations on highly correlated ionizing systems, of a modified finite element discrete variable representation (FE-DVR) appended with a Gauss-Radau-Laguerre element. The appended element includes exterior complex scaling (ECS) to impose outgoing wave boundary conditions on treatments of processes involving continuum electrons. In this "infinite range" ECS (irECS), the complications that introduce reflections from the end of the grid when the last ECS finite element has finite range are avoided by the use of the Laguerre-weighted exponentially decaying tails, while outgoing wave boundary conditions are still imposed via the ECS transformation. For highly correlated systems in the absence of strong external fields we find that accurate two-electron integrals are essential in this modified FE-DVR. To accurately compute the two-electron integrals over the entire ECS contour, we present a detailed examination of the implications from the boundary terms that arise in a solution of Poisson's equation with the Radau-Laguerre basis. A boundary term correction is necessary, and when included, the Radau-Laguerre DVR can accurately describe highly correlated states such as the doubly excited states of helium over the entire ECS contour.
Optimizing Reorganization Energies of Small Conjugated Molecules Using an Inverse-Design Approach.
Xu H, Springborg M
J Phys Chem A · 2026 Jun · PMID 42252610 · Publisher ↗
During the past years, organic solar cells (OSCs) have been at the center of a significant scientific interest. However, it is still a big challenge to identify organic molecules with the best performances due to the alm...
During the past years, organic solar cells (OSCs) have been at the center of a significant scientific interest. However, it is still a big challenge to identify organic molecules with the best performances due to the almost inexhaustible chemical space. We recently proposed an inverse-design approach, i.e.. poor man's materials optimization (PooMa), which combines global optimization based on genetic algorithms (GA) and a fast, efficient parametrized density-functional tight-binding (DFTB) method for the calculation of the electronic structure for each individual structure. Here, we applied this method to identify conjugated systems with low reorganization energies for charge-transfer processes. We have considered benzene and pyridine as two different simple test systems and selected 20 different functional groups for the construction of organic molecules. The results showed that our approach (PooMa) is able to construct numerous new structures and successfully identify molecules with good performance at a low computational cost. Furthermore, we present a simple theory that can capture the main results of our study.
POET: A Software Suite for Mapping the Site-Specific Electronic Origins of Magnetic Anisotropy.
Navrátil J, Błoński P
J Phys Chem A · 2026 Jun · PMID 42249855 · Full text
The growing energy demand of global information technologies motivates the development of sustainable materials capable of retaining and processing information at the atomic scale. Resolving the site- and orbital-specifi...
The growing energy demand of global information technologies motivates the development of sustainable materials capable of retaining and processing information at the atomic scale. Resolving the site- and orbital-specific origins of magnetic anisotropy energy (MAE) is key to establishing the physical principles required for the rational design of tailored atomic-scale magnets. However, these contributions remain obscured in the output of noncollinear density functional theory calculations incorporating spin-orbit coupling. We present the Palacký OptoElectronic Toolkit (POET), an open-access suite that decomposes complex simulation data into intuitive graphical representations that map electronic reorganization onto atomic and orbital contributions and elucidates the interplay between bonding and magnetic anisotropy. To demonstrate its utility, we investigate two complementary strategies for achieving electric-field tunable MAE in transition-metal-functionalized graphene on various substrates. We show that iodination of Pt adatoms on nitrogen-decorated single-vacancy graphene on MgO creates strong, field-tunable in-plane anisotropy, while OsPt and OsPd heterodimers yield exceptional perpendicular MAE of ∼150 meV. The microscopic insights enabled by POET facilitate the rational control of magnetic anisotropy through chemical engineering, paving the way for energy-efficient information storage and processing at the atomic limit.
How to Accurately Predict the Fluorescent Property of Multiresonance TADF: Density Functional Theory or Machine Learning?
Zhao Y
J Phys Chem A · 2026 Jun · PMID 42249848 · Publisher ↗
Organic thermally activated delayed fluorescence (TADF) emitters have attracted considerable attention in organic light-emitting diode (OLED) applications, owing to their potential for 100% exciton utilization. Among the...
Organic thermally activated delayed fluorescence (TADF) emitters have attracted considerable attention in organic light-emitting diode (OLED) applications, owing to their potential for 100% exciton utilization. Among them, multiple-resonance (MR) TADF compounds have emerged as a particularly promising class of emitters, offering superior color purity and high photoluminescence quantum yields (PLQYs) compared to conventional donor-acceptor-type TADF materials. With the increasing demand for ultrahigh-definition display technologies, exemplified by the BT.2020 color gamut standard, the development of MR-TADF materials featuring narrowband emission and high efficiency has become a key research priority. In this study, we systematically assessed the predictive capabilities of density functional theory (DFT) and machine learning (ML) methods in determining the luminescence properties of MR-TADF materials. While ML models offer rapid prediction capabilities, their generalization ability remains limited, primarily constrained by the quality and size of the available training data set. In contrast, DFT-based approaches, although more computationally demanding, exhibit reliable accuracy and generalization performance when calibrated using appropriate regression models. Using a B3LYP-based calibration model, we further predicted the optical properties of the designed MR-TADF molecules and identified a promising candidate exhibiting deep-blue emission. Overall, the findings underscore the complementary roles of DFT and ML approaches in MR-TADF research and provide valuable theoretical guidance for the rational design of high-performance MR-TADF emitters tailored to next-generation OLED applications.
Discrimination of Chiral BCPs by Möbius Cyclacene: A DFT Study.
Abbas A, Huzaifa M, Ghayur MN … +3 more , Aman A, Nur-E-Alam M, Ul-Haq Z
J Phys Chem A · 2026 Jun · PMID 42244267 · Publisher ↗
Chiral discrimination plays a pivotal role in the development of enantiomerically pure pharmaceuticals, as stereochemistry directly influences both the pharmacological efficacy and safety profiles. In this study, a twist...
Chiral discrimination plays a pivotal role in the development of enantiomerically pure pharmaceuticals, as stereochemistry directly influences both the pharmacological efficacy and safety profiles. In this study, a twisted nitrogen-substituted cyclacene comprising 15 fused benzene rings was employed as a host framework for the enantiomeric recognition of chiral bicyclo[1.1.1]pentane (BCP) analogues. Unlike conventional diastereomer-based chiral resolution methods, the cyclacene host enables direct enantiomeric discrimination of BCP analogues. The interaction behavior of four R/S enantiomeric pairs of BCP derivatives with the cyclacene host was systematically explored by using density functional theory (DFT) simulations. Key interaction parameters, including binding energies, hydrogen bonding, noncovalent interaction (NCI), and electron density difference (EDD) maps, were examined in detail. The results indicate that the R enantiomers exhibit stronger binding affinities and enhanced chiral selectivity relative to their S counterparts, as evidenced by a greater number of hydrogen bonds and more favorable host-guest interaction distances. Remarkably, BCP3 and BCP4 demonstrated the highest degrees of chiral discrimination, highlighting the nitrogen-substituted twisted cyclacene as a promising candidate for enantioselective separation. Collectively, these findings provide valuable theoretical insights into the rational design and development of advanced nanostructured host systems for selective chiral recognition and separation technologies. Furthermore, this study may contribute to the advancement of next-generation supramolecular materials for pharmaceutical purification, asymmetric sensing, and molecular-scale recognition systems, thereby supporting broader efforts toward safer drug development and more efficient enantiomeric separation methodologies.
Geometry-Modulated Linear and Third-Order Nonlinear Optical Responses in Atomic Clusters.
Zhong Q
J Phys Chem A · 2026 Jun · PMID 42241506 · Publisher ↗
Atomic clusters are promising candidates for nonlinear optical (NLO) materials owing to their diverse geometric configurations and tunable electronic structures. With the development of preparation techniques, various at...
Atomic clusters are promising candidates for nonlinear optical (NLO) materials owing to their diverse geometric configurations and tunable electronic structures. With the development of preparation techniques, various atomic clusters, particularly carbon clusters, have been experimentally synthesized. However, understanding how cluster geometries relate to NLO response remains limited, largely because their geometric and electronic complexity prevents direct transfer of insights from traditional molecular systems. In this work, the linear and third-order NLO responses were investigated using carbon clusters as model systems, revealing that geometric evolution─from cage and planar-porous to cyclic to linear structures─continuously enhances both responses, driven by increased electron delocalization along less confined directions. Consequently, linear structures represent the optimal geometry to maximize these responses in carbon clusters. Additionally, unlike the strong dependence between optical response and the HOMO-LUMO gap that exists in traditional organic systems, predicting the linear and third-order NLO responses in carbon clusters via the HOMO-LUMO gap is infeasible owing to the absence of crucial excited states in generic geometric systems. These insights are further confirmed in multielement boron nitride cluster systems. This study provides deep insights into the linear and third-order NLO behaviors of atomic clusters and a clear design guideline for developing these materials.
Separability of the Coupled-Cluster Excited State Equations: The Case of Excitonic Couplings.
Köhn A
J Phys Chem A · 2026 Jun · PMID 42241504 · Full text
Coupled-cluster theory provides an accurate description of electronic ground and excited states. While it has been rigorously established that the coupled-cluster ground state energy is size extensive and local excitatio...
Coupled-cluster theory provides an accurate description of electronic ground and excited states. While it has been rigorously established that the coupled-cluster ground state energy is size extensive and local excitations are size intensive in the thermodynamic limit, the separability of other properties in coupled-cluster theory is subject to known limitations. In particular, it was shown [Stanton, , 8928-8937] that multicenter two-particle density matrices are not asymptotically separable in general. In the present work, an analogous analysis is applied to the excitonic coupling of local excitations of two separate molecules, using both the equation-of-motion (EOM) and linear-response (LR) formalism. It is shown that for both formalisms the two-electron transition density associated with the excitonic coupling does not exactly separate into a product of local one-electron transition densities. Numerical examples are provided for Ne, (CO), and (CH), using coupled-cluster expansions up to single, double, triple, and quadruple excitations (CCSDTQ). Small but noticeable deviations on the order of 5-10% are reported for approximations like CCSD and its second-order approximate variant CC2. As soon as triple excitations are included, the separability error becomes significantly smaller. Exploratory computations for the coupling of larger chromophores such as perylene or cumarine dyes using the CC2 method indicate that the separability error can grow up to orders of 20%. While these deviations are typically below the error margin of this method, the separability error could be of relevance in the design and benchmarking of local coupled-cluster approaches for excited states.
On The van Vleck Paramagnetic Susceptibility of d() Transition Metal Ions.
González-Barajas BA, Alfonso-Ortíz HL, Pérez-Torres JF
J Phys Chem A · 2026 Jun · PMID 42240555 · Publisher ↗
In this work, we derive a mathematical expression for the van Vleck paramagnetic susceptibility of d ions embedded in octahedral crystal-field environments. The formula is derived within relativistic crystal-field theory...
In this work, we derive a mathematical expression for the van Vleck paramagnetic susceptibility of d ions embedded in octahedral crystal-field environments. The formula is derived within relativistic crystal-field theory and involves two parameters: the energy difference Δ between the ground state Γ(t) and the first excited state Γ(t), and a dimensionless quantity ϵ that incorporates the crystal-field strength , the spin-orbit coupling constant ξ, and the relativistic ratio /. The parameter ϵ also determines the effective Bohr magneton number . Within the strong-field approximation, we show that ξ can be directly estimated from Δ using standard SQUID magnetometry, bypassing the need for specialized optical spectroscopy. While the excellent agreement for ReF demonstrates the intrinsic link between magnetic and optical properties, we also show that for nondilute or distorted systems, such as the double perovskite BaNaOsO, the model's deviations from optical data serve as a sensitive diagnostic tool to reveal the onset of ion-ion interactions or symmetry breaking. Our formalism thus provides a practical route for experimentalists to characterize relativistic effects and identify departures from ideal single-ion behavior using macroscopic magnetic measurements.
A Wave Function Theory and Density Functional Theory Investigation of Ground and Excited States of HfSi.
Ariyarathna IR
J Phys Chem A · 2026 Jun · PMID 42237707 · Publisher ↗
In this work, the reaction between Hf and Si atoms was explored using high-level ab initio multireference configuration interaction (MRCI), coupled-cluster wave function methods, and density functional theory (DFT), in c...
In this work, the reaction between Hf and Si atoms was explored using high-level ab initio multireference configuration interaction (MRCI), coupled-cluster wave function methods, and density functional theory (DFT), in conjugation with large correlation-consistent basis sets. A comprehensive set of potential energy curves (PECs), spin-orbit curves, permanent dipole moment (PDM) curves, transition dipole moment (TDM) curves, chemical bonding patterns, spectroscopic parameters, and energy-related properties were determined. The two most stable electronic states of HfSi (1Π and 1Σ) exhibit 1σ2σ3σ1π and 1σ2σ1π electron configurations, respectively, with approximate charge localization of HfSi. Both coupled-cluster and MRCI built on state-specific complete active space self-consistent field (CASSCF) wave functions predicted the 1Σ state to be slightly more stable than the 1Π state. The spin-orbit ground state of HfSi is an Ω = 0 state composed of 52% of 1Π and 41% 1Σ character. Considering a few low-lying states, the rovibrational line spectrum of HfSi was investigated at 700 and 3000 K, and the spin-orbit effects were found to be significant. The dissociation energy () of HfSi, estimated by accounting for higher-order electron correlation effects, complete basis set corrections, and spin-orbit effects, [CBS-C-CCSD(T)+δT(Q)+SO], is 2.855 eV, in good agreement with the experimental value. The bond energies and electron populations of the isovalent HfX (X = C, Ge, Sn, Pb) were also investigated, showing a decrease in bond energy from X = C to X = Pb. A strong linear relationship between bond energy and ionicity was observed across the series. Finally, the performance of 16 exchange-correlation functionals, spanning three rungs of Jacob's ladder of density functional approximations (DFAs), was assessed against the coupled-cluster results.
Measurements of the OH + Octanal and OH, Cl, and Br + Nonanal Reaction Rate Coefficients and the Nonanal Aldehydic Site Branching Ratio.
Spanoudaki MI, Novelli A, Orlando JJ … +2 more , Papadimitriou VC, Burkholder JB
J Phys Chem A · 2026 Jun · PMID 42236164 · Publisher ↗
In this study, rate coefficients, (T), for the gas-phase OH radical reaction with octanal, (273-370 K), CH(CH)CHO, and nonanal, CH(CH)CHO, (296-370 K), were measured using a pulsed laser photolysis-laser-induced fluoresc...
In this study, rate coefficients, (T), for the gas-phase OH radical reaction with octanal, (273-370 K), CH(CH)CHO, and nonanal, CH(CH)CHO, (296-370 K), were measured using a pulsed laser photolysis-laser-induced fluorescence absolute method. A relative rate (RR) method, with Fourier transform infrared (FTIR) detection, was used to measure the rate coefficient for the Cl-atom, (298 K), and Br-atom, (298 K), + nonanal reaction, and the Br-atom + pentanal, (298 K) reaction. The obtained room-temperature rate coefficients and Arrhenius expressions are (cm molecule s): (296 K) = (3.33 ± 0.30) × 10, (T) = (1.62 ± 0.25) × 10 exp (+(217 ± 40)/); (296 K) = (3.84 ± 0.35) × 10, (T) = (1.97 ± 0.25) × 10 exp (+(198 ± 30)/), where the quoted uncertainties are 2σ and the (296 K) and -factor in the Arrhenius fit include estimated systematic errors. The Cl-atom and Br-atom reaction rate coefficients were measured relative to -2-butene, propene, and diethyl ether (Cl-atom reaction) and -2-butene (Br-atom reactions) to be (cm molecule s): (295 K) = (5.2 ± 0.7) × 10; (295 K) = (1.9 ± 0.3) × 10; (295 K) = (1.1 ± 0.2) × 10, where the quoted uncertainty includes estimated systematic errors. OH radical initiated oxidation experiments performed in the presence of excess NO were used to derive a nonanal aldehydic H-atom abstraction site-specific branching ratio of 66 ± 8% at 295 K. Results from this study are discussed in terms of the OH + -aldehyde reaction mechanism, reactivity trends of -aldehydes, and structure activity relationship (SAR) parametrizations to be used in air quality modeling.
Insights into Tetrazine-Benzene Cycloadditions.
Demuth T, Svatunek D
J Phys Chem A · 2026 Jun · PMID 42233179 · Full text
Arene-tetrazine cycloadditions involving benzene as the dienophile represent an exceptionally rare class of Diels-Alder reactions. In this study, we computationally investigate the origin of the intrinsic reactivity of s...
Arene-tetrazine cycloadditions involving benzene as the dienophile represent an exceptionally rare class of Diels-Alder reactions. In this study, we computationally investigate the origin of the intrinsic reactivity of simple aromatic systems toward tetrazines and identify the factors that enable successful cycloaddition. Our results show that only highly activated tetrazines can overcome the unfavorable orbital overlap associated with the compact π-system of benzene. Energy decomposition analysis reveals that reactivity is governed by a combination of frontier molecular orbital interactions and charge-control effects. Substituent effects strongly influence the activation barrier, particularly when they promote a shift toward a more asynchronous and polar transition state. These findings provide mechanistic insight into a rare mode of arene reactivity and clarify the electronic requirements for enabling tetrazine Diels-Alder reactions with aromatic dienophiles.
Direct Absorption Spectroscopy at 1.1 THz: New Measurements of CrH (XΣ).
Ravi R, Nemchick DJ, Drouin BJ … +1 more , Ziurys LM
J Phys Chem A · 2026 Jun · PMID 42231045 · Publisher ↗
The pure rotational spectrum of the CrH radical in its XΣ state at 1.1 THz has been recorded with direct absorption methods using a cascaded chain multiplier source. The = 3 ←2 transition has been measured in multiple s...
The pure rotational spectrum of the CrH radical in its XΣ state at 1.1 THz has been recorded with direct absorption methods using a cascaded chain multiplier source. The = 3 ←2 transition has been measured in multiple spin components for Δ = 0, ± 1, and the proton hyperfine structure resolved. The transition was also recorded for the Cr isotopologue in natural abundance and both the proton and Cr hyperfine splittings measured─the first observation of this species by rotational spectroscopy. The data, including previously measured = 1 ← 0 and 2 ← 1 frequencies for the main isotopologue, were analyzed with a Hund's case (b) Hamiltonian that included third-order spin-rotation and fourth-order spin-spin terms. Rotational, fine structure, and proton hyperfine constants were determined for CrH, as well as Cr hyperfine parameters for CrH. In particular, the higher-order parameters γ and θ were established to a high degree of accuracy. The hyperfine constants for both nuclei are consistent with the unpaired electrons being principally located on the chromium nucleus. The Fermi contact term for the Cr nucleus suggests that the unpaired σ electron occupies an orbital mostly 3d in character.
PCS2-Based Schemes for Geometry Optimization and Frequency Calculations in Criegee Intermediate-Water Reactions.
Gao P, Crisci L, Xie C … +2 more , Fu P, Long B
J Phys Chem A · 2026 Jun · PMID 42230326 · Publisher ↗
The accurate prediction of thermochemical and kinetic parameters for Criegee intermediate reactions with water is essential for atmospheric modeling, yet the unusual electronic structure of these species poses significan...
The accurate prediction of thermochemical and kinetic parameters for Criegee intermediate reactions with water is essential for atmospheric modeling, yet the unusual electronic structure of these species poses significant challenges to standard quantum chemical methods. Here, we present a benchmark study validating new approaches for the reactions of formaldehyde oxide (CHOO), -CHCHOO, and -CHCHOO with water, where geometry optimizations and harmonic frequency calculations are performed at a computationally affordable level while accurate energies are obtained via the high-level W2X composite method. In particular, we assess the Pisa Composite Scheme (PCS2), which combines explicitly correlated coupled-cluster theory [CCSD(T)-F12b] with core-valence correlation corrections; an analytical-gradient variant, in which the CABS singles correction is replaced by a Hartree-Fock level basis set incompleteness term, provides equivalent accuracy while enabling more efficient harmonic frequency calculations. Both formulations show excellent agreement with CCSD(T)-F12a/cc-pVTZ-F12 reference geometries (bond length RMSD < 0.001 Å) and reproduce activation enthalpies within 0.12 kcal/mol, demonstrating that basis set convergence is achieved at the double-ζ F12 level. This systematic benchmark was enabled by an automated, black-box workflow that minimizes the number of optimization iterations even for weakly bound hydrated complexes, allowing efficient exploration of multiple low-level/high-level combinations. The results reveal that the choice of the low-level method is far from trivial: TST rate constants show that geometry and zero-point energy differences can propagate into rate-constant errors of 30-60% for systems with very low activation enthalpies. This validation establishes dual-level PCS-based approaches as reliable tools for atmospheric chemistry applications involving Criegee intermediates, while highlighting the importance of careful low-level method selection.
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