An efficient and near linear scaling pair natural orbital based local coupled cluster method

Matrix-isolation IR spectra of iodotrifluoroethylene (C2F3I)

The infrared spectra of iodotrifluoroethylene (ITFE) recorded under matrix-isolation (MI) conditions in para-hydrogen, neon and argon were investigated. The experimental spectra were analyzed by comparison with computed anharmonic spectra obtained in the second-order vibrational perturbation theory (VPT2) framework at the MP2 and revDSD-PBEP86-D3BJ levels of theory. In para-hydrogen and neon matrices, the experimentally observable bands in the range of 1800-650 cm-1 could be assigned to vibrational transitions of monomeric ITFE. The spectral resolution even allowed assignments of transitions arising from 13C-isotopologues and the observation of various higher-order resonances in the range up to ∼3550 cm-1. A comprehensive series of MI experiments in argon obtained by varying several experimental parameters revealed a dependence of the spectra on the deposition temperature. The spectra generally showed strong site-splitting effects due to the existence of different local environments around the ITFE molecule. Detailed analysis of the experimental spectra resulted in the identification of bands which are differently affected by matrix annealing. This observation led to the conclusion that ITFE occupies two major matrix sites of different stability. Calculations on ITFE dimers confirmed that spectral changes during annealing are due to the formation of dimers, which are stabilized through π-π interactions.

Coordination isomerism in dioxophosphorane cyanides

The 1,3-phosphaazaallene DippTerP = C=NtBu (DippTer = 2,6-(2,6-iPr2C6H3)2-C6H3) is thermally labile towards iso-butene elimination and formation of the corresponding cyanophosphine DippTerP(H)CN (1). In previous work we have shown facile deprotonation of 1 with K[N(SiMe3)2 and formation of cyanophosphide [(DippTerPCN)K]. We now present the alkali metal tethered cyanophosphides [(DippTerPCN)M(crown)] (M = Na, K; crown = 15-c-5, 18-c-6) and their structural diversity in the solid state depending on the metal (M) and the crown ether. Facile oxidation of [DippTerPCN][M(crown)] with O2 yields the formal cyanide adducts of dioxophosphoranes [DippTerPO2(CN)]-. Surprisingly, [DippTerPO2(CN)]- is obtained as a mixture of the cyanide and isocyanide isomers, indicating a coordination isomerism. This phenomenon is corroborated by experimental and theoretical studies revealing the cyanide isomer to be thermodynamically more stable. The oxidation with elemental sulphur gave the corresponding dithiophosphorane cyanide adduct [DippTerPS2(CN)]-, in which no isomerism was observed. This points to a crucial role of triplet oxygen in the isomerisation process. Monooxidation occurs when [DippTerPO2(CN)]- salts were treated with N2O, giving formal anionic phoshinidene monoxide adducts.

Systematic discrepancies between reference methods for noncovalent interactions within the S66 dataset

The accurate treatment of noncovalent interactions is necessary to model a wide range of applications, from molecular crystals to surface catalysts to aqueous solutions and many more. Quantum diffusion Monte Carlo (DMC) and coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)] are considered two widely trusted methods for treating noncovalent interactions. However, while they have been well-validated for small molecules, recent work has indicated that these two methods can disagree by more than 7.5 kcal/mol for larger systems. The origin of this discrepancy remains unknown. Moreover, the lack of systematic comparisons, particularly for medium-sized complexes, has made it difficult to identify which systems may be prone to such disagreements and the potential scale of these differences. In this work, we leverage the latest developments in DMC to compute interaction energies for the entire S66 dataset, containing 66 medium-sized complexes with a balanced representation of dispersion and electrostatic interactions. Comparison to previous CCSD(T) references reveals systematic trends, with DMC predicting stronger binding than CCSD(T) for electrostatic-dominated systems, while the binding becomes weaker for dispersion-dominated systems. We show that the relative strength of this discrepancy is correlated to the ratio of electrostatic and dispersion interactions, as obtained from energy decomposition analysis methods. Finally, we have pinpointed model systems: the hydrogen-bonded acetic acid dimer (ID 20) and dispersion-dominated uracil-cyclopentane dimer (ID 42), where these discrepancies are particularly prominent. These systems offer cost-effective benchmarks to guide future developments in DMC, CCSD(T), as well as the wider electronic structure theory community.

Linear-scaling quadruple excitations in local pair natural orbital coupled-cluster theory

We present a fast, asymptotically linear-scaling implementation of the perturbative quadruples energy correction in coupled-cluster theory using local natural orbitals. Our work follows the domain-based local pair natural orbital (DLPNO) approach previously applied to lower levels of excitations in coupled-cluster theory. Our DLPNO-CCSDT(Q) algorithm uses converged doubles and triples amplitudes from a preceding DLPNO-CCSDT computation to compute the quadruples amplitude and energy in the quadruples natural orbital (QNO) basis. We demonstrate the compactness of the QNO space, showing that more than 95% of the (Q) correction can be recovered using relatively loose natural orbital cutoffs, compared to the tighter cutoffs used in pair and triples natural orbitals at lower levels of coupled-cluster theory. We also highlight the accuracy of our algorithm in the computation of relative energies, which yields deviations of sub-kJ mol-1 in relative energy compared to the canonical CCSDT(Q). Timings are conducted on a series of growing linear alkanes (up to 10 carbons and 608 basis functions) and water clusters (up to 49 water molecules and 2842 basis functions) to establish the asymptotic linear-scaling of our DLPNO-(Q) algorithm.

Origin of Intramolecular versus Intermolecular C-H Arene Activation Selectivity by Cyclopentadienyl-Triphenylphosphine Iridium

Photolysis of (η5-C5Me5)Ir(PPh3)(H)2 in benzene generates the 16-electron (16e-) complex (η5-C5Me5)Ir(PPh3) that undergoes competitive intramolecular ortho-metalation with a phenyl group from -PPh3 and intermolecular C-H activation with benzene. Previous density functional theory (DFT) studies identified the intramolecular π-complex and the intermolecular benzene π-complex intermediates and their corresponding C-H activation transition states. However, neither the mechanism of interconversion between these intermediates nor the origin of intramolecular versus intermolecular pathway selectivity has been established. Here, we characterized the open-shell 16e- iridium species and extensively mapped out the energy landscape for intramolecular ortho-metalation of -PPh3 versus intermolecular benzene C-H activation. Also, we performed DFT-based direct dynamics simulations, and the results suggest that the intramolecular versus intermolecular pathway selectivity is determined dynamically within picoseconds as the 16e- iridium species evolves into a coordinatively saturated structure. During this process, the π-complexes are formed concurrently with, instead of prior to, the iridium hydrides, which could not be explained by kinetic models that assume C-H cleavage as the rate-limiting step. These findings demonstrate that dynamics simulations in addition to DFT calculations are needed for a more complete mechanistic understanding of photoinduced C-H activation reactions, of which the product selectivity can be influenced by atomic motion.

Discriminating High from Low Energy Conformers of Druglike Molecules: An Assessment of Machine Learning Potentials and Quantum Chemical Methods

Accurate and efficient prediction of high energy ligand conformations is important in structure-based drug discovery for the exclusion of unrealistic structures in docking-based virtual screening and de novo design approaches. In this work, we constructed a database of 140 solution conformers from 20 druglike molecules of varying size and chemical complexity, with energetics evaluated at the DLPNO-CCSD(T)/complete basis set (CBS) level. We then assessed a selection of machine learning potentials and semiempirical quantum mechanical models for their ability to predict conformational energetics. The GFN2-xTB tight binding density functional method correlates with reference conformer energies, yielding a Kendall's τ of 0.63 and mean absolute error of 2.2 kcal/mol. As putative internal energy filters for screening, we find that the GFN2-xTB, ANI-2x and MACE-OFF23(L) models perform well in identifying low energy conformer geometries, with sensitivities of 95 %, 89 % and 95 % respectively, but display a reduced ability to exclude high energy conformers, with respective specificities of 80 %, 61 % and 63 %. The GFN2-xTB method therefore exhibited the best overall performance and appears currently the most suitable of the three methods to act as an internal energy filter for integration into drug discovery workflows. Enrichment of high energy conformers in the training of machine learning potentials could improve their performance as conformational filters.

Stacking Interactions of Druglike Heterocycles with Nucleobases

Stacking interactions contribute significantly to the interaction of small molecules with RNA, and harnessing the power of these interactions will likely prove important in the development of RNA-targeting inhibitors. To this end, we present a comprehensive computational analysis of stacking interactions between a set of 54 druglike heterocycles and the natural nucleobases. We first show that heterocycle choice can tune the strength of stacking interactions with nucleobases over a large range and that heterocycles favor stacked geometries that cluster around a discrete set of stacking loci characteristic of each nucleobase. Symmetry-adapted perturbation theory results indicate that the strengths of these interactions are modulated primarily by electrostatic and dispersion effects. Based on this, we present a multivariate predictive model of the maximum strength of stacking interactions between a given heterocycle and nucleobase that depends on molecular descriptors derived from the electrostatic potential. These descriptors can be readily computed using density functional theory or predicted directly from atom connectivity (e.g., SMILES). This model is used to predict the maximum possible stacking interactions of a set of 1854 druglike heterocycles with the natural nucleobases. Finally, we show that trivial modifications of standard (fixed-charge) molecular mechanics force fields reduce errors in predicted stacking interaction energies from around 2 kcal/mol to below 1 kcal/mol, providing a pragmatic means of predicting more reliable stacking interaction energies using existing computational workflows. We also analyze the stacking interactions between ribocil and a bacterial riboswitch, showing that two of the three aromatic heterocyclic components engage in near-optimal stacking interactions with binding site nucleobases.

Involving Carbene or Not? Mechanism of Corey-Winter Reaction

The mechanism of the Corey-Winter reaction is unclear, even though this reaction, discovered more than 60 years ago, has evolved as an indispensable method of synthesizing olefins from diols. We report a computational study of the mechanism of the Corey-Winter reaction, ruling out three well-known pathways of this reaction and proposing a new pathway consistent with the experimental observations. This new pathway starts from nucleophilic addition of phosphite to the carbon (not the sulfur) atom of the cyclic thionocarbonates. This step is followed by the intramolecular addition of S to P, generating thiaphosphirane intermediates. Elimination of phosphorothioates then takes place, giving oxygen-heterocyclic carbenes (OHCs). Finally, the cycloreversion of the OHCs gives carbon dioxide and the target olefin products. Calculations support that the OHC intermediates are involved in the Corey-Winter reaction. In addition, the Woodward-Hoffmann orbital correlation and electron flow pattern for the cycloreversion process of the OHCs to olefins and carbon dioxide have been revised. Moreover, why diazaphospholidine can accelerate the Corey-Winter reaction has been rationalized in this study by multivariable linear regression analysis.

Relaxation and Photochemistry of Nitroaromatic Compounds: Intersystem Crossing through 1ππ* to Higher 3ππ* States, and NO• Dissociation in 9-Nitroanthracene─A Theoretical Study

Determination of the photodegradation pathways of nitroaromatic compounds, known for their mutagenic properties and toxicity, is a relevant topic in atmospheric chemistry. In the present theoretical study, mechanisms for the photophysical relaxation and NO• dissociation of 9-nitroanthracene (9-NA) are proposed that challenge the commonly assumed pathways based on the El-Sayed rules. The analysis of the stationary points on the potential energy surfaces obtained with multiconfigurational methods indicates that after light absorption and subsequent relaxation of the S1 state, the system undergoes ultrafast intersystem crossing to T2, which serves as a gate-state to the triplet manifold due to favorable energetic couplings. This occurs despite the nature of the singlet and triplet states being 1ππ* and 3ππ*, where the receiver triplet involves NO2 orbitals that are tilted from the polyaromatic plane, with no involvement of the 3nπ state in the process. After the singlet to triplet manifold crossing, the system evolves along two possible trajectories. One leads to the global minimum of T1 (phosphorescent end state) and the other involves the dissociation into antryloxy and NO• radicals. Overall, the information obtained is in agreement with steady-state and time-resolved spectroscopic data reported for 9-NA. Furthermore, it suggests that the deactivation mechanism of nitroaromatic compounds can take place between 1ππ* and 3ππ* states, which opens a new landscape for the rationalization of the photophysics of these and other systems.

Conformational isomerization in Co(acac)2via spin-state switch: a computational study

Conformational dynamics of ligands in transition metal complexes can give rise to interesting physical, chemical, spectroscopic, and magnetic properties of the complexes. The changing ligand environment often affects the d-orbital splitting pattern that allows multiple possible ways of electron arrangement in the frontier molecular orbitals, resulting in several closely spaced electronic states with different orbital and spin symmetries. The system can explore these states with either thermal or photophysical means. In the present work, we demonstrate the possibility of a spin transition in Co(acac)2 assisted by a conformational rearrangement of the ligand. Electronic structure calculations show that the complex adopts a square-planar and tetrahedral geometry with low-spin and high-spin electronic configurations, respectively. A spin-conserved conformational change involves a larger energy barrier in both high- and low-spin states. On the other hand, a low-lying minimum-energy-crossing point exists between the two spin-states that provides a low-energy pathway for conformational isomerization between the two isomers. While the spin-assisted isomerization from a tetrahedral to square planar form requires crossing a 10 kcal mol-1 barrier, the reverse barrier is only 2 kcal mol-1. The calculation of the magnetic properties of the complex reveals a large magnetic anisotropy barrier of 57.6 cm-1 for this complex in the high-spin state.

Decentralized Metal-Metal Bonding in the AuNi(CO)4- Anion Described Equally Well with Dative Bonding as with Electron-Sharing Bonding

The heterodinuclear AuNi(CO)4- complex is scrutinized in the gas phase by using mass-selected anionic photoelectron velocity-map imaging spectroscopy in conjunction with theoretical computations. The ground state of AuNi(CO)4- is characterized to have an Au-Ni bonded structure, consisting of an AuCO fragment attached to the Ni center of the Ni(CO)3 fragment. Comprehensive quantum chemical studies reveal that the AuNi(CO)4- complex at equilibrium structure features a decentralized bonding scenario, where the exotic metal-metal σ bonding may be equally well described with dative bonding as with electron-sharing bonding between two fragments.

N-Heterocyclic Carbenes: A Benchmark Study on their Singlet-Triplet Energy Gap as a Critical Molecular Descriptor

N-heterocyclic carbenes (NHCs) are used extensively in modern chemistry and materials science. The in-depth understanding of their electronic structure and their metal complexes remains an important topic of research and of experimental and theoretical interest. Herein, the adiabatic singlet-triplet gap as a superior, quantifiable critical descriptor, sensitive to the nature and the structural diversity of the NHCs, for a successful rationalization of experimental observations and computationally extracted trends is established. The choice is supported by a benchmark study on the electronic structures of NHCs, using high-level ab initio methods, that is, complete active space self-consistent field, n-electron valence second-order perturbation theory, multireference configuration interaction + singles + doubles, and domain-based local pair natural orbital-coupled cluster method with single-, double-, and perturbative triple excitations along with density functional theory methods such as BP86, M06, and M06-L, B3LYP, PBE0, TPSSh, CAM-B3LYP, and B2PLYP. In contrast to the adiabatic singlet-triplet (S-T) gap preferred as descriptor, the highest occupied molecular orbital-lowest unoccupied molecular orbital gap or the S-T vertical gap that has been used in the past occasionally leads to controversial results; some of these are critically discussed below. Extrapolation of these ideas to a group of copper-NHC complexes is also described.

Room-Temperature Decomposition of the Ethaline Deep Eutectic Solvent

Environmentally benign, nontoxic electrolytes with combinatorial design spaces are excellent candidates for green solvents, green leaching agents, and carbon capture sources. We examine ethaline, a 2:1 molar ratio of ethylene glycol and choline chloride. Despite its touted green credentials, we find partial decomposition of ethaline into toxic chloromethane and dimethylaminoethanol at room temperature, limiting its sustainable advantage. We experimentally characterize these decomposition products and computationally develop a general, quantum-chemically accurate workflow to understand its decomposition. We find that fluctuations in the hydrogen bonds bind chloride near reaction sites, initiating the reaction between choline cations and chloride anions. The strong hydrogen bonds formed in ethaline are resistant to thermal perturbations, entrapping Cl in high-energy states and promoting the uphill reaction. In the design of stable green solvents, we recommend detailed evaluation of the hydrogen-bonding potential energy landscape as a key consideration for generating stable solvent mixtures.

Binding Energy Calculations of Anthracene and Rhodamine 6G H-Type Dimers: A Comparative Study of DFT and SMD Methods

With the ever-growing need to study systems of increased size and complexity, modern density functional theory (DFT) methods often encounter problems arising from the growing computational demands. In this work, we have presented a comprehensive DFT validation of the steered molecular dynamics (SMD) approach in estimating the binding energies of aromatic dimers. By performing DFT calculations on optimized and unoptimized anthracene and rhodamine 6G (R6G) dimers using functionals with progressively enhanced exchange-correlation energy description and comparing the obtained results with SMD-predicted values, it was found that SMD predictions are in good agreement with the results obtained from hybrid DFT calculations. The average binding energies for optimized anthracene dimers were found to be 6.46 kcal/mol using DFT at ωB97X-D4/def2-QZVPP and 7.64 ± 1.61 kcal/mol as predicted by the SMD. For the R6G H-type dimer, the binding energies were 17.48 and 19.02 ± 2.22 kcal/mol, respectively. The study also revealed that due to the lack of explicit terms accounting for electron-electron interactions in MD force fields, the proposed method tends to overbind dimers. It is anticipated that the presented method can be applied to more complex dimers, potentially accelerating the calculations of binding energies. Moreover, this study further validates the accuracy of the CHARMM36 FF.

Efficient Computational Strategies of the Cluster-in-Molecule Local Correlation Approach for Interaction Energies of Large Host-Guest Systems

We propose a heterogeneously accelerated reduced cluster-in-molecule (CIM) local correlation approach for calculating host-guest interaction energies. The essence of this method is to compute only the clusters that make significant contributions to the interaction energies while approximately neglecting those clusters with smaller contributions. Benchmark calculations at the CIM resolution-of-identity second-order Mo̷ller-Plesset perturbation (CIM-RI-MP2) or CIM spin-component-scaled RI-MP2 (CIM-SCS-RI-MP2) levels, involving three medium-sized protein-ligand structures, demonstrate that the reduced CIM method achieves over 48% time savings without compromising accuracy, as the interaction energy error remains within 0.5 kcal/mol compared to the full CIM method. To further enhance cluster computation efficiency, we developed a heterogeneous parallel version of the CIM-(SCS-)RI-MP2 method. It achieves over 93% internode parallel efficiency and over 98% multi-GPU card parallel efficiency for the tested large complexes. Ultimately, the hardware-accelerated reduced CIM-(SCS-)RI-MP2 method is applied to calculate the interaction energies of six protein-ligand systems, ranging from 913 to 1425 atoms. Remarkably, the method requires only 4.3-22.8% of the clusters to achieve accurate results, and under the condition of using only a single node, the wall time is within 2 days. Additionally, the reduced CIM domain-based local pair natural orbital coupled cluster with singles, doubles, and perturbative triples [CIM-DLPNO-CCSD(T)] method is successfully applied to the calculation of a 1425-atom protein-ligand system. These computations demonstrate the capability of a specific electronic structure to accurately calculate interaction energies for large host-guest systems.

Efficient Implementation of the Random Phase Approximation with Domain-Based Local Pair Natural Orbitals

We present an efficient implementation of the direct random phase approximation (RPA) for molecular systems within the domain-based local pair natural orbital (DLPNO) framework. With recommended loose, normal, and tight parameter settings, DLPNO-RPA achieves approximately 99.7-99.95% accuracy in the total correlation energy compared to a canonical implementation, enabling highly accurate reaction energies and potential energy surfaces to be computed while substantially reducing computational costs. As an application, we demonstrate the capability of DLPNO-RPA to efficiently calculate basis set-converged binding energies for a set of large molecules, with results showing excellent agreement with high-level reference data from both coupled cluster and diffusion Monte Carlo. This development paves the way for the routine use of RPA-based methods in molecular quantum chemistry.

A Generally Applicable Method for Disentangling the Effect of Individual Noncovalent Interactions on the Binding Energy

We introduce the fragment-pairwise Local Energy Decomposition (fp-LED) scheme for precise quantification of individual interactions contributing to the binding energy of arbitrary chemical entities, such as protein-ligand binding energies, lattice energies of molecular crystals, or association energies of large biomolecular assemblies. Using fp-LED, we can assess whether the contribution to the binding energy arising from noncovalent interactions between pairs of molecular fragments in any chemical system is attractive or repulsive, and accurately quantify its magnitude at the coupled cluster level - commonly considered as the "gold standard" of computational chemistry. Such insights are crucial for advancing molecular and material design strategies in fields like catalysis and therapeutic development. Illustrative applications across diverse fields demonstrate the versatility and accuracy of this theoretical framework, promising profound implications for fundamental understanding and practical applications.

Key Kinetic Interactions between NOX and Unsaturated Hydrocarbons: H Atom Abstraction from C3-C7 Alkynes, Dienes, and Trienes by NO2

An adequate understanding of the NOx interacting chemistry is a prerequisite for a smoother transition to carbon-lean and carbon-free fuels such as ammonia and hydrogen. In this regard, this study presents a comprehensive study on the H atom abstraction by NO2 from C3 to C7 alkynes, dienes, and trienes forming 3 HNO2 isomers (i.e., TRANS_HONO, HNO2, and CIS_HONO), encompassing 8 hydrocarbons and 24 reactions. Through a combination of high-level quantum chemistry computation, electronic structures, single-point energies, C-H bond dissociation energies, and 1-D hindered rotor potentials of the reactants, transition state (TS), complexes, and products involved in each reaction are determined at DLPNO-CCSD(T)/cc-pVDZ//M06-2X/6-311++g(d,p), from which potential energy surfaces and energy barriers for each reaction are determined. Following this, the rate coefficients for all studied reactions, over a temperature range from 298 to 2000 K, are computed based on TS theory using the Master Equation System Solver program by considering unsymmetric tunneling corrections. Comprehensive analysis of branching ratios elucidates the diversity and similarities between different species, different HNO2 isomers, and different abstraction sites. Incorporating the calculated rate parameters into a recent chemistry model reveals the significant influences of this type of reaction on model performance, where the updated model is consistently more reactive for all the alkynes, dienes, and trienes studied in predicting autoignition characteristics. Sensitivity and flux analyses are further conducted, through which the importance of H atom abstractions by NO2 is highlighted. With the updated rate parameters, the branching ratios in fuel consumption clearly shift toward H atom abstractions by NO2 while away from H atom abstractions by ȮH. The obtained results emphasize the need for adequately representing these kinetics in new alkyne, diene, and triene chemistry models to be developed by using the rate parameters determined in this study, and call for future efforts to experimentally investigate NO2 blending effects on alkynes, dienes, and trienes.

Linear-Scaling Local Natural Orbital-Based Full Triples Treatment in Coupled-Cluster Theory

We present an efficient, asymptotically linear-scaling implementation of the canonically O ( N 8 ) coupled-cluster method with singles, doubles, and full triples excitations (CCSDT) method. We apply the domain-based local pair natural orbital (DLPNO) approach for computing CCSDT amplitudes. Our method, called DLPNO-CCSDT, uses the converged coupled-cluster amplitudes from a preceding DLPNO-CCSD(T) computation as a starting point for the solution of the CCSDT equations in the local natural orbital basis. To simplify the working equations, we t1-dress our two-electron integrals and Fock matrices, allowing our equations to take on the form of CCDT. With appropriate parameters, our method can recover more than 99.99% of the total canonical CCSDT correlation energy. In addition, we demonstrate that our method consistently yields sub-kJ mol-1 errors in relative energies when compared to canonical CCSDT, and, likewise, when computing the difference between CCSDT and CCSD(T). Finally, to highlight the low scaling of our algorithm, we present timings on linear alkanes (up to 30 carbons and 730 basis functions) and water clusters (up to 131 water molecules and 3144 basis functions).

Another Angle on Benchmarking Noncovalent Interactions

For noncovalent interactions, the CCSD(T)-coupled cluster method is widely regarded as the "gold standard". With localized orbital approximations, benchmarks for ever larger complexes are being published, yet FN-DMC (fixed-node quantum Monte Carlo) intermolecular interaction energies diverge to a progressively larger degree from CCSD(T) as the system size grows, particularly when π-stacking is involved. Unfortunately, post-CCSD(T) methods like CCSDT(Q) are cost-prohibitive, which requires us to consider alternative means of estimating post-CCSD(T) contributions. In this work, we take a step back by considering the evolution of the correlation energy with respect to the number of subunits for such π-stacked sequences as acene dimers and alkadiene dimers. We show it to be almost perfectly linear and propose the slope of the line as a probe for the behavior of a given electron correlation method. By going further into the coupled cluster expansion and comparing with CCSDT(Q) results for benzene and naphthalene dimers, we show that CCSD(T) does slightly overbind but not as strongly as suggested by the FN-DMC results.

CH Bond Activation Mechanism by a High-Valent Dinuclear Copper Complex: Unraveling the Effect of a Base by a Theoretical Study

Recently, an electrochemically monooxidized dinuclear copper(II) complex [Cu2(L)(μ-OH)2]2+ with the dipyridylethane naphthyridine ligand (L) has been shown to activate the recalcitrant aliphatic Csp 3H bond of toluene (bond dissociation free energy, BDFE = 87.0 kcal mol-1) at room temperature. The mechanistic pathway turns from stoichiometric to catalytic upon addition of a base (2,6-lutidine), suggesting a modification of the reactive species. Herein, we report theoretical calculations to characterize the reactive species and obtain a detailed understanding of the reactivity. Since different electronic structures are possible for these high valent systems, we perform DFT calculations coupled to CCSD(T) ones using the DLPNO-CCSD(T) scheme. Our results show that the presence of a base will impact the nature of the reactive species but also the type of mechanism involved in the CH activation.

Conformational Profile of Galactose-α-1,3-Galactose (α-Gal) and Structural Basis of Its Immunological Response

Small carbohydrates present a rich and complex conformational landscape whose accurate description is a significant challenge for computational molecular science, yet essential for understanding their physicochemical properties, biological roles, and medical implications. Galactose-α-1,3-galactose (α-Gal) is a notable example of a disaccharide that remains insufficiently characterized despite being implicated in the life-threatening anaphylactic response known as alpha-Gal syndrome. Here we present a thorough conformational analysis of α-Gal using a unique combination of techniques, ranging from classical dynamics to a staged automatic conformer generation and screening using a quantum-mechanics-based protocol elaborated in the present work. The results reveal a remarkably constrained and rigid conformational profile that is minimally responsive to solvation. Subsequently, we study the binding of α-Gal to the M86 antibody using multiscale hybrid (QM/MM) calculations. Quantum mechanical analysis of the binding in terms of non-covalent interactions, local energy decomposition, and quantities derived from the quantum theory of atoms in molecules, enable us to identify and quantify the key interactions that form the structural basis of α-Gal's immunological response.

Ph3PC - A Monosubstituted C(0) Atom in Its Triplet State

This study introduces a novel class of carbon-centered diradicals: a monosubstituted C atom stabilized by a phosphine. The diradical Ph3P→C was photochemically generated from a diazophosphorus ylide precursor (Ph3PCN2) and characterized by EPR and isotope-sensitive ENDOR spectroscopy at low temperatures. Ph3P→C features an axial zero-field splitting parameter D=0.543 cm-1 with a vanishingly small rhombicity |E|/D=0.002. Time- and temperature-dependent measurements confirm a triplet ground state with a lifetime of approximately 10 min at 127 K in toluene-d8. Multireference electronic structure calculations predict a clear triplet ground state with a singlet-triplet gap greater than 20 kcal/mol. In contrast to divalent C(0) compounds, such as Ph3P→C←PPh3, in which carbon needs excitation into a highly-excited closed-shell 2s02p4 configuration, Ph3P→C can be explained by direct involvement of carbon in its natural 3P state arising from the 2s22p2 configuration.

SpectroIBIS: Automated Data Processing for Multiconformer Quantum Chemical Spectroscopic Calculations

Quantum chemical spectroscopic calculations have grown increasingly popular in natural products research for aiding the elucidation of chemical structures, especially their stereochemical configurations. These calculations have become faster with modern computational speeds, but subsequent data handling, inspection, and presentation remain key bottlenecks for many researchers. In this article, we introduce the SpectroIBIS computer program as a user-friendly tool to automate tedious tasks commonly encountered in this workflow. Through a simple graphical user interface, researchers can drag and drop Gaussian or ORCA output files to produce Boltzmann-averaged ECD, VCD, UV-vis and IR data, optical rotations, and/or 1H and 13C NMR chemical shifts in seconds. Also produced are formatted, publication-quality supplementary data tables containing conformer energies and atomic coordinates, saved to a DOCX file compatible with Microsoft Word and LibreOffice. Importantly, SpectroIBIS can assist researchers in finding common calculation issues by automatically checking for redundant conformers and imaginary frequencies. Additional useful features include recognition of conformer energy recalculations at a higher theory level, and automated generation of input files for quantum chemistry programs with optional exclusion of high-energy conformers. Lastly, we demonstrate the applicability of SpectroIBIS with spectroscopic calculations for five natural products. SpectroIBIS is open-source software available as a free desktop application (https://github.com/bbulcock/SpectroIBIS).

Computational Study of Hypervalent Chalcogen Bond Catalysis on the Hydroarylation of Styrene with Phenol: O-Activation vs π-Activation

Chalcogen bond catalysis is gaining recognition in organocatalysis due to its environmental benignity and relatively low cost. The hypervalent selenium salts can drive the hydroarylation of styrene and phenol, and hypervalent chalcogen···π catalysis has been proposed [Zhang, Q. Angew. Chem., Int. Ed. 2022, 61, e202208009]. In this work, the hydroarylation of styrene and phenol catalyzed by cyclic hypervalent selenium-based catalysts is investigated by density functional theory (DFT) calculations, and two activation modes are observed: one is on the styrene (π-activation mode), and the other is on the phenol (O-activation mode). The energy barriers via the O-activation mode are lower than those of the π-activation mode, and our proposed O-activation mode in this work may be more favorable. For the O-activation mode, energy barriers for the ortho-hydroarylation are lower than those for the para-hydroarylation, which is consistent with the experimental observation that the ortho-hydroarylation product is the major product and supports our proposed O-activation mode. Further investigation revealed that the stronger electrostatic interaction is the main factor leading to the ortho-hydroarylation in the O-activation mode compared to the para-hydroarylation. Moreover, the substituent effect of cyclic hypervalent selenium-based catalysts on the reactivity was investigated. This work would provide a valuable perspective on expanding applications for chalcogen bond catalysis.

A framework for designing main-group single-molecule magnets

Single-molecule magnets (SMMs) are molecular entities with strongly anisotropic magnetic moment. As a result, SMMs display slow relaxation of magnetization at the macroscopic scale. Up to date all experimentally characterized SMMs are based on either d- or f-block metals with lanthanides proving to be the most successful. In the present work, a framework for constructing SMMs consisting purely of main-group elements will be outlined by computational and theoretical means. The proposed main-group SMMs utilize the strong spin-orbit coupling of a single heavy p-block atom or ion that can lead to strong magnetic anisotropy and pronounced SMM properties. A theoretical crystal-field model is developed to describe the magnetic properties of p-block SMMs with a minimal set of parameters related to the chemical structure of the SMMs. The model is used to establish which p-block elements and oxidation states can lead to SMM behavior. A large number of model structures are studied to establish general features of optimal chemical structures. These include one- and two-coordinate structures involving ligands with different coordination modes and all group 13 to 17 elements in periods 4 to 6. The results show that the most viable structures are based on mono-coordinated complexes of bismuth in oxidation state 0 with σ-donor ligands. Structures with bulkier ligands that sterically protect the bismuth atoms are then proposed as a starting point for the practical realization of main-group SMMs. The calculations show that minimizing the anagostic interactions with the bismuth atom is essential in the ligand design, which along with the low oxidation state of bismuth introduces significant synthetic challenges. The results do, however, show that main-group SMMs are plausible from a practical point of view within a limited set of heavier p-block elements in specific oxidation states. Furthermore, the proposed SMMs display much larger energy barriers for the relaxation of magnetization than even the best lanthanide-based SMMs do. This indicates that it is possible that main-group SMMs can supersede even the best currently known SMMs based on d- or f-block elements.

Direct measurement of fluorocarbon radicals in the thermal destruction of perfluorohexanoic acid using photoionization mass spectrometry

Thermal destruction is a critical cornerstone of addressing the rampant contamination of natural resources with per- and polyfluoroalkyl substances (PFAS). However, grave concerns associated with stack emissions from incineration exist because mechanistic studies have thus far relied on ex situ analyses of end products and theoretical calculations. Here, we used synchrotron-based vacuum ultraviolet photoionization mass spectrometry to study the pyrolysis of a representative PFAS-perfluorohexanoic acid-and provide direct evidence of fluorocarbon radicals and intermediates. A key reaction pathway from perfluorocarboxylic acids to ketenes via acyl fluorides is proposed. We furthermore propose CF2/CF3 radical-centered pyrolysis mechanisms and explain their roles in the formation of other products that may form in full-scale incinerators. These results have not only unveiled the role of radicals and intermediates in thermal PFAS decomposition and recombination mechanisms but also provide unique insight into improving the safety and viability of industrial PFAS incineration.

Overview of Developments in the MRCC Program System

mrcc is a versatile suite of quantum chemistry programs designed for accurate ab initio and density functional theory (DFT) calculations. This contribution outlines the general features and recent developments of the package. The most popular features include the open-ended coupled-cluster (CC) code, state-of-the-art CC singles and doubles with perturbative triples [CCSD(T)], second-order algebraic-diagrammatic construction, and combined wave function theory-DFT approaches. Cost-reduction techniques are implemented, such as natural orbital (NO), local NO (LNO), and natural auxiliary function approximations, which significantly decrease the computational demands of these methods. This paper also details the method developments made over the past five years, including efficient schemes to approach the complete basis set limit for CCSD(T) and the extension of our LNO-CCSD(T) method to open-shell systems. Additionally, we discuss the new approximations introduced to accelerate the self-consistent field procedure and the cost-reduction techniques elaborated for analytic gradient calculations at various levels. Furthermore, embedding techniques and novel range-separated double-hybrid functionals are presented for excited-state calculations, while the extension of the theories established to describe core excitations and ionized states is also discussed. For academic purposes, the program and its source code are available free of charge, and its commercial use is also facilitated.

Determining the Molecular Shape of Progesterone: Insights from Laser Ablation Rotational Spectroscopy

Herein, we present the first experimental observation of isolated progesterone, an endogenous steroid, placed in the gas phase by laser ablation and characterized in a supersonic expansion by Fourier transform microwave techniques. Guided by quantum-chemical calculations, we assigned the rotational spectrum of the most stable structure. The internal rotation of the acetyl methyl group led to the observation of A-E doublets in the spectrum, which were analyzed, resulting in a V3 barrier of 2.4425 ± 0.0025 kJ mol-1. By fitting over 250 transitions, we determined accurate rotational constants that enabled us to compare the gas phase geometrical parameters with those of crystalline forms and complexes with progesterone receptors. Our results indicate that the A ring of progesterone that contains the ketone group is surprisingly flexible, despite its rigid appearance. This finding is particularly significant, since this ring is an active biological site that is involved in strong intermolecular interactions. Notably, progesterone C21H30O2 is the largest molecule investigated using laser ablation rotational spectroscopy.

On the nature of high-spin forms in the S2 state of the oxygen-evolving complex

The Mn4CaO x cluster of the oxygen-evolving complex (OEC) in photosystem II, the site of biological water oxidation, adopts different forms as it progresses through the catalytic cycle of S i states (i = 0-4) and within each S i state itself. This has been amply documented by spectroscopy, but the structural basis of spectroscopic polymorphism remains debated. The S2 state is extensively studied by magnetic resonance spectroscopies. In addition to the common type of g ≈ 2 multiline EPR signal attributed to a low-spin (S = 1/2) form of the manganese cluster, other signals at lower fields (g ≥ 4) associated with the S2 state arise from higher-spin forms. Resolving the structural identity of the high-spin species is paramount for a microscopic understanding of the catalytic mechanism. Hypotheses explored by theoretical studies implicate valence isomerism, proton tautomerism, or coordination change with respect to the low-spin form. Here we analyze structure-property correlations for multiple formulations employing a common high-level protocol based on multiscale models that combine a converged quantum mechanics region embedded within a large protein region treated semiempirically with an extended tight-binding method (DFT/xTB), surpassing conventional quantum mechanics/molecular mechanics (QM/MM) approaches. Our results provide a comprehensive comparison of magnetic topologies, spin states and energetics in relation to experimental observations. Crucial predictions are made about 14N hyperfine coupling constants and X-ray absorption Mn K-pre-edge features as criteria for discriminating between different models. This study updates our view on a persistent mystery of biological water oxidation, while providing a refined and transferable computational platform for future theoretical studies of the OEC.

Beyond CCSD(T) Accuracy at Lower Scaling with Auxiliary Field Quantum Monte Carlo

We introduce a black-box auxiliary field quantum Monte Carlo (AFQMC) approach to perform highly accurate electronic structure calculations using configuration interaction singles and doubles (CISD) trial states. This method consistently provides more accurate energy estimates than coupled cluster singles and doubles with perturbative triples (CCSD(T)), often regarded as the gold standard in quantum chemistry. This level of precision is achieved at a lower asymptotic computational cost, scaling as O(N6) compared to the O(N7) scaling of CCSD(T). We provide numerical evidence supporting these findings through results for challenging main group and transition metal-containing molecules.

CO2 Reduction at a Borane-Modified Iron Complex: A Secondary Coordination Sphere Strategy

This work addresses fundamental questions that deepen our understanding of secondary coordination sphere effects on carbon dioxide (CO2) reduction using derivatized hydride analogues of the type, [Cp*Fe(diphosphine)H] (Cp* = C5Me5 -) - a well-studied family of organometallic complex - as models. More precisely, we describe the general reactivity of [(Cp*-BR2)Fe(diphosphine)H], which contains an intramolecularly positioned Lewis acid, and its cooperative reactivity with CO2. Control experiments underscore the critical nature of borane incorporation for transforming CO2 to reduced products, a reaction that does not occur for unfunctionalized [Cp*Fe(diphosphine)H]. Additional experiments highlight relevance of borane hybridization and substituent effects. Mechanistic studies performed in the presence and absence of CO2 emphasize the significance of carbonyl substrate to catalyst longevity. Lessons from these reactions were also transferable - with such borane-containing complexes enabling the chemoselective reduction of aldehydes in the presence of alkenes. These findings provide valuable insights into metal-ligand cooperative design strategies for carbonyl reduction and illustrate the versatility of intramolecularly positioned Lewis acids for otherwise challenging chemical transformations.

Curcumin Electroanalysis at a Disposable Graphite Electrode

Curcumin (CU, turmeric), a polyphenolic phytochemical that is largely used as a food spice, has benefits for human health, which have led to increased interest in its therapeutic applications and its analysis from different matrices. The two guaiacol moieties of CU are responsible for its antioxidant properties and allow for its voltammetric quantification. Cyclic and differential pulse voltammetry (DPV) investigations at a single-use pencil graphite electrode (PGE) emphasized complex pH-dependent electrode processes, involving an equal number of protons and electrons. Theoretical calculations predicted a folded geometry for the β-diketone CU conformers, which interact with the PGE surface, exposing the electroactive moieties of only one aromatic ring. The Gibbs energy variations of the structures involved in CU electro-oxidation and the theoretical electrochemical potential values were calculated. CU's DPV cathodic peak intensity recorded at an HB-type PGE in 0.05 mol × L-1 H2SO4 varied linearly in the range 5.00 × 10-8-5.00 × 10-6 mol × L-1 CU. The method's detection and quantification limits were 2.12 × 10-8 mol × L-1 and 6.42 × 10-8 mol × L-1, respectively. The practical applicability of the developed method, successfully tested by CU assessment in dietary supplements, provided a recovery of 99.28 ± 2.04%.

Intermolecular Forces in Bioactive Resveratrol Complexes with Alcohols: A Study of Stability and Electronic Structure

Noncovalent interactions, particularly hydrogen bonding, play a pivotal role in determining the structural stability and functional properties of molecules, including bioactive compounds like resveratrol. This study focuses on the hydrogen-bonding behavior and other noncovalent interactions in gas-phase resveratrol-ethanol (EtOH) and resveratrol-methanol (MtOH) complexes, referred to as System 1 and System 2, respectively. These systems were optimized using the ωB97XD functional and cc-pVDZ basis set, providing a detailed picture of their stability and intermolecular interactions. By employing advanced methods such as Domain-Based Local Pair Natural Orbital Coupled Cluster (DLPNO-CCSD)(T) for energy decomposition, natural bond orbital (NBO) for charge analysis, atoms in molecule (AIM) for electron density topology, and noncovalent interaction (NCI) techniques, we decompose interaction energies into meaningful components like electrostatic, dispersion, and exchange-repulsion. The findings indicate that, while hydrogen bonding contributes to the stability of these complexes, London dispersion and other attractive interactions are substantial factors as well. The resveratrol-EtOH and resveratrol-MtOH systems demonstrate a robust electronic environment with significant contributions from various intermolecular forces, underscoring the importance of noncovalent interactions in stabilizing bioactive compounds. This study adds to our understanding of molecular interactions in resveratrol complexes, with potential implications for medicinal chemistry and material science, particularly where solvation effects are critical.

Vibrationally Assisted Tunneling through the Bread of a Proton Sandwich─Connections to Dynamic Matching

Proton sandwiches are unusual nonclassical carbocations characterized by a five-center, four-electron bonding array which rapidly isomerize to lower energy isomers with three-center, two-electron bonding arrays via hydrogen migration transition states. These reactions are suspected to involve significant contributions from tunneling, even at relatively high temperatures, where tunneling effects are usually minimal. Machine-learning-accelerated ring-polymer, quasiclassical, and classical ab initio molecular dynamics simulations were used to investigate the effects of a flavor of dynamic matching that involves coupling of vibrational modes of the reactant to the transition structure mode with an imaginary frequency, and how quantum mechanical tunneling affects this coupling. These nonstatistical dynamic effects were quantified by analysis of momentum in the molecular dynamics simulations. We show the importance of momentum for reactivity with and without tunneling, how tunneling amplifies these benefits, and that vibrational modes can be leveraged to generate beneficial momentum.

Metal-Metal Bonding in Tri-Actinide Clusters: A DFT Study of [An3Cl6]z (z=1-6) and [An3Cl6Cp3]z (z=-2-+3; An=Ac, Th, Pa, U, Np, Pu)

The actinide-actinide bonding in tri-actinide clusters [An₃Cl₆]z (An=Ac-Pu, z=1-6) and [An₃Cl₆Cp₃]z (z=-2-+3; Cp=(η5-C5H5)) is studied using density functional theory. We find 3-centre bonding similar to the tri-thorium cluster [{Th(η⁸-C₈H₈)(μ₃-Cl)₂}₃{K(THF)₂}₂]∞, as we previously reported (Nature 2021, 598, 72-75). The population of 3-centre molecular orbitals (3c-MOs) by zero, one or two electrons correlates with shortening of the An-An bond lengths, which also decrease with increasing actinide atomic number, consistent with the contraction of the actinide valence atomic orbitals. Mulliken analyses indicate that these 3c-MOs predominantly involve An 6d and 5 f orbitals. Various methods evidence the presence of An-An bonding in most systems with populated 3c-MOs, including bond orders (Mayer and Wiberg), quantum theory of atoms in molecules metrics (ρ, ∇2ρ, -G/V, H, delocalization indices), electron localization function, and electron density assessments. Additionally, we explore the effect of Cp ligand substitution on uranium complexes, finding that bulkier Cp ligands can induce U-U bond distortions and result in slightly longer U-U bonds. Overall, this study advances our understanding of metal-metal bonding in tri-actinide clusters, highlighting its effects on geometric and electronic structures.

Evaluation of Density-Functional Tight-Binding Methods for Simulation of Protic Molecular Ion Pairs

In this work, we benchmark the accuracy of the density-functional tight-binding (DFTB) method, namely the long-range corrected second-order (LC-DFTB2) and third-order (DFTB3) models, for predicting energetics of imidazolium-based ionic liquid (IL) ion pairs. We compare the DFTB models against popular density functionals such as LC-ωPBE and B3LYP, using ab initio domain-based local pair-natural orbital coupled cluster (DLPNO-CC) energies as reference. Calculations were carried out in the gas phase, as well as in aqueous solution using implicit solvent methods. We find that the LC-DFTB2 model shows excellent performance in the gas phase and agrees well with reference energies in implicit solvent, often outperforming DFTB3 predictions for complexation energetics. Our study identifies a range of opportunities for use of the LC-DFTB method and quantifies its sensitivity to protonation states and the types of chemical interactions between ion pairs.

Synthetically Relevant Post-Transition State Bifurcation Leading to Diradical and Zwitterionic Intermediates: Controlling Nonstatistical Kinetic Selectivity through Solvent Effects

A post-transition state surface intersection (PTSSI) between radical and zwitterionic states that causes a bifurcation in the reaction pathway was discovered through density functional theory calculations on potential energy surfaces and ab initio molecular dynamics simulations of cycloadditions between a bicyclobutane and a triazolinedione (BCB-TAD). It was predicted that changes to the solvent polarity would enable control over the dynamic selectivity in this system; indeed, experimental evidence supported this prediction. This work not only provides new insights into an unusual type of post-transition state bifurcation, but also demonstrates how the nonstatistical dynamic effects that control selectivity for such reactions can be manipulated rationally to increase the yields of synthetically useful reactions.

Phenoxazinone Synthase-Like Catalytic Activity of Bi- and Trinuclear Copper(II) Complexes with 2-Benzylethanolamine: Experimental and Theoretical Investigations

The self-assembly reaction of 2-benzylaminoethanol (Hbae) with CuCl2 or Cu(NO3)2 leads to the formation of binuclear [Cu2(bae)2(Cl)2] (1) and [Cu2(Hbae)2(bae)2](NO3)2 (2) complexes, while the trinuclear [Cu3(Hbae)2(bae)2(dmba)2](NO3)2 (3) compound was obtained using the auxiliar bulky substituted 2,2-dimethylbutyric acid (Hdmba). Crystallographic studies reveal the molecular structures of 1 and 2 based on the similar {Cu2(μ-O)2} core, while the structure of 3 features the {Cu3(μ-O)2} core with consecutive arranement of the metal centres, supported by the additional carboxylate bridges. The strong intermolecular hydrogen bonds join the molecular structures into 1D (for 1 and 3) or 2D (for 2) architectures. All three compounds act as catalysts for the aerobic oxidation of 2-aminophenol to the phenoxazinone chromophore (phenoxazinone synthase-like activity) with the maximum reaction rates up to 2.3×10-8 M s-1. The substrate scope involves methyl-, nitro- and chloro-substituted 2-aminophenols, disclosing the negligible activity of nitro-derivatives, while the 6-amino-m-cresol substrate shows the highest activity with the initial reaction rate of 5.8×10-8 M s-1. The mechanism of the rate-limiting reaction step (copper-catalysed formation of 2-aminophenoxyl radicals) was investigated at the DFT level. The combined DFT and CASSCF studies of the copper superoxo CuII-OO⋅ radical species as possible unconventional reaction intermediates resulted in a rational mechanism of H-atom abstraction, where the activation energies follow the experimental reactivity of substituted 2-aminophenols. The TDDFT and STEOM-DLPNO-CCSD theoretical calculations of the absorption spectra of substrates, phenoxazinone chromophores and putative polynuclear species containing 2-aminophenoxo ligand are reported.

Influence of counterion substitution on the properties of imidazolium-based ionic liquid clusters

Due to their unique physiochemical properties that may be tailored for specific purposes, ionic liquids (ILs) have been investigated for various applications, including chemical separations, catalysis, energy storage, and space propulsion. The different cations and anions comprising ILs may be selected to optimize a range of desired properties, such as thermal stability, ionic conductivity, and volatility, leading to the designation of certain ILs as designer "green" solvents. The effect of counterions on the properties of ILs is of both fundamental scientific interest and technological importance. Herein, we report a systematic experimental and theoretical investigation of the size, charge, stability toward dissociation, and geometric/electronic structure of 1-ethyl-3-methyl imidazolium (EMIM)-based IL clusters containing two different atomic counterions (i.e., bromide [Br-] and iodide [I-]). This work extends our studies of EMIM+ cations with atomic chloride (Cl-) and molecular tetrafluoroborate (BF4-) anions reported previously by Baxter et al. [Chem. Mater. 34, 2612 (2022)] and Zhang et al. [J. Phys. Chem. Lett. 11, 6844 (2020)], respectively. Distributions of anionic IL clusters were generated in the gas phase using electrospray ionization and characterized by high mass resolution mass spectrometry, energy-resolved collision-induced dissociation, and negative ion photoelectron spectroscopy experiments. The experimental results reveal anion-dependent trends in the size distribution, relative abundance, ionic charge state, stability toward dissociation, and electron binding energies of the IL clusters. Complementary global optimization theory provides molecular-level insights into the bonding and electronic structure of a selected subset of clusters, including their low energy structures and electrostatic potential maps, and how these fundamental characteristics are influenced by anion substitution. Collectively, our findings demonstrate how the fundamental properties of ILs, which determine their suitability for many applications, may be tuned by substituting counterions. These observations are critical in the sub-nanometer cluster size regime where phenomena do not scale predictably to the bulk phase, and each atom counts toward determining behavior.

Theoretical Study on the Kinetics of Secondary Oxygen Addition Reactions for N-Butyl Radicals

Chemical kinetics for second oxygen addition reactions (·QOOH + O2) of long-chain alkanes are of great importance in low-temperature combustion technologies. However, kinetic data for key reactions of ·QOOH + O2 systems are often difficult to obtain experimentally and are primarily estimated or calculated by using theoretical methods. In this work, barrier heights (BHs), reaction energies (ΔEs), and relative energies (REs) of stationary points for key reactions of two representative ·QOOH + O2 systems in the low-temperature oxidation of n-butyl as well as pressure-dependent rate constants for the involved reactions are calculated with the high-level quantum chemical method CCSD(T)-F12b/CBS. These results can be employed in the development of low-temperature combustion mechanisms for n-butane and longer straight-chain alkanes. In addition, the performance of some quantum chemistry methods with a lower computational cost on BHs, ΔEs, and REs as well as rate constants is also investigated. Our results indicate that the maximum error on these energies with PNO-LCCSD(T)-F12a is within 1 kcal/mol, and rate constants with this method are in the best agreement with reference values, with a maximum relative error of about half the reference values. Due to its low computational cost and memory requirements, this method is strongly recommended for studying low-temperature combustion reactions involving larger hydrocarbon fuels.

Isolation of a Staudinger-type Intermediate Utilizing a Five-Membered Phosphorus-Centered Biradicaloid

The Staudinger reaction provides chemists with a valuable tool for the reduction of azides, which are notoriously unstable and can decompose explosively. By providing a controlled method for the conversion of azides to amines, the reaction opened up new avenues for the synthesis of various amine-containing compounds that are widely used in natural products, pharmaceuticals and polymers. The Staudinger reaction begins with the nucleophilic attack of a trivalent phosphine (usually triphenylphosphine), leading to the formation of a triazenide intermediate, a highly reactive species. Here we report how a divalent phosphorus-centered biradicaloid reacts with covalent azides and show that it is possible to capture and fully characterize the transient intermediate. The experimental data is supported by quantum chemical calculations of the reaction paths and in terms of thermodynamics and chemical bonding.

Predicting Carbonic Anhydrase Binding Affinity: Insights from QM Cluster Models

A systematic series of QM cluster models has been developed to predict the trend in the carbonic anhydrase binding affinity of a structurally diverse dataset of ligands. Reference DLPNO-CCSD(T)/CBS binding energies were generated for a cluster model and used to evaluate the performance of contemporary density functional theory methods, including Grimme's "3c" DFT composite methods (r2SCAN-3c and ωB97X-3c). It is demonstrated that when validated QM methods are used, the predictive power of the cluster models improves systematically with the size of the cluster models. This provided valuable insights into the key interactions that need to be modeled quantum mechanically and could inform how the QM region should be defined in hybrid quantum mechanics/molecular mechanics (QM/MM) models. The use of r2SCAN-3c on the largest cluster model composed of 16 residues appears to be an economical approach to predicting binding trends compared with using more robust DFT methods such as ωB97M-V and provides a significant improvement compared with docking.

Simulating Ionized States in Realistic Chemical Environments with Algebraic Diagrammatic Construction Theory and Polarizable Embedding

Theoretical simulations of electron detachment processes are vital for understanding chemical redox reactions, semiconductor and electrochemical properties, and high-energy radiation damage. However, accurate calculations of ionized electronic states are very challenging due to their open-shell nature, importance of electron correlation effects, and strong interactions with chemical environment. In this work, we present an efficient approach based on algebraic diagrammatic construction theory with polarizable embedding that allows to accurately simulate ionized electronic states in condensed-phase or biochemical environments (PE-IP-ADC). We showcase the capabilities of PE-IP-ADC by computing the vertical ionization energy (VIE) of thymine molecule solvated in bulk water. Our results show that the second- and third-order PE-IP-ADC methods combined with the basis of set of triple-ζ quality yield a solvent-induced shift in VIE of -0.92 and -0.93 eV, respectively, in an excellent agreement with experimental estimate of -0.9 eV. This work demonstrates the power of PE-IP-ADC approach for simulating charged electronic states in realistic chemical environments and motivates its further development.

Application of modern artificial intelligence techniques in the development of organic molecular force fields

The molecular force field (FF) determines the accuracy of molecular dynamics (MD) and is one of the major bottlenecks that limits the application of MD in molecular design. Recently, artificial intelligence (AI) techniques, such as machine-learning potentials (MLPs), have been rapidly reshaping the landscape of MD. Meanwhile, organic molecular systems feature unique characteristics, and require more careful treatment in both model construction, optimization, and validation. While an accurate and generic organic molecular force field is still missing, significant progress has been made with the facilitation of AI, warranting a promising future. In this review, we provide an overview of the various types of AI techniques used in molecular FF development and discuss both the advantages and weaknesses of these methodologies. We show how AI methods provide unprecedented capabilities in many tasks such as potential fitting, atom typification, and automatic optimization. Meanwhile, it is also worth noting that more efforts are needed to improve the transferability of the model, develop a more comprehensive database, and establish more standardized validation procedures. With these discussions, we hope to inspire more efforts to solve the existing problems, eventually leading to the birth of next-generation generic organic FFs.

An Open-Shell FeIV Nitrido

We report the photogeneration and characterization of an open-shell, terminal iron nitrido (EmL)Fe(N) using a sterically encumbered dipyrrin ligand environment. The Fe-N distance in the solid-state, zero-field 57Fe Mössbauer spectrum, and computational analysis are consistent with a triplet electronic ground state of the iron nitrido. Notably, the attenuation of Fe-N multiple bond character through occupying π*Fe-N enables (i) primary C(sp3)-H amination, (ii) H2 cleavage, (iii) aromatic C-C cleavage, and (iv) photocatalytic N-atom transfer reactivity. These modes of reactivity have not previously been observed in low-spin Fe(N) analogues.

Accurate Enthalpies of Formation for Bioactive Compounds from High-Level Ab Initio Calculations with Detailed Conformational Treatment: A Case of Cannabinoids

Our recently developed approach based on the local coupled-cluster with single, double, and perturbative triple excitation [LCCSD(T)] model gives very efficient means to compute the ideal-gas enthalpies of formation. The expanded uncertainty (95% confidence) of the method is about 3 kJ·mol-1 for medium-sized compounds, comparable to typical experimental measurements. Larger compounds of interest often exhibit many conformations that can significantly differ in intramolecular interactions. Although the present capabilities allow processing even a few hundred distinct conformer structures for a given compound, many systems of interest exhibit numbers well in excess of 1000. In this study, we investigate how to reduce the number of expensive LCCSD(T) calculations for large conformer ensembles while controlling the error of the approximation. The best strategy found was to correct the results of the lower-level, surrogate model (density functional theory, DFT) in a systematic manner. It was also found that the error in the conformational contribution introduced by a surrogate model is mainly driven by a systematic (bias) rather than a random component of the DFT energy deviation from the LCCSD(T) target. This distinction is usually overlooked in DFT benchmarking studies. As a result of this work, the enthalpies of formation for 20 cannabinoid and cannabinoid-related compounds were obtained. Comprehensive uncertainty analysis suggests that the expanded uncertainties of the obtained values are below 4 kJ·mol-1.

Activation and C-C Coupling of Aryl Iodides via Bismuth Photocatalysis

Within the emerging field of bismuth redox catalysis, the catalytic formation of C-C bonds using aryl halides would be highly desirable; yet such a process remains a synthetic challenge. Herein, we present a chemoselective bismuth-photocatalyzed activation and subsequent coupling of (hetero)aryl iodides with pyrrole derivatives to access C(sp2)-C(sp2) linkages through C-H functionalization. This unique reactivity is the result of the bismuth complex featuring two redox state-dependent interactions with light, which 1) activates the Bi(I) complex for oxidative addition via MLCT, and 2) promotes the homolytic cleavage of aryl Bi(III) intermediates through a LLCT process.

Homologation of Alkenyl Carbonyls via a Cyclopropanation/Light-Mediated Selective C-C Cleavage Strategy

We report herein our studies on the direct photoactivation of carbonyl cyclopropanes to give biradical intermediates, leading to selective cleavage of the more substituted carbon-carbon bond. Depending on the substrate structure, extended alkenes were isolated or directly reacted in a photo-Nazarov process to give bicyclic products. Based on these results, a unified reductive ring-opening reaction was developed by using diphenyl disulfide as a hydrogen atom transfer (HAT) reagent. By performing a sequential cyclopropanation/selective ring opening reaction, we achieved a CH2 insertion into the α,β bond of both acyclic and cyclic unsaturated carbonyl compounds. Our protocol provides a further tool for the modification of the carbon framework of organic compounds, complementing the recent progress in "skeletal editing".

Spatial Signatures of Electron Correlation in Least-Squares Tensor Hypercontraction

Least-squares tensor hypercontraction (LS-THC) has received some attention in recent years as an approach to reduce the significant computational costs of wave function-based methods in quantum chemistry. However, previous work has demonstrated that LS-THC factorization performs disproportionately worse in the description of wave function components (e.g., cluster amplitudes T̂2) than Hamiltonian components (e.g., electron repulsion integrals (pq|rs)). This work develops novel theoretical methods to study the source of these errors in the context of the real-space T̂2 kernel, and reports, for the first time, the existence of a "correlation feature" in the errors of the LS-THC representation of the "exchange-like" correlation energy EX and T̂2 that is remarkably consistent across ten molecular species, three correlated wave functions, and four basis sets. This correlation feature portends the existence of a "pair point kernel" missing in the usual LS-THC representation of the wave function, which critically depends upon pairs of grid points situated close to atoms and with interpair distances between one and two Bohr radii. These findings point the way for future LS-THC developments to address these shortcomings.