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

Mechanism and optimization of ruthenium-catalyzed oxalamide synthesis using DFT

The oxalamide skeleton is a common structural motif in many biologically active molecules. These scaffolds can be synthesized via ruthenium pincer complex-catalyzed acceptorless dehydrogenative coupling of ethylene glycol and amines. In this study, we elucidate the mechanism of this oxalamide synthesis using density functional theory calculations. The rate-determining state is identified as the formation of molecular hydrogen following the oxidation of hydroxyacetamide to oxoacetamide. In predictive catalysis exercises, various modifications to the ruthenium pincer catalyst were investigated to assess their impact on the reactivity.

The divide expand consolidate scheme for unrestricted second order Møller-Plesset perturbation theory ground state energies

The linear scaling divide-expand-consolidate (DEC) framework is expanded to include unrestricted Hartree-Fock references. By partitioning the orbital space and employing local molecular orbitals, the full molecular calculation can be performed as independent calculations on individual fragments, making the method well-suited for massively parallel implementations. This approach also incorporates error control through the fragment optimization threshold (FOT), which maintains precision and consistency throughout the calculations. A benchmark was conducted for correlation energies of open-shell systems and the relative energies of both open- and closed-shell molecules at the MP2 level of theory. The full calculation result is achieved as the FOT approaches zero. For correlation energies, an FOT of 10-3 is sufficient to recover over 98% of the full result in all cases. However, for relative energies and the electronic energy component of oxidation potentials, a tighter FOT of 10-4 is required to keep the DEC error within 10% for both open- and closed-shell molecules. This is likely due to a lack of systematic error cancellation for the molecules with vastly different chemical natures. Therefore, for accurate relative energies, the FOT should be an order of magnitude lower, and additional caution is needed, particularly for large systems. The DEC method extension to unrestricted references maintains favorable features of linear scaling and can be implemented in a massively parallel algorithm to calculate correlation energies for large open-shell systems.

Cluster perturbation theory. XI. Excitation-energy series using a variational excitation-energy function

Traditionally, excitation energies in coupled-cluster (CC) theory have been calculated by solving the CC Jacobian eigenvalue equation. However, based on our recent work [Jørgensen et al., Sci. Adv. 10, eadn3454 (2024)], we propose a reformulation of the calculation of excitation energies where excitation energies are determined as a conventional molecular property. To this end, we introduce an excitation-energy function that depends on the CC Jacobian and the right and left eigenvectors for the Jacobian eigenvalue problem. This excitation-energy function is variational with respect to the right and left eigenvectors but not with respect to the cluster amplitudes. Instead, the cluster amplitudes satisfy the cluster-amplitude equations, and we set up an excitation-energy Lagrangian by adding to the excitation-energy function the cluster-amplitude equations with an undetermined multiplier for each cluster-amplitude constraint. The excitation-energy Lagrangian is variational in all its parameters. Based on the variational property of the Lagrangian, we have determined two quadratically convergent excitation-energy series: the total-order cluster-perturbation (tCP) and variational cluster-perturbation (vCP) excitation-energy series. Calculations of the excitation energies of three small molecules have shown that the vCP series is to be preferred over the tCP series. The test calculations have been carried out for CPS(D) expansions [targeting the CC singles-and-doubles (CCSD) wave function from the CC singles wave function] and the CPSD(T) expansion [targeting the CC singles-doubles-triples (CCSDT) wave function from the CCSD wave function]. For the S(D) and SD(T) orbital excitation space calculations, we obtain in the second vCP iteration excitation energies with a mean deviation from CCSD excitation energies of about 0.04 eV for the S(D) orbital spaces, and for the SD(T) orbital space calculation, we obtain a mean deviation from the CCSDT excitation energies of 0.001 eV.

Modeling Local Aerosol Surface Environments: Clustering of Pyruvic Acid Analogs, Water, and Na+, Cl- Ions

Pyruvic acid is an omnipresent compound in nature and is found both in the gas phase and in the particle phase of the atmosphere as well as in aqueous solution in the hydrosphere. Despite much literature on the photochemical degradation and stability of pyruvic acid in different chemical environments, the study of simultaneous interactions between gas-phase pyruvic acid or similar carboxylic acids with water and ions is not well-understood. Here, we present a study of microhydrated molecular clusters containing pyruvic acid and the structurally analogous carboxylic acids lactic acid, propionic acid, and 2,2-dihydroxypropanoic acid by probing geometries, binding free energies, hydrate distributions, as well as their infrared (IR) absorption spectra. We performed a meticulous configurational sampling protocol for the various hydrated clusters ranging from low level of theory to high level of theory to identify the lowest free energy structure. We find that cluster geometries and especially their water structure are highly sensitive to the presence and character of ions. We show that the hydration of the studied organic acids is thermodynamically unfavorable in the gas phase and ions are necessary for mediating interactions between organic acids and water thus stabilizing the clusters. Finally, we find a clear correlation between decreasing pyruvic acid carboxylic O-H stretching frequencies, increasing intensity when adding more water to the clusters, and a correlation between increasing redshifting of the O-H frequencies upon addition of ions to the clusters. The observations done in this study could pave the way to unravel the mechanisms behind the transitioning of organic acids from the gas phase to the particle phase.

Dyotropic Rearrangement of β-Lactams: Reaction Development, Mechanistic Study, and Application to the Total Syntheses of Tricyclic Marine Alkaloids

An unprecedented dyotropic rearrangement of β-lactams has been developed, which provides an enabling tool for the synthesis of structurally diverse γ-butyrolactams. Unlike the well-established dyotropic rearrangements of β-lactones, the present reaction probably proceeds through a dual-activation mode, and thus displays unusual reactivity and chemoselectivity. The combined computational and experimental results suggest that the dyotropic rearrangement of β-lactams may proceed through different mechanisms depending on the nature of migrating groups (H, alkyl, or aryl). Hinging on a chemoselective H-migration dyotropic rearrangement of β-lactams, we have completed the divergent synthesis of tricyclic marine alkaloids (-)-lepadiformine A, (+)-cylindricine C, and (-)-fasicularin within 11-12 longest linear steps.

Triazolide Complexes of Sodium and Potassium in the Gas Phase

Aromatic organometallic complexes, such as ferrocene and the "inverse sandwich complex" [Na2Cp]+, are stabilized via charge-transfer (C-T) interactions and cation-π interactions (i.e., charge-induced dipole and charge-quadrupole interactions). Much effort has gone into investigating systems that contain organic moieties, such as benzene or cyclopentadienyl ligands, but less attention has been paid to aromatic systems that contain heteroatoms (e.g., N), possibly owing to the complexity arising from a lowering of symmetry and the introduction of electron lone pair density and dipole moments. Here we investigate sodiated and potassiated clusters of 1,2,3-triazolide, [Mx (123T)x-1]+ (x = 3, 4; M = Na, K), and 1,2,4-triazolide, [Mx (124T)x-1]+ (x = 3, 4; M = Na, K), using a combination of infrared ion spectroscopy (IRIS) and DFT calculations. Cluster structures are strongly influenced by charge-dipole interactions and C-T interactions from N lone pairs to the metal cations. IRIS spectra indicated that the geometries of the triazolide moieties are essentially unperturbed by the interaction with the metal ions. Additional spectral features, not predicted by DFT calculations, that are observed in the 1500-1800 cm-1 region seem to be associated with combination bands involving C-H wagging and ring torsion motions.

From boom to bloom: synthesis of diazidodifluoromethane, its stability and applicability in the 'click' reaction

Diazidodifluoromethane was prepared from dibromodifluoromethane, sodium azide and an alkanethiolate initiator. It represents the first example of a diazidomethane that is stable enough to be used in synthesis. The stability of (poly)azidomethanes was explored with ab initio calculations. Copper(I)-catalysed azide-alkyne cycloaddition of the title azide with alkynes afforded difluoromethylene-containing bis(1,2,3-triazoles)amenable to Rh(II)-catalysed transannulation with nitriles to difluoromethylene bis(imidazoles).

Impact of Intracellular Proteins on μ-Opioid Receptor Structure and Ligand Binding

Chronic pain is a prevalent problem affecting approximately one out of every five adults in the U.S. The most effective way to treat chronic pain is with opioids, but they cause dangerous side effects such as tolerance, addiction, and respiratory depression, which makes them quite deadly. Opioids, such as fentanyl, target the μ-opioid receptor (MOR), which can then bind to the intracellular Gi protein or the β-arrestin protein. The Gi pathway is primarily responsible for pain relief and potential side effects, but the β-arrestin pathway is chiefly responsible for the unwanted side effects. Ideally, an effective pain medication without side effects would bind to MOR, which would bias signaling solely through the Gi pathway. We used the Bio3D library to conduct principal component analysis to compare the cryo-electron microscopy MOR structure in complex with the Gi versus an X-ray crystallography MOR structure with a nanobody acting as a Gi mimic. Our results agree with a previous study by Munro, which concluded that nanobody-bound MOR is structurally different than Gi-bound MOR. Furthermore, we investigated the structural diversity of opioids that can bind to MOR. Quantum mechanical calculations show that the low energy solution structures of fentanyl differ from the one bound to MOR in the experimental structure, and pKa calculations reveal that fentanyl is protonated in aqueous solution. Glide docking studies show that higher energy structures of fentanyl in solution form favorable docking complexes with MOR. Our calculations show the relative abundance of each fentanyl conformation in solution as well as the energetic barriers that need to be overcome to bind to MOR. Docking studies confirm that multiple fentanyl conformations can bind to the receptor. Perhaps a variety of conformations of fentanyl can stabilize multiple conformations of the MOR, which can explain why fentanyl can induce different intracellular signaling and multiple physiological effects.

Chemical Bond Overlap Descriptors From Multiconfiguration Wavefunctions

The chemical bond is a fundamental concept in chemistry, and various models and descriptors have evolved since the advent of quantum mechanics. This study extends the overlap density and its topological descriptors (OP/TOP) to multiconfigurational wavefunctions. We discuss a comparative analysis of OP/TOP descriptors using CASSCF and DCD-CAS(2) wavefunctions for a diverse range of molecular systems, including X-O bonds in X-OH (XH, Li, Na, H2B, H3C, H2N, HO, F) and Li-X' (XF, Cl, and Br). Results show that OP/TOP aligns with bonding models like the quantum theory of atoms in molecules (QTAIM) and local vibrational modes theory, revealing insights such as overlap densities shifting towards the more electronegative atom in polar bonds. The Li-F dissociation profile using OP/TOP descriptors demonstrated sensitivity to ionic/neutral inversion during Li-F dissociation, highlighting their potential for elucidating complex bond phenomena and offering new avenues for understanding multiconfigurational chemical bond dynamics.

Improving the Efficiency of Electrostatic Embedding Using the Fast Multipole Method

This paper reports the improvement in the efficiency of embedded-cluster model (ECM) calculations in ORCA thanks to the implementation of the fast multipole method. Our implementation is based on state-of-the-art algorithms and revisits certain aspects, such as efficiently and accurately handling the extent of atomic orbital shell pairs. This enables us to decompose near-field and far-field terms in what we believe is a simple and effective manner. The main result of this work is an acceleration of the evaluation of electrostatic potential integrals by at least one order of magnitude, and up to two orders of magnitude, while maintaining excellent accuracy (always better than the chemical accuracy of 1 kcal/mol). Moreover, the implementation is versatile enough to be used with molecular systems through QM/MM approaches. The code has been fully parallelized and is available in ORCA 6.0.

Ligand Many-Body Expansion as a General Approach for Accelerating Transition Metal Complex Discovery

Methods that accelerate the evaluation of molecular properties are essential for chemical discovery. While some degree of ligand additivity has been established for transition metal complexes, it is underutilized in asymmetric complexes, such as the square pyramidal coordination geometries highly relevant to catalysis. To develop predictive methods beyond simple additivity, we apply a many-body expansion to octahedral and square pyramidal complexes and introduce a correction based on adjacent ligands (i.e., the cis interaction model). We first test the cis interaction model on adiabatic spin-splitting energies of octahedral Fe(II) complexes, predicting DFT-calculated values of unseen binary complexes to within an average error of 1.4 kcal/mol. Uncertainty analysis reveals the optimal basis, comprising the homoleptic and mer symmetric complexes. We next show that the cis model (i.e., the cis interaction model solved for the optimal basis) infers both DFT- and CCSD(T)-calculated model catalytic reaction energies to within 1 kcal/mol on average. The cis model predicts low-symmetry complexes with reaction energies outside the range of binary complex reaction energies. We observe that trans interactions are unnecessary for most monodentate systems but can be important for some combinations of ligands, such as complexes containing a mixture of bidentate and monodentate ligands. Finally, we demonstrate that the cis model may be combined with Δ-learning to predict CCSD(T) reaction energies from exhaustively calculated DFT reaction energies and the same fraction of CCSD(T) reaction energies needed for the cis model, achieving around 30% of the error from using the CCSD(T) reaction energies in the cis model alone.

Monomer size effect in inelastic collisional dynamics of non-equilibrium soot nucleation

Molecular dynamics (MD) simulations of the collisional dynamics of the coronene-acepyrene and coronene radical-acepyrene pairs have been carried out to investigate the size effect of monomers of polycyclic aromatic hydrocarbons (PAH) on their non-equilibrium dimerization. The results compared to the previous MD simulations of the smaller pyrene-acepyrene and pyrenyl-acepyrene systems corroborate the non-equilibrium hypothesis of crosslinking PAH dimerization enhanced by physical interaction between the monomers. The phenomenon of inelastic collisional dynamics responsible for non-equilibrium van der Waals dimerization, which fosters a covalent bond formation between the monomers, amplifies with increasing PAH size. The increase in the size of the colliding monomers enhances the non-equilibrium effects as the growing pool of low-frequency modes provides a larger sink for the energy of the colliding PAH monomers. Based on the direct count of the crosslinking reaction events observed in the MD simulations, the forward rate constant for the coronene radical-acepyrene association is estimated at ∼10-11 cm3 molecule-1 s-1, showing a 15-fold increase with respect to the value from the statistical Rice-Ramsperger-Kassel-Marcus calculations. A comparison with the eightfold increase reported previously for the pyrenyl-acepyrene system shows that the statistical (equilibrium-based) calculations increasingly underestimate the reaction rate with the increasing size of the interacting PAHs from pyrene to coronene. The total increase of the MD-assessed rate constant for the coronene radical-acepyrene dimerization reaction as compared to pyrenyl-acepyrene is a factor of 2.4, with the overall collision efficiency to produce dimerized products growing by 30%.

Au-Catalyzed 5C Reaction of Type II Diene-Ynenes toward Dihydrosemibullvalenes: Reaction Development and Mechanistic Study

We report an unexpected gold-catalyzed 5C reaction of type II diene-ynenes to synthesize dihydrosemibullvalenes, which are potential bioisosteres for drug discovery. This 5C reaction occurs through a sequence of elementary reactions of cyclopropanation/Cope rearrangement/carbon shift/cyclopropanation/C-H insertion (shortened here as the 5C reaction), supported by quantum chemistry calculations. Mechanistic studies have also been applied to answer why type-II diene-ynenes cannot access seven-membered carbocycles-embedded bridged molecules under the gold catalysis, finding that the chair-like Cope rearrangement transition state (not the traditional boat-like transition state) is the key to the change of regiochemistry.

Local Wave Function Embedding: Correlation Regions in PNO-LCCSD(T)-F12 Calculations

Many chemical reactions affect only a rather small number of bonds, leaving the largest part of the chemical and geometrical structure of the molecules nearly unchanged. In this work we extended the previously proposed region method [J. Chem. Phys. 128, 144106 (2008)] to PNO-LCCSD(T)-F12. Using this method, we investigate whether accurate reaction energies for larger systems can be obtained by correlating only the electrons in a region of localized molecular orbitals close to the reaction center at high-level (PNO-LCCSD(T)-F12). The remainder is either treated at lower level (PNO-LMP2-F12) or left uncorrelated (Hartree-Fock frozen core). It is demonstrated that indeed the computed reaction energies converge rather quickly with the size of the correlation regions toward the results of the full calculations. Typically, 2-3 bonds from the reacting atoms need to be included to reproduce the results of the full calculations to within ±0.2 kcal/mol. We also computed spin-state energy differences in a large transition metal complex, where a factor of 15 in computation time could be saved, still yielding a result that is within ±0.1 kcal/mol of the one obtained in a full PNO-LCCSD(T)-F12 calculation.

Enantioselective interactions of aminonitrile dimers

Enantioenrichment of amino acids is essential during the early chemical evolution leading to the origin of life. However, the detailed molecular mechanisms remain unsolved. Dimerization of enantiomers is the first molecular process in the nucleation of deposition and crystallization, which are both essential for enantioenrichment. Here, we report the enantioselective interactions of dimers of chiral intermediates, i.e., aminonitriles, in both gas and water environments based on density functional theory (DFT) and more accurate coupled-cluster (CC) calculations. We show that all the aminonitriles stabilize the homochiral dimer preferentially to the heterochiral dimer in the gas phase, while this trend was not observed in water. The energies of the enantioselective interactions in aminonitriles are substantially lower compared to those in amino acids, especially isovaline. These results suggest that prebiotic enhancements of enantiomeric excess are more likely to occur in amino acids than in the aminonitrile intermediates.

Phenyl Radical Activates Molecular Hydrogen Through Protium and Deuterium Tunneling

Activating dihydrogen, H2, is a challenging endeavor typically achieved using transition metal centers. Pure main-group compounds capable of this are rare and have emerged in recent decades. These systems rely on synergistic donor-acceptor interactions with H2's antibonding σ* and bonding σ orbital. An alternative (hydrocarbon) radical-mediated activation is problematic because the H-H bond is stronger (104.2 kcal mol-1) than most C-H bonds. Here, we explore using the phenyl radical to tackle this, forming benzene with a C-H bond energy (112.9 kcal mol-1) that provides a thermodynamic driving force. We mainly observe a benzene-HI complex upon photolysis of iodobenzene in an H2-doped neon matrix at 4.4 K despite a barrier of 7.6 kcal mol-1, while phenyl radical forms in case of the heavier D2 isotopologue. When D2 molecules are allowed to diffuse, mono-deuterated benzene accumulates within hours. Computations using path integral-based instanton theory highlight that primarily the transferred hydrogen atom is moving during the reaction which greatly increases the tunneling probability. In excellent agreement with the experimental results, we predict significant tunneling rate constants for both isotopologues, H2 and D2, featuring a strong kinetic isotope effect of up to four orders of magnitude at the lowest temperatures.

Study of Sterically Crowded Alkanes: Assessment of Non-Empirical Density Functionals Including Double-Hybrid (Cost-Effective) Methods

We theoretically study here the homolytic dissociation reactions of sterically crowded alkanes of increasing size, carrying three different (bulky) substituents such as tert-butyl, adamantyl, and [1.1.1]propellanyl, employing a family of parameter-free functionals ranging from semi-local, to hybrid and double-hybrid models. The study is complemented with the interaction between a pair of HC(CH3)3 molecules at repulsive and attractive regions, as an example of a system composed by a pair of weakly bound sterically crowded alkanes. We also assessed the effect of incorporating reliable dispersion corrections (i. e., D4 or NL) for all the functionals assessed, as well as the use of a tailored basis set (DH-SVPD) for non-covalent interactions, which provides the best trade-off between accuracy and computational cost for a seemingly extended applications to branched or crowded systems. Overall, the PBE-QIDH/DH-SVPD and r2SCAN-QIDH/DH-SVPD methods represent an excellent compromise providing relatively low, and thus very competitive, errors at a fraction of the cost of other quantum-chemical methods in use.

Elementary reactions for glycine production in hot and dense interstellar media from CH 3 COOH , HCOOH, and NH 2 CH

CONTEXT

In this work, we investigate three elementary reactions involved in the production of glycine in the interstellar medium (ISM) employing trustworthy electronic structure and chemical kinetics methodologies. We considered three elementary reactions: HCOOH + NH 2 CH → NH 2 CH 2 COOH (R 1 ),CH 3 COOH + NH → CH 2 COOH + NH 2 (R 2 ) andCH 2 COOH + NH 2 → NH 2 CH 2 COOH ( R 3 ) under conditions consistent with hot molecular cores of massive star-forming regions. Our results indicate that the elementary reactions are feasible in these environments, with reaction barriers of 18.8 ( R 1 ) and 18.4 kcal · mol - 1 ( R 2 ). The rate coefficients for these reactions were calculated to be 1.4 × 10 - 17 and 9.3 × 10 - 16 cm 3 · molecule - 1 · s - 1 at 1000 K. Additionally, if the products of ( R 2 ) couple on a singlet surface, R 3 connects to the ground state of glycine via a barrierless path presenting a rate coefficient equal to 8.7 × 10 - 9 cm 3 · molecule - 1 · s - 1 at 298.15 K. Given that the molecules involved in these reactions have been detected in regions such as Sgr B2, our findings suggest that these elementary reactions should be included in mechanisms to study the production of glycine in such locations.

METHODS

The single-reference electronic structure calculations were carried out with the ORCA 4.1.2 package while the multi-reference calculations were performed with the COLUMBUS 7.0 package. The DFT functionals employed were M06-2X, ω B97X, and ω B97X-D3, with the 6-31+G* and def2-TZVP, and for the wave function-based calculations, the CCSD(T), DLPNO-CCSD(T), MRCI, and CASSF methods were employed using the aug-cc-pVDZ, aug-cc-pVTZ, and aug-cc-pVQZ basis sets. The chemical kinetic calculations for the elementary reactions with well-defined saddle points were performed using the Pilgrim package employing the TST, CVT, and CVT/SCT approaches.

A Benchmark Study of DFT-Computed p-Block Element Lewis Pair Formation Enthalpies Against Experimental Calorimetric Data

The quantification of Lewis acidity is of fundamental and applied importance in chemistry. While the computed fluoride ion affinity (FIA) is the most widely accepted thermodynamic metric, only sparse experimental values exist. Accordingly, a benchmark of methods for computing Lewis pair formation enthalpies, also with a broader set of Lewis bases against experimental data, is missing. Herein, we evaluate different density functionals against a set of 112 experimentally determined Lewis acid/base binding enthalpies and gauge influences such as solvation correction in structure optimization. From that, we can recommend r2SCAN-3c for robust quantification of this omnipresent interaction.

Pyrolytic conversion of glucose into hydroxymethylfurfural and furfural: Benchmark quantum-chemical calculations

Quantum chemical methods have been intensively applied to study the pyrolytic conversion of glucose into hydroxymethylfurfural (HMF) and furfural (FF). Herein, we collect the most relevant mechanistic proposals from the recent literature and organize them into a single reaction network. All the transition structures (TSs) and intermediates are characterized using highly accurate ab initio methods and the possible reaction pathways are assessed in terms of the Gibbs energies of the TSs and intermediates with respect to β-glucopyranose, selecting a 2D ideal-gas standard state at 773 K to represent the pyrolysis conditions. Several pathways can lead to the formation of both HMF and FF passing through rate-determining TSs that have ΔG‡ values of ~49-50 kcal/mol. Both water-assisted mechanisms and nonspecific environmental effects have a minor impact on the Gibbs energy profiles. We find that the HMF → FF + CH2O fragmentation has a small ΔrxnG value and an accessible ΔG‡ barrier. Our computational results, which are in consonance with the kinetic parameters derived from lumped models, the results of isotopic labeling experiments and the reported HMF/FF molecular ratios, could be useful for modeling studies including on nonequilibrium kinetic effects that may render more information about product yields and the relevance of the various pathways.

Generation and interception of bicyclo[3.2.1]oct-2-yne: an experimental and theoretical mechanistic study

Bicyclo[3.2.1]oct-2-yne was generated from the Fritsch-Buttenberg-Wiechell rearrangement of 2-norbornylidene carbene. The rearrangement preferentially involves migration of a tertiary carbon over a secondary carbon, a trend that contrasts with rearrangements of acyclic carbenes and which may be attributable to hyperconjugative effects promoted by the bridged structure of the carbene.

Theoretical elucidation of the structure, bonding, and reactivity of the CaMn4Ox clusters in the whole Kok cycle for water oxidation embedded in the oxygen evolving center of photosystem II. New molecular and quantum insights into the mechanism of the O-O bond formation

This paper reviews our historical developments of broken-symmetry (BS) and beyond BS methods that are applicable for theoretical investigations of metalloenzymes such as OEC in PSII. The BS hybrid DFT (HDFT) calculations starting from high-resolution (HR) XRD structure in the most stable S1 state have been performed to elucidate structure and bonding of whole possible intermediates of the CaMn4Ox cluster (1) in the Si (i = 0 ~ 4) states of the Kok cycle. The large-scale HDFT/MM computations starting from HR XRD have been performed to elucidate biomolecular system structures which are crucial for examination of possible water inlet and proton release pathways for water oxidation in OEC of PSII. DLPNO CCSD(T0) computations have been performed for elucidation of scope and reliability of relative energies among the intermediates by HDFT. These computations combined with EXAFS, XRD, XFEL, and EPR experimental results have elucidated the structure, bonding, and reactivity of the key intermediates, which are indispensable for understanding and explanation of the mechanism of water oxidation in OEC of PSII. Interplay between theory and experiments have elucidated important roles of four degrees of freedom, spin, charge, orbital, and nuclear motion for understanding and explanation of the chemical reactivity of 1 embedded in protein matrix, indicating the participations of the Ca(H2O)n ion and tyrosine(Yz)-O radical as a one-electron acceptor for the O-O bond formation. The Ca-assisted Yz-coupled O-O bond formation mechanisms for water oxidation are consistent with recent XES and very recent time-resolved SFX XFEL and FTIR results.

Enzymatic synthesis of azide by a promiscuous N-nitrosylase

Azides are energy-rich compounds with diverse representation in a broad range of scientific disciplines, including material science, synthetic chemistry, pharmaceutical science and chemical biology. Despite ubiquitous usage of the azido group, the underlying biosynthetic pathways for its formation remain largely unknown. Here we report the characterization of an enzymatic route for de novo azide construction. We demonstrate that Tri17, a promiscuous ATP- and nitrite-dependent enzyme, catalyses organic azide synthesis through sequential N-nitrosation and dehydration of aryl hydrazines. Through biochemical, structural and computational analyses, we further propose a plausible molecular mechanism for azide synthesis that sets the stage for future biocatalytic applications and biosynthetic pathway engineering.

A refreshing approach to understanding the action on DNA of vanadium (IV) and (V) complexes derived from the anticancer VCp2Cl2

A computational study based on derivatives of the anticancer VCp2Cl2 compound and their interaction with representative models of deoxyribonucleic acid (DNA) is presented. The derivatives were obtained by substituting the cyclopentadienes of VCp2Cl2 with H2O, NH3, OH-, Cl-, O2- and C2O42- ligands. The oxidation states IV and V of vanadium were considered, so a total of 20 derivative complexes are included. The complexes interactions with DNA were studied using two different models, the first model considers the interactions of the complexes with the pair Guanine-Cytosine (G-C) and the second involves the interaction of the complexes with adjacent pairs, that is, d(GG). This study compares methodologies based on density functional theory with coupled cluster like calculations (DLPNO-CCSD(T)), the gold standard of electronic structure methods. Furthermore, the change in the electron density of the hydrogen bonds that keep bonded the G-C pair and d(GG) pairs, due to the presence of vanadium (IV) and (V) complexes is rationalize. To this aim, quantities obtained from the topology of the electron densities are inspected, particularly the value of the electron density at the hydrogen bond critical points. The approach allowed to identify vanadium complexes that lead to significant changes in the hydrogen bonds indicated above, a key aspect in the understanding, development, and proposal of mechanisms of action between metal complexes and DNA.

Solvent-Directed Social Chiral Self-Sorting in Pd2L4 Coordination Cages

A family of Pd2L4 cages prepared from ligands based on an axially chiral diamino-[1,1'-biazulene] motif (serving as a unique azulene-based surrogate of the ubiquitous BINOL moiety) is reported. We show that preparing a cage starting from the racemate of a shorter bis-monodentate ligand derivative, equipped with pyridine donor groups, leads to integrative ("social") chiral self-sorting, exclusively yielding the meso-trans product, but only in a selection of solvents. This phenomenon is driven by individual solvent molecules acting as hydrogen bonding tethers between the amino groups of neighboring ligands, thereby locking the final coordination cage in a single isomeric form. The experimental (solvent-dependent NMR, single-crystal X-ray diffraction) observations of this cooperative interaction could be explained by computational analyses only when explicit solvation was considered. Furthermore, we prepared a larger chiral ligand with isoquinoline donors, which, unlike the first one, does not undergo social self-sorting from its racemic mixture, further highlighting the importance of solvents bridging short distances between the amino groups. Homochiral cages formed from this larger ligand, however, furnish a cavity that can bind anionic and neutral metal complexes such as [Pt(CN)6]2- and Cr(CO)6 and discriminate between the two enantiomers of chiral guest camphor sulfonate.

Finite-field Cholesky decomposed coupled-cluster techniques (ff-CD-CC): theory and application to pressure broadening of Mg by a He atmosphere and a strong magnetic field

For the interpretation of spectra of magnetic stellar objects such as magnetic white dwarfs (WDs), highly accurate quantum chemical predictions for atoms and molecules in finite magnetic field are required. Especially the accurate description of electronically excited states and their properties requires established methods such as those from coupled-cluster (CC) theory. However, respective calculations are computationally challenging even for medium-sized systems. Cholesky decomposition (CD) techniques may be used to alleviate memory bottlenecks. In finite magnetic field computations, the latter are increased due to the reduction of permutational symmetry within the electron-repulsion-integrals (ERIs) as well as the need for complex-valued data types. CD enables a memory-efficient, approximate description of the ERIs with rigorous error control and thus the treatment of larger systems at the CC level becomes feasible. In order to treat molecules in a finite magnetic field, we present in this work the working equations of the left and right-hand side equations for finite field (ff)-EOM-CD-CCSD for various EOM flavours as well as for the approximate ff-EOM-CD-CC2 method. The methods are applied to the study of the modification of the spectral lines of a magnesium transition by a helium atmosphere that can be found on magnetic WD stars.

Tricyanomethane or Dicyanoketenimine-Silylation makes the Difference

Pseudohalides such as tricyanomethanide, [C(CN)3]-, are well known in chemistry, biochemistry and industrial chemistry. The protonated species HC(CN)3, a classic hydrogen pseudohalide Brønsted acid, is a very strong acid with a pKa value of -5. However, HC(CN)3 is difficult to handle as it tends to decompose rapidly or, more precisely, to oligo- and polymerize. Therefore, silylated pseudohalide compounds with the [Me3Si]+ as the "big organometallic proton" have become interesting, exhibiting similar chemical properties but better kinetic protection. Here, the stepwise silylation of the pseudohalide anion [C(CN)3]- is reported, forming the heavier homologue of HC(CN)3, namely [Me3Si][C(CN)3], and in presence of two additional [Me3Si]+ cations even the dicationic species [(Me3Si-NC)3C]2+ as stable [B(C6F5)4]- salt. Surprisingly, in contrast to the protonated species HC(CN)3, in which the proton is bound to the central carbon atom of [C(CN)3]-, silylation of the [C(CN)3]- anion occurs at one of the three terminal nitrogen atoms, thus forming the long-sought dicyanoketenimine [Me3Si-NC-C(CN)2]. All further silylation steps take place exclusively on the terminal N atoms of the three CN groups and not on the central carbon atom, until the intriguing, highly symmetrical dication, [(Me3Si-NC)3C]2+, is finally generated. The experimental data are supported by quantum chemical calculations in terms of thermodynamics and chemical bonding.

Calculated ionization energies, orbital eigenvalues (HOMO), and related QSAR descriptors of organic molecules: a set of 61 experimental values enables elimination of systematic errors and provides realistic error estimates

Ionization energies (IEs) of organic compounds come in different forms-adiabatic, vertical, as electrode potentials, or as orbital eigenvalues via Koopmans' theorem. They have been linked to the reactivity towards electrophiles and have been used to quantitatively describe electron transfer processes. The de novo prediction of IEs is only meaningful when an estimate of the prediction's uncertainty is included. Pulsed-field ionization (PFI) experiments have quantified adiabatic IEs with unprecedented precision. In this work, a new set of PFI-derived IEs is compiled from the literature as a benchmark for prediction methods. This set includes many common functional groups, a size range from diatomics to two aromatic rings, and IEs between 7 and 14 eV. The first-principles CCSD(T)/CBS protocol presently used reproduces these values within 0.05 eV. For adiabatic IEs and vertical IEs/orbital eigenvalues predicted using approximate density functional theory (DFT), linear regression models are proposed, so that IEs calculated using different methods can be directly compared on a physical scale. This elimination of systematic errors improves the error statistics and allows the performance of predicted IEs to be evaluated if used in quantitative structure-property or -activity relationships, as the latter implicitly correct a descriptor's bias. Owing to the structural scope of the test set, the minimum and maximum deviations from experiment should correspond to those expected for common organic molecules. Deviations from reference values found for orbital eigenvalues but also for IEs calculated explicitly with HF or semi-empirical MO methods were as large as 0.5 eV to 2.0 eV. Such large errors could also propagate into quantitative structure-property models, as shown in illustrative examples of oxidation rate constants in solution.

Spectroscopic and Quantum Chemical Evidence of Amine-CO2 and Alcohol-CO2 Interactions: Confirming an Intriguing Affinity of CO2 to Monoethanolamine (MEA)

A recent broadband rotational spectroscopic investigation of the cross-association mechanisms of CO2 with monoethanolamine (MEA) in molecular beams [F. Xie et al., Angew. Chem. Int. Ed., 2023, 62, e202218539] revealed an intriguing affinity of CO2 to the hydroxy group. These findings have triggered the present systematic vibrational spectroscopic exploration of weakly bound amine··CO2 and alcohol··CO2 van der Waals cluster molecules embedded in inert "quantum" matrices of neon at 4.2 K complemented by high-level quantum chemical conformational analyses. The non-covalent interactions formed between the amino and hydroxy groups and the electron-deficient carbon atom of CO2 are demonstrated to lift the degeneracy of the doubly degenerate intramolecular CO2-bending fundamental significantly with characteristic observed spectral splittings for the amine··CO2 (≈35-45 cm-1) and alcohol··CO2 (≈20-25 cm-1) interactions, respectively, despite the almost identically predicted total association energies (≈12-14 kJ·mol-1) for these van der Waals contacts, as revealed by benchmark Domain-based Local Pair Natural Orbital Coupled Cluster DLPNO-CCSD(T) theory. These high-level theoretical predictions reveal significantly higher "geometry preparation energies" for the amine··CO2 systems leading to a more severe distortion of the CO2 linearity upon complexation in agreement with the infrared spectroscopic findings. The systematic combined spectroscopic and quantum chemical evidences for cross-association between CO2 and amines/alcohols in the present work unambiguously confirm an intriguing binding preference of CO2 to the hydroxy group of the important carbon capture agent MEA, with an accurate vibrational zero-point energy corrected association energy (D0) of 13.5 kJ·mol-1 at the benchmark DLPNO-CCSD(T)/aug-cc-pV5Z level of theory.

Rationalizing polymorphism with local correlation-based methods: a case study of pnictogen molecular crystals

A computational workflow is proposed to quantify and rationalize the relative stability of different structures of molecular crystals using cluster models and quantum chemical methods. The Hartree-Fock plus London Dispersion (HFLD) scheme is used to estimate the lattice energy of molecular crystals in various structural arrangements. The fragment-pairwise Local Energy Decomposition (fp-LED) scheme is then employed to quantify the key intermolecular interactions responsible for the relative stability of different crystal structures. The fp-LED scheme provides also in-depth chemical insights by decomposing each interaction into energy components such as dispersion, electrostatics, and exchange. Notably, this analysis requires only a single interaction energy computation per structure on a suitable cluster model. As a case study, two polymorphs of each of the following are considered: naphthyl-substituted dipnictanes (with As, Sb, and Bi as the pnictogen atom) and tris(thiophen-2-yl)bismuthane. The approach outlined offers high accuracy as well as valuable insights for developing design principles to engineer crystal structures with tailored properties, opening up new avenues in the study of molecular aggregates, potentially impacting diverse fields in materials science and beyond.

Cluster perturbation theory. X. A parallel implementation of Lagrangian perturbation series for the coupled cluster singles and doubles ground-state energy through fifth order

We describe an efficient implementation of cluster perturbation and Møller-Plesset Lagrangian energy series through the fifth order that targets the coupled cluster singles and doubles energy utilizing the resolution of the identity approximation. We illustrate the computational performance of the implementation by performing ground state energy calculations on systems with up to 1200 basis functions using a single node and by comparison to conventional coupled cluster singles and doubles calculations. We further show that our hybrid message passing interface/open multiprocessing parallel implementation that also utilizes graphical processing units can be used to obtain fifth order energies on systems with almost 1200 basis functions with a 90 min "time to solution" running on Frontier at Oak Ridge National Laboratory.

"Best" Iterative Coupled-Cluster Triples Model? More Evidence for 3CC

To follow up on the unexpectedly good performance of several coupled-cluster models with approximate inclusion of 3-body clusters [Rishi, V.; Valeev, E. F. J. Chem. Phys. 2019, 151, 064102.] we performed a more complete assessment of the 3CC method [Feller, D. . J. Chem. Phys. 2008, 129, 204105.] for accurate computational thermochemistry in the standard HEAT framework. New spin-integrated implementation of the 3CC method applicable to closed- and open-shell systems utilizes a new automated toolchain for derivation, optimization, and evaluation of operator algebra in many-body electronic structure. We found that with a double-ζ basis set the 3CC correlation energies and their atomization energy contributions are almost always more accurate (with respect to the CCSDTQ reference) than the CCSDT model as well as the standard CCSD(T) model. The mean absolute errors in cc-pVDZ {3CC, CCSDT, and CCSD(T)} electronic (per valence electron) and atomization energies relative to the CCSDTQ reference for the HEAT data set [Tajti, A. . J. Chem. Phys. 2004, 121, 11599-11613.], were {24, 70, 122} μEh/e and {0.46, 2.00, 2.58} kJ/mol, respectively. The mean absolute errors in the complete-basis-set limit {3CC, CCSDT, and CCSD(T)} atomization energies relative to the HEAT model reference, were {0.52, 2.00, and 1.07} kJ/mol, The significant and systematic reduction of the error by the 3CC method and its lower cost than CCSDT suggests it as a viable candidate for post-CCSD(T) thermochemistry applications, as well as the preferred alternative to CCSDT in general.

Formation of atmospheric molecular clusters containing nitric acid with ammonia, methylamine, and dimethylamine

This study investigates the formation of atmospheric molecular clusters containing ammonia (NH3, A), methylamine (CH3NH2, MA), or dimethylamine (CH3NHCH3, DMA) with nitric acid (HNO3, NA) using quantum mechanics. The Atmospheric Cluster Dynamic Code (ACDC) was employed to simulate the total evaporation rate, formation rate, and growth pathways of three types of clusters under dry and hydrated conditions. This study evaluates the enhancing potential of A/MA/DMA for NA-based new particle formation (NPF) at parts per trillion (ppt) levels. The results indicate that A/MA/DMA can enhance NA-based NPF at high nitric acid concentrations and low temperatures in the atmosphere. The enhancing potential of MA is weaker than that of DMA but stronger than that of A. Cluster growth predominantly follows the lowest free energy pathways on the acid-base grid, with the formation of initial acid-base dimers (NA)(A), (NA)(MA), and (NA)(DMA) being crucial. Hydration influences the evaporation rate and formation rate of clusters, especially for initial clusters. When the humidity is at 100%, the formation rate for NA-A, NA-MA, and NA-DMA clusters can increase by approximately 109, 107, and 104-fold compared to the corresponding unhydrated clusters, respectively. These results highlight the significance of nitric acid nucleation in NPF events in low-temperature, high-humidity atmospheres, particularly in regions like China with significant automobile exhaust pollution.

Automatic Orbital Pair Selection for Multilevel Local Coupled-Cluster Based on Orbital Maps

We present an automatic, orbital-map based orbital-pair selection scheme for multilevel local coupled-cluster approaches that exploits the locality of chemical reactions by focusing on the part of the molecule directly involved in the reaction. The previously introduced pair-selected multilevel extension to domain-based local pair natural orbital coupled-cluster with singles, doubles, and semicanonical perturbative triples [DLPNO-CCSD(T0)] partitions the orbital pairs according to relative changes in pair correlation energies [Bensberg, M.; Neugebauer, J. Orbital pair selection for relative energies in the domain-based local pair natural orbital coupled-cluster method. J. Chem. Phys. 2022, 157, 064102. 10.1063/5.0100010]. To this end, maps between localized orbitals are required which in turn require maps between the atoms of structures along reaction paths. So far, these atom maps have been manually determined, which can be a (human) time-consuming procedure. Here, we present an automatic atom mapping algorithm based on the principle of minimum chemical distance that incorporates orientation dependence through dihedral angles. A similar strategy is then introduced to obtain orbital maps, which proves advantageous over the previously used direct orbital selection. Along with a modified orbital pair prescreening, this results in an improved variant of the pair-selected multilevel DLPNO-CCSD(T0) method. The performance of this approach is demonstrated for various reaction types showing a significant efficiency gain and accurate results due to beneficial, systematic error cancellation. The presented method operates in a black-box manner due to its fully automatized algorithms with only the need to specify a single target-accuracy parameter. Additionally, we demonstrate that basis set extrapolation techniques can be applied. In this context, the approach shows deficiencies for the use of large basis sets, especially with diffuse basis functions, which can be traced back to the semicanonical triples correction.

Progress in Multiscale Modeling of Silk Materials

As a result of their hierarchical structure and biological processing, silk fibers rank among nature's most remarkable materials. The biocompatibility of silk-based materials and the exceptional mechanical properties of certain fibers has inspired the use of silk in numerous technical and medical applications. In recent years, computational modeling has clarified the relationship between the molecular architecture and emergent properties of silk fibers and has demonstrated predictive power in studies on novel biomaterials. Here, we review advances in modeling the structure and properties of natural and synthetic silk-based materials, from early structural studies of silkworm cocoon fibers to cutting-edge atomistic simulations of spider silk nanofibrils and the recent use of machine learning models. We explore applications of modeling across length scales: from quantum mechanical studies on model peptides, to atomistic and coarse-grained molecular dynamics simulations of silk proteins, to finite element analysis of spider webs. As computational power and algorithmic efficiency continue to advance, we expect multiscale modeling to become an indispensable tool for understanding nature's most impressive fibers and developing bioinspired functional materials.

Reduced Scaling Correlated Natural Transition Orbitals for Multilevel Coupled Cluster Calculations

Multilevel coupled cluster theory offers reduced scaling computation of intensive properties in systems that are too large for standard coupled cluster calculations. A significant benefit of the multilevel coupled cluster framework is the possibility of calculating intensive properties that are not tightly localized if an appropriate set of active orbitals is used. Correlated natural transition orbitals (CNTOs) are tailored to describe excitation processes. For multilevel coupled cluster singles and doubles (MLCCSD) and singles and perturbative doubles (MLCC2) calculations, the construction of CNTOs generally becomes the computational bottleneck. Here, we demonstrate how CNTOs can be obtained with O ( N 3 ) operations, eliminating the O ( N 5 ) -scaling steps involved in the original approach. This reduction in scaling moves the bottleneck of MLCC2 and MLCCSD calculations from the active orbital space preparation to the MLCC2 and MLCCSD equations with O ( N 4 ) -scaling.

Comparing coupled cluster and composite quantum chemical methods for computing activation energies and reaction enthalpies of radical propagation reactions

Accurate determination of activation energies and reaction enthalpies is essential for understanding the propagation step in free radical polymerization, as it significantly affects polymer chain length and structure. In this study, we compare DLPNO-CCSD(T) to canonical CCSD(T) for 17 radical addition activation energies and 18 reaction enthalpies from Radom and Fischer's test set. Additionally, we compare the computationally efficient composite methods G3(MP2)-RAD and CBS-RAD against CCSD(T)/aug-cc-pVTZ and DLPNO-CCSD(T)/CBS methods. Compared to the CCSD(T)/aug-cc-pVTZ reference, our results indicate that DLPNO-CCSD(T)/CBS with unrestricted Hartree-Fock (UHF) or UB3LYP reference orbitals and NormalPNO parameters consistently achieves chemical accuracy, with mean absolute deviations of 3.5 kJ mol-1 for activation energies and 1.5 kJ mol-1 for reaction enthalpies. Comparing the two composite methods shows that CBS-RAD agrees most closely with coupled cluster reaction enthalpies, while G3(MP2)-RAD tracks the activation energies most closely.

Design of Anti-Hund Organic Emitters Based on Heptazine

Hund's rule, which is powerful in governing the first excited states of closed-shell organic materials, can hardly be violated to get inverted singlet-triplet gap (INVEST) molecules with negative singlet-triplet energy gaps (ΔEST), although INVEST materials have shown extraordinary photophysical properties and promising device performance especially in light-emitting diodes. Here, we propose a facile strategy to construct emissive INVEST molecules by introducing different types of substituents to heptazine in various modes, which can effectively tune the ΔEST to be negative with the enlarged oscillator strength (f) for the high fluorescence rate of the heptazine derivatives. Systematic computational studies show that the double substitution of electron-donating units with another nonconjugated substituent in hybrid substitution mode is the most favorable way in achieving slightly negative ΔEST and large f values; the conjugated substituent will compete with heptazine to make the molecule deviate from the INVEST feature. Especially, a series of high-performance heptazine-based INVEST emitters were constructed, exhibiting ΔEST low to -0.362 eV, f up to 0.0436, as well as a wide range emission color from 339 to 716 nm. Also, the designed molecules were predicted to have fluorescence radiative rates up to 106 s-1, along with efficient reverse intersystem crossing rates reaching 108 s-1. Importantly, the figure of merit (FM) was first proposed as a parameter to wholly evaluate the performance of INVEST emitters, and the highest FM of 0.198 was found in the triazine and double nonconjugated amine-substituted heptazine. These results highlight the great potential of the heptazine chromophore in constructing INVEST emitters, revealing fundamental structure-property understandings for the material design of efficient anti-Hund organic molecules with improved emission properties.

Intermolecular interaction energies with AROFRAG-A systematic approach for fragmentation of aromatic molecules

Intermolecular interactions with polycyclic aromatic hydrocarbons (PAHs) represent an important area of physisorption studies. These investigations are often hampered by a size of interacting PAHs, which makes the calculation prohibitively expensive. Therefore, methods designed to deal with large molecules could be helpful to reduce the computational costs of such studies. Recently we have introduced a new systematic approach for the molecular fragmentation of PAHs, denoted as AROFRAG, which decomposes a large PAH molecule into a set of predefined small PAHs with a benzene ring being the smallest unbreakable unit, and which in conjunction with the Molecules-in-Molecules (MIM) approach provides an accurate description of total molecular energies. In this contribution we propose an extension of the AROFRAG, which provides a description of intermolecular interactions for complexes composed of PAH molecules. The examination of interaction energy partitioning for various test cases shows that the AROFRAG3 model connected with the MIM approach accurately reproduces all important components of the interaction energy. An additional important finding in our study is that the computationally expensive long-range electron-correlation part of the interaction energy, that is, the dispersion component, is well described at lower AROFRAG levels even without MIM, which makes the latter models interesting alternatives to existing methods for an accurate description of the electron-correlated part of the interaction energy.

A static quantum embedding scheme based on coupled cluster theory

We develop a static quantum embedding scheme that utilizes different levels of approximations to coupled cluster (CC) theory for an active fragment region and its environment. To reduce the computational cost, we solve the local fragment problem using a high-level CC method and address the environment problem with a lower-level Møller-Plesset (MP) perturbative method. This embedding approach inherits many conceptual developments from the hybrid second-order Møller-Plesset (MP2) and CC works by Nooijen [J. Chem. Phys. 111, 10815 (1999)] and Bochevarov and Sherrill [J. Chem. Phys. 122, 234110 (2005)]. We go beyond those works here by primarily targeting a specific localized fragment of a molecule and also introducing an alternative mechanism to relax the environment within this framework. We will call this approach MP-CC. We demonstrate the effectiveness of MP-CC on several potential energy curves and a set of thermochemical reaction energies, using CC with singles and doubles as the fragment solver, and MP2-like treatments of the environment. The results are substantially improved by the inclusion of orbital relaxation in the environment. Using localized bonds as the active fragment, we also report results for N=N bond breaking in azomethane and for the central C-C bond torsion in butadiene. We find that when the fragment Hilbert space size remains fixed (e.g., when determined by an intrinsic atomic orbital approach), the method achieves comparable accuracy with both a small and a large basis set. Additionally, our results indicate that increasing the fragment Hilbert space size systematically enhances the accuracy of observables, approaching the precision of the full CC solver.

Assessing the Noncovalent Interaction of Deucravacitinib and Ethanol with Special Reference to an Independent Gradient Model Based on Hirshfeld Partition

The current study begins by optimizing the deucravacitinib molecule in the gas phase at the ωB97XD/cc-pVDZ level of theory using density functional theory and proceeds to study its intramolecular interactions. Further, a molecule of EtOH was introduced at different locations on the deucravacitinib molecule, and the noncovalent interactions arising from them were also investigated using several computational tools. In this way, eight deucravacitinib-EtOH systems (1-8) were identified and their electronic environment was studied after evaluating their binding energy. Using natural bond orbital analysis, the localization of charges between the donor and acceptor fragments in these interacting systems was examined. The nature of interactions was analyzed using the reduced gradient approach (NCI analysis), and few hydrogen bonding interactions (intermolecular and intramolecular) were found in each system. The strength of these hydrogen bonding interactions was further investigated by using theoretical tools such as atoms in molecules analysis and independent gradient model based on Hirshfeld partition analysis. The binding energy of deucravacitinib with EtOH was decomposed into energy components based on the domain-based local pair natural orbital coupled cluster technique using LED analysis. The results from the hydrogen bonding interaction analysis using different computational tools were found to be consistent with the calculated order of binding energy of systems 1-8 and they also pointed toward the higher stability of system 3.

Reductive Elimination From Tetra-Alkyl Cuprates [MenCu(CF3)4-n]- (n=0-4): Beyond Simple Oxidation States

In recent years, the electronic structures of organocuprates in general and the complex [Cu(CF3)4]- in particular have attracted significant interest. A possible key indicator in this context is the reactivity of these species. Nonetheless, this aspect has received only limited attention. Here, we systematically study the series of tetra-alkyl cuprates [MenCu(CF3)4-n]- and their unimolecular reactivity in the gas phase, which includes concerted formal reductive eliminations as well as radical losses. Through computational studies, we characterize the electronic structures of the complexes and show how these are connected to their reactivity. We find that all [MenCu(CF3)4-n]- ions feature inverted ligand fields and that the distinct reactivity patterns of the individual complexes arise from the interplay of different effects.

Prospects for rank-reduced CCSD(T) in the context of high-accuracy thermochemistry

Obtaining sub-chemical accuracy (1 kJ mol-1) for reaction energies of medium-sized gas-phase molecules is a longstanding challenge in the field of thermochemical modeling. The perturbative triples correction to coupled-cluster single double triple [CCSD(T)] constitutes an important component of all high-accuracy composite model chemistries that obtain this accuracy but can be a roadblock in the calculation of medium to large systems due to its O(N7) scaling, particularly in HEAT-like model chemistries that eschew separation of core and valence correlation. This study extends the work of Lesiuk [J. Chem. Phys. 156, 064103 (2022)] with new approximate methods and assesses the accuracy of five different approximations of (T) in the context of a subset of molecules selected from the W4-17 dataset. It is demonstrated that all of these approximate methods can achieve sub-0.1 kJ mol-1 accuracy with respect to canonical, density-fitted (T) contributions with a modest number of projectors. The approximation labeled Z̃T appears to offer the best trade-off between cost and accuracy and shows significant promise in an order-of-magnitude reduction in the computational cost of the CCSD(T) component of high-accuracy model chemistries.

Improving Predictions of Spin-Crossover Complex Properties through DFT Calculations with a Local Hybrid Functional

We conducted a study on the performance of the local hybrid exchange-correlation functional PBE0r for a set of 95 experimentally characterized iron spin-crossover (SCO) complexes [Vennelakanti, V.; J. Chem. Phys. 2023, 159, 024120]. The PBE0r functional is a variant of PBE0 where the exchange correction is restricted to on-site terms formulated on the basis of local orbitals. We determine the free parameters of the PBE0r functional against the experimental data and other hybrid functionals. With a Hartree-Fock (HF) exchange factor of 4%, the PBE0r functional accurately reproduces the electronic and free-energy trends predicted in prior DFT studies for these 95 complexes by using the B3LYP functional. Larger values of HF exchange stabilize high-spin states. The PBE0r-predicted bond lengths tend to exceed the experimental bond lengths, although bond lengths are less sensitive to HF exchange than in global hybrids. The predicted SCO transition temperatures T1/2 from PBE0r correlate moderately with the experimental transition temperatures, showing a slight improvement compared to the previous modB3LYP-predicted T1/2. This study suggests that the PBE0r functional is computationally cost-effective and offers the possibility of simulating larger complexes with accuracy comparable to global hybrid functionals, provided the HF-exchange parameter is carefully optimized.

Photoelectron Spectroscopy and Theoretical Studies on the Structures and Properties of Ge4C- and Ge4CH- Clusters

We investigate the structures and properties of Ge4C-/0 and Ge4CH-/0 clusters using anion photoelectron spectroscopy and theoretical calculations. Our calculations show that the first two low-lying isomers coexist in the experiments of Ge4C- and Ge4CH-. The first two low-lying isomers of Ge4C- have trigonal bipyramidal structures with the C atom on the equatorial plane and the top vertex, respectively. It is found that the first two low-lying isomers of Ge4CH- can be obtained by adding an H atom to the top and equatorial C atoms of Ge4C-, respectively. The AdNDP analyses reveal that the C atom in Ge4C forms one 4c-2e σ bond, two 4c-2e π bonds, and one 5c-2e σ bond with Ge atoms. The C atom in Ge4C interacts with an H- forming a C-H σ bond in Ge4CH-. AIMD simulation results indicate high dynamic stabilities of Ge4C and Ge4CH- at 300 and 500 K. Our results show that the structures and chemical bonding of Ge4B- and Ge4N+ are similar to those of Ge4C, while those of Ge4BH2- and Ge4NH resemble those of Ge4CH-.

A comprehensive understanding of the mechanism of the biomimetic total synthesis of brevianamide A

Recently, several studies on the chemical synthesis of brevianamide A (BA) were reported. In particular, a highly efficient and remarkably selective synthetic strategy was reported by Lawrence's group. However, a unified mechanistic understanding of these results is still lacking. We have carried out a DFT study and proposed a unified mechanism to understand these experimental results. Starting from intermediate 2, the most favorable reaction sequence is a fast tautomerization, followed by a σ-migration of the base moiety, and a final inverse-electron demanding Diels-Alder reaction, resulting in the formation of the BA product stereoselectively. This reaction mechanism can also be applied to understand the biosynthesis of BA that involves enzymatic catalysis.

From unbound to bound states: Ab initio molecular dynamics of ammonia clusters with an excess electron

Ab initio molecular dynamics simulations of negatively charged clusters of 2-48 ammonia molecules were performed to elucidate the electronic stability of the excess electron as a function of cluster size. We show that while the electronic stability of finite temperature clusters increases with cluster size, as few as 5-7 ammonia molecules can bind an excess electron, reaching a vertical binding energy slightly less than half of the bulk value for the largest system studied. These results, which are in agreement with previous studies wherever available, allowed us to analyze the excess electron binding patterns in terms of its radius of gyration and shape anisotropy and provide a qualitative interpretation based on a particle-in-a-spherical-well model.

Assessment of the similarity-transformed equation of motion (STEOM) for open-shell organic and transition metal molecules

This study benchmarks the newly re-implemented single-reference excited-state methods, IP-EOM-CCSD, EA-EOM-CCSD, and STEOM-CCSD, in ORCA6.0, with a focus on open-shell systems. We compare STEOM against EOM-CCSD, CC3, and CCSDT across a range of systems, including small organic radicals, hydrated transition metal (TM) ions, and TM diatomic systems with both closed and open-shell configurations. For organic radicals, STEOM and EOM-CCSD show comparable performance, aligning closely with CC3 and CCSDT results. In the case of hydrated TM ions, IP-EOM closely matches DLPNO-CCSD results, while deviations from DLPNO-CCSD(T) are consistent. For open-shell TM systems, IP-EOM exhibits a blueshift relative to both the DLPNO-CCSD methods, while EA-EOM-CCSD shows better agreement. When comparing STEOM and CC3 to CCSDT, STEOM shows slightly larger deviations in closed-shell systems but shows excellent agreement in open-shell systems. Computational efficiency is also assessed, revealing a significant speedup in ORCA 6.0 compared to ORCA 5.0, with optimizations improving computation times. This study provides valuable insights into the performance and efficiency of STEOM in various chemical environments, highlighting its strengths and limitations.

Catalytic prenyl conjugate additions for synthesis of enantiomerically enriched PPAPs

Polycyclic polyprenylated acylphloroglucinols (PPAPs) are a class of >400 natural products with a broad spectrum of bioactivity, ranging from antidepressant and antimicrobial to anti-obesity and anticancer activity. Here, we present a scalable, regio-, site-, and enantioselective catalytic method for synthesis of cyclic β-prenyl ketones, compounds that can be used for efficient syntheses of many PPAPs in high enantiomeric purity. The transformation is prenyl conjugate addition to cyclic β-ketoesters promoted by a readily accessible chiral copper catalyst and involving an easy-to-prepare and isolable organoborate reagent. Reactions reach completion in just a few minutes at room temperature. The importance of this advance is highlighted by the enantioselective preparation of intermediates previously used to generate racemic PPAPs. We also present the enantioselective synthesis of nemorosonol (14 steps, 20% yield) and its one-step conversion to another PPAP, garcibracteatone (52% yield).

Exploring the potential of natural orbital functionals

In recent years, Natural Orbital Functional (NOF) theory has gained increasing significance in quantum chemistry, successfully addressing one of the field's most challenging problems: providing an accurate and balanced description of systems with strong electronic correlation. The quest for NOFs that strike the delicate balance between computational tractability and predictive accuracy represents a holy grail for researchers. Today, NOFs provide an alternative formalism to both density functional and wavefunction-based methods, with their appeal rooted in a wonderfully simple conceptual framework. This perspective outlines the basic concepts, strengths and weaknesses, and current status of NOFs, while offering suggestions for their future development.