Efficient, approximate and parallel Hartree–Fock and hybrid DFT calculations. A ‘chain-of-spheres’ algorithm for the Hartree–Fock exchange

The structure of Mn(II)-bound Rubisco from Spinacia oleracea

The rate of photosynthesis and, thus, CO2 fixation, is limited by the rate of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Not only does Rubisco have a relatively low catalytic rate, but it also is promiscuous regarding the metal identity in the active site of the large subunit. In Nature, Rubisco binds either Mg(II) or Mn(II), depending on the chloroplastic ratio of these metal ions; most studies performed with Rubisco have focused on Mg-bound Rubisco. Herein, we report the first crystal structure of a Mn-bound Rubisco, and we compare its structural properties to those of its Mg-bound analogues.

Carbon-sulfur bond elongation as the promoting reaction coordinate in the efficient sub-nanosecond intersystem crossing in thianaphthene derivatives

Thiophene derivatives have become integral to OLEDs, photovoltaics, and photodynamic therapy research. A deeper understanding of their excited state dynamics and electronic relaxation mechanisms is expected to provide important physical insights of direct relevance for these applications. In this study, thianaphthene (TN), 2-methylbenzothiophene (2MBT), and 3-methylbenzothiophene (3MBT) are investigated using femtosecond broadband transient absorption and steady-state spectroscopy techniques along with time-dependent density functional calculations in cyclohexane and acetonitrile. The photophysical properties and electronic relaxation mechanisms of these derivatives are elucidated. Small fluorescence quantum yields ranging from 0.4 to 1.1% are measured. It is demonstrated that excitation of TN at 290 nm leads primarily to intersystem crossing to the triplet manifold with a lifetime of 400 ± 15 ps in either solvent, whereas four- to twofold shorter intersystem crossing lifetimes are measured for 2MBT and 3MBT depending on whether cyclohexane or acetonitrile is used. Linear interpolation of internal coordinates evidence that elongation of the S-C bonds enables ultrafast intersystem crossing in these thiophene derivatives involving singlet and triplet states with ππ* and πσ* characters. Excitation at 266 nm results in an additional 5 ± 1 ps lifetime, which is assigned to intramolecular vibrational relaxation dynamics occurring in the excited singlet state.

Molecular interactions of photosystem I and ZIF-8 in bio-nanohybrid materials

Bio-nanohybrid devices featuring natural photocatalysts bound to a nanostructure hold great promise in the search for sustainable energy conversion. One of the major challenges of integrating biological systems is protecting them against harsh environmental conditions while retaining, or ideally enhancing their photophysical properties. In this mainly computational work we investigate an assembly of cyanobacterial photosystem I (PS I) embedded in a metal-organic framework (MOF), namely the zeolitic imidazolate framework ZIF-8. This complex has been reported experimentally [Bennett et al., Nanoscale Adv., 2019, 1, 94] but so far the molecular interactions between PS I and the MOF remained elusive. We show via absorption spectroscopy that PS I remains intact throughout the encapsulation-release cycle. Molecular dynamics (MD) simulations further confirm that the encapsulation has no noticeable structural impact on the photosystem. However, the MOF building blocks frequently coordinate to the Mg2+ ions of chlorophylls in the periphery of the antenna complex. High-level quantum mechanical calculations reveal charge-transfer interactions, which affect the excitonic network and thereby may reversibly change the fluorescence properties of PS I. Nevertheless, our results highlight the stability of PS I in the MOF, as the reaction center remains unimpeded by the heterogeneous environment, paving the way for applications in the foreseeable future.

Multi-GPU RI-HF Energies and Analytic Gradients─Toward High-Throughput Ab Initio Molecular Dynamics

This article presents an optimized algorithm and implementation for calculating resolution-of-the-identity Hartree-Fock (RI-HF) energies and analytic gradients using multiple graphics processing units (GPUs). The algorithm is especially designed for high throughput ab initio molecular dynamics simulations of small and medium size molecules (10-100 atoms). Key innovations of this work include the exploitation of multi-GPU parallelism and a workload balancing scheme that efficiently distributes computational tasks among GPUs. Our implementation also employs techniques for symmetry utilization, integral screening, and leveraging sparsity to optimize memory usage. Computational results show that the implementation achieves significant performance improvements, including over 3 × speedups in single GPU AIMD throughput compared to previous GPU-accelerated RI-HF and traditional HF methods. Furthermore, utilizing multiple GPUs can provide superlinear speedup when the additional aggregate GPU memory allows for the storage of decompressed three-center integrals.

Use of Multigrids to Reduce the Cost of Performing Interpolative Separable Density Fitting

In this article, we present an interpolative separable density fitting (ISDF)-based algorithm to calculate the exact exchange in periodic mean field calculations. In the past, decomposing the two-electron integrals into the tensor hypercontraction (THC) form using ISDF was the most expensive step of the entire mean field calculation. Here, we show that by using a multigrid-ISDF algorithm, both the memory and the CPU cost of this step can be reduced. The CPU cost is brought down from cubic scaling to quadratic scaling with a low computational prefactor which reduces the cost by almost 2 orders of magnitude. Thus, in the new algorithm, the cost of performing ISDF is largely negligible compared to other steps. Along with the CPU cost, the memory cost of storing the factorized two-electron integrals is also reduced by a factor of up to 35. With the current algorithm, we can perform Hartree-Fock calculations on a diamond supercell containing more than 17,000 basis functions and more than 1500 electrons on a single node with no disk usage. For this calculation, the cost of constructing the exchange matrix is only a factor of 4 slower than the cost of diagonalizing the Fock matrix. Augmenting our approach with linear scaling algorithms can further speed up the calculations.

Levamisole Based Co(II) Single-Ion Magnet

A new Co(II) complex, [Co(NCS)2(L)2] (1) has been synthesized based on levamisole (L) as a new ligand. Single-crystal X-ray diffraction analyses confirm that the Co(II) ion is having a distorted tetrahedral coordination geometry in the complex. Notably strong intramolecular S⋅⋅⋅S and S⋅⋅⋅N interactions has been confirmed by employing Quantum Theory of Atoms in Molecules (QTAIM). These intramolecular interactions occur among the sulfur and nitrogen atoms of the levamisole ligands and also the nitrogen atoms of the thiocyanate. Direct current (dc) magnetic analyses reveal presence of zero field splitting (ZFS) and large magnetic anisotropy on Co(II). Detailed ab initio ligand field theory calculations quantitatively predicted the magnitude of ZFS. Prominent field-induced single-ion magnet (SIM) behavior was observed for 1 from dynamic magnetization measurements. Slow magnetic relaxation follows an Orbach mechanism with the effective energy barrier Ueff=29.6 (7) K and relaxation time τo=1.4 (4)×10-9 s.

Unraveling the Roles of Ionic Size and Hydrogen Bonding in Electric Field-Driven Ion Emission from Hydroxylamine Nitrate-Based Ionic Liquids

Ionic size and hydrogen bonding (HB) may play significant roles in controlling ion emission from HAN (hydroxylamine nitrate)-based ionic liquids (ILs) but have received little attention. In this paper, the ion emission behavior and mechanism in an external electric field are meticulously investigated using the molecular dynamics (MD) method and density functional theory. We find that the higher the proportion of ionic HAN in the blend of ILs, the longer the delay time of the ion start-up emission. In the positive mode, cations can evaporate directly from the surface of the studied ILs and manifest exclusively as the [EMIM]+ monomers within the simulation time scale, whereas in the negative mode, a variety of complicated anion clusters are emitted. As a result, the average charge-to-mass ratio of the positively charged species remarkably exceeds that of the negatively charged species. This large difference is attributed to the relatively larger size of the [EMIM]+ ion and the absence of substantial HB interactions between the [EMIM]+ ion and any other monomer, leading to diminished binding energies. Conversely, the strong HB interactions, primarily constituted by N-H--O and O-H--O hydrogen bonds, are clearly found in the [EtSO4]--based and HAN-based clusters. In addition, the [NO3]- and [EtSO4]- ions tend to combine with the small-sized [HA]+ ions to form large anion clusters rather than with the [EMIM]+ ions. The energy decomposition results further elucidate that the orbital interaction plays a pivotal role in the [NO3]- and [EtSO4]--based clusters. The findings clearly elucidate the experimental phenomena observed in previous studies and have implications for the formulation of multimode IL propellants.

Synthesis, structural and magnetic properties of cobalt(ii) complexes with pyridine-based macrocyclic ligand containing two pyridine pendant arms

With the aim of tuning the magnetic anisotropy, a series of Co(ii) complexes with the general formula of complex cations [Co(L)X]+, where X = Br- (1); I- (2); NCO- (3); NCS- (4a); N3 - (5), and [Co(L)(NCS)2] (4b), (L = a 17-membered pyridine-based N3O2-macrocyclic ligand containing two pyridin-2-ylmethyl pendant arms) were prepared and thoroughly characterized. The molecular structures for all complexes showed strongly distorted geometry in between octahedral and trigonal prismatic. The magnetic studies confirmed substantial magnetic anisotropy with positive values of D, the axial zero-field splitting parameter, but E/D ratios close to 1/3. This was supported by theoretical CASSCF calculations showing no significant effect of the co-ligands. Complex 4b was found to behave as a field-induced SMM.

Application of the Adiabatic Connection Random Phase Approximation to Electron-Nucleus Hyperfine Coupling Constants

The electron-nucleus hyperfine coupling constant is a challenging property for density functional methods. For accurate results, hybrid functionals with a large amount of exact exchange are often needed and there is no clear "one-for-all" functional which describes the hyperfine coupling interaction for a large set of nuclei. To alleviate this unfavorable situation, we apply the adiabatic connection random phase approximation (RPA) in its post-Kohn-Sham fashion to this property as a first test. For simplicity, only the Fermi-contact and spin-dipole terms are calculated within the nonrelativistic and the scalar-relativistic exact two-component framework. This requires to solve a single coupled-perturbed Kohn-Sham equation to evaluate the relaxed density matrix, which comes with a modest increase in computational demands. RPA performs remarkably well and substantially improves upon its Kohn-Sham (KS) starting point while also reducing the dependence on the KS reference. For main-group systems, RPA outperforms global, range-separated, and local hybrid functionals─at similar computational costs. For transition-metal compounds and lanthanide complexes, a similar performance as for hybrid functionals is observed. In contrast, related post-Hartree-Fock methods such as Møller-Plesset perturbation theory or CC2 perform worse than semilocal density functionals.

State-of-the-art local correlation methods enable affordable gold standard quantum chemistry for up to hundreds of atoms

In this feature, we review the current capabilities of local electron correlation methods up to the coupled cluster model with single, double, and perturbative triple excitations [CCSD(T)], which is a gold standard in quantum chemistry. The main computational aspects of the local method types are assessed from the perspective of applications, but the focus is kept on how to achieve chemical accuracy (i.e., <1 kcal mol-1 uncertainty), as well as on the broad scope of chemical problems made accessible. The performance of state-of-the-art methods is also compared, including the most employed DLPNO and, in particular, our local natural orbital (LNO) CCSD(T) approach. The high accuracy and efficiency of the LNO method makes chemically accurate CCSD(T) computations accessible for molecules of hundreds of atoms with resources affordable to a broad computational community (days on a single CPU and 10-100 GB of memory). Recent developments in LNO-CCSD(T) enable systematic convergence and robust error estimates even for systems of complicated electronic structure or larger size (up to 1000 atoms). The predictive power of current local CCSD(T) methods, usually at about 1-2 order of magnitude higher cost than hybrid density functional theory (DFT), has become outstanding on the palette of computational chemistry applicable for molecules of practical interest. We also review more than 50 LNO-based and other advanced local-CCSD(T) applications for realistic, large systems across molecular interactions as well as main group, transition metal, bio-, and surface chemistry. The examples show that properly executed local-CCSD(T) can contribute to binding, reaction equilibrium, rate constants, etc. which are able to match measurements within the error estimates. These applications demonstrate that modern, open-access, and broadly affordable local methods, such as LNO-CCSD(T), already enable predictive computations and atomistic insight for complicated, real-life molecular processes in realistic environments.

Real-space energy decomposition analysis method for qualitative and quantitative interpretations

In the work, a real-space energy decomposition analysis method, called DM-EDA(RS), is introduced based on our recently developed DM-EDA method [Zhang et al., J. Chem. Phys. 160, 174101 (2024)]. The EDA terms in DM-EDA(RS), including electrostatic, exchange, repulsion, polarization, and correlation, are expressed as the summations of grid-based energy density in real-space. This method is able to interpret intermolecular interactions in a unified qualitative and quantitative way. DM-EDA(RS) results provide not only comprehensive explanations for intermolecular interactions but also insights for sub-region interactions involving different functional groups.

A switchable green emitting dye and its phenomenal properties: implications for the photoluminescence features of carbon dots

New molecular fluorophores are constantly being discovered in post-synthetic mixtures of carbon dots (CDs), prompting researchers to elucidate their role in the optical properties of these nanomaterials. It has been reported that the green-emitting fluorophore that forms during the synthesis of popular citric acid/urea CDs is HPPT (4-hydroxy-1H-pyrrolo[3,4-c]pyridine-1,3,6(2H,5H)-trione). However, due to the low concentration of HPPT-like molecules within the structure of CDs, their actual binding and contribution to the optical properties of CDs has not so far been convincingly confirmed. In this joint experimental and quantum chemical study, we show that HPPT is a strong acid and only its anionic form, HPPT-, present in solutions of pH 0-10, is emissive. Next, its fluorescence can be switched off rapidly in strongly alkaline environments as a result of HPPT- hydrolysis, leading to the opening of its pyrrole ring and formation of CDPC (3-carbamoyl-2,6-dihydroxypyridine-4-carboxylic acid), existing as the CDPC2- dianion under these conditions. Eventually, we found that the ring opening hydrolysis is reversible and the green emission may be restored in acidic environments. The kinetics and mechanism of this hydrolysis were also revealed. The optical features of citric acid (CA)-urea CDs under various conditions were compared with a simpler CD system prepared by treating the CDs obtained from CA solely with HPPT- (HPPT@CDs). Our results indicate the feasibility of the post-synthetic modification of HPPT- present in the structures of CA-urea CDs and HPPT@CDs. Without HPPT- they emit blue fluorescence only. Thus, this makes the nanosystem switch the PL emission colour reversibly from green to blue owing to the opening and closing of the pyrrole ring in HPPT-like molecules. More importantly, the latter process may be considered a first step toward genuine fine tuning of the PL emission colour from CDs. These findings are of general importance to the further development of citric acid-based CDs with tailored properties.

A Phosphanyl Phosphagermene and its Reactivity

Reaction of a nucleophilic germylene Ge[CH(SiMe3)2]2 with the phosphanyl phosphaketene [{(H2C)(NDipp)}2P]PCO induces decarbonylation to form a phosphanyl phosphagermene [{(H2C)(NDipp)}2P]P=Ge[CH(SiMe3)2]2 (1; Dipp=2,6-diisopropyl-phenyl). Addition of CO2 or MeCN to 1 results in [3+2]-cycloaddition reactions to afford five-membered heterocycles. This mode of reactivity is reminiscent of that observed for frustrated Lewis pairs, with the pendant phosphanyl group acting as a base and the germanium center as a Lewis acid. Contrastingly, 1,2-addition across the P=Ge bond was observed when using ammonia, small primary amines (NH2 nP), or metal complexes (e. g. Au(PPh3)Cl and ZnEt2). These latter reactions allow for the one-step synthesis of metal phosphide complexes.

Electronic and Molecular Structures of a Series of Nickel Bis-1,1-Dithiolates

A series of homoleptic Ni bis-1,1-dithiolates, [Ni(S2C2RR')2]2- (R=CN, R'=CN, CO2Et, CONH2, Ph, Ph-4-Cl, Ph-4-OMe, Ph-4-NO2, Ph-3-CF3, Ph-4-CF3, Ph-4-CN; R=NO2, R'=H; R=R'=CO2Et) have been synthesized from the reaction of the alkali metal salt of the ligand and nickel chloride, and isolated as tetraphenylphosphonium or tetrabutylammonium salts. The complexes were characterized by X-ray crystallography, high-resolution mass spectrometry, and infrared (IR), nuclear magnetic resonance (NMR) and electronic absorption spectroscopies. The molecular structures show a rigidly square planar Ni(II) center linking two four-membered chelate rings whose dimensions are constant across the series. The electronic effect of the ligand substituent is revealed in the 13C NMR and electronic spectra, and corroborated by density functional calculations. Electron withdrawing groups deshield the low-field CS2 resonance, and the signature charge transfer band in the visible region is red-shifted. These observables have been accurately reproduced computationally, and revealed the Ni contribution to the ground state diminishes with decreasing electron withdrawing capacity of the ligand substituent. In contrast to 1,2-dithiolates, the redox inactivity afforded by 1,1-dithiolates stems from the smaller chelate ring and substantially reduced sulfur content that is key to stabilizing the radical form.

A solid noncovalent organic double-helix framework catalyzes asymmetric [6 + 4] cycloaddition

Whereas [4 + 2] cycloadditions are among the most powerful tools in the chemist's synthetic arsenal, controlling reactivity and selectivity of [6 + 4] cycloadditions has proven to be extremely challenging. Such transformations, especially if compatible with simple hydrocarbon-based substrates, could ultimately provide a general approach to highly valuable and otherwise difficult to access 10-membered rings. We report here that highly acidic and confined imidodiphosphorimidate catalysts do not catalyze this reaction under homogeneous conditions. Notably, however, they can spontaneously precipitate an insoluble and double helix-shaped noncovalent organic framework, which acts as a distinctively reactive and stereoselective catalyst of [6 + 4] cycloadditions of simple dienes with tropone.

Extension of the D3 and D4 London dispersion corrections to the full actinides series

Efficient dispersion corrections are an indispensable component of modern density functional theory, semi-empirical quantum mechanical, and even force field methods. In this work, we extend the well established D3 and D4 London dispersion corrections to the full actinides series, francium, and radium. To keep consistency with the existing versions, the original parameterization strategy of the D4 model was only slightly modified. This includes improved reference Hirshfeld atomic partial charges at the ωB97M-V/ma-def-TZVP level to fit the required electronegativity equilibration charge (EEQ) model. In this context, we developed a new actinide data set called AcQM, which covers the most common molecular actinide compound space. Furthermore, the efficient calculation of dynamic polarizabilities that are needed to construct CAB6 dispersion coefficients was implemented into the ORCA program package. The extended models are assessed for the computation of dissociation curves of actinide atoms and ions, geometry optimizations of crystal structure cutouts, gas-phase structures of small uranium compounds, and an example extracted from a small actinide complex protein assembly. We found that the novel parameterizations perform on par with the computationally more demanding density-dependent VV10 dispersion correction. With the presented extension, the excellent cost-accuracy ratio of the D3 and D4 models can now be utilized in various fields of computational actinide chemistry and, e.g., in efficient composite DFT methods such as r2SCAN-3c. They are implemented in our freely available standalone codes (dftd4, s-dftd3) and the D4 version will be also available in the upcoming ORCA 6.0 program package.

Chitosan-azo dye bioplastics that are reversibly resoluble and recoverable under visible light irradiation

Biopolymer composite materials were prepared by combining bio-sourced cationic water-soluble chitosan with bi-functional water-soluble anionic azo food dyes amaranth (AMA) or allura red (ALR) as ionic cross-linkers, mixing well in water, and then slow-drying in air. The electrostatically-assembled ionically-paired films showed good long-term stability to dissolution, with no re-solubility in water, and competitive mechanical properties as plastic materials. However, upon exposure of the bioplastics to low power light at sunlight wavelengths and intensities stirring in water, the stable materials photo-disassembled back to their water-soluble and low-toxicity (edible) constituent components, via structural photo-isomerization of the azo ionic crosslinkers. XRD, UV-vis, and IR spectroscopy confirmed that these assemblies are reversibly recoverable and so can in principle represent fully recyclable, environmentally degradable materials triggered by exposure to sunlight and water after use, with full recovery of starting components ready for re-use. A density functional theory treatment of the amaranth azo dye identified a tautomeric equilibrium favouring the hydrazone form and rationalized geometrical isomerization as a mechanism for photo-disassembly. The proof-of-principle suitability of films of these biomaterial composites as food industry packaging was assessed via measurement of mechanical, water and vapour barrier properties, and stability to solvent tests. Tensile strength of the composite materials was found to be 25-30 MPa, with elongation at break 3-5%, in a range acceptable as competitive for some applications to replace oil-based permanently insoluble non-recyclable artificial plastics, as fully recyclable, recoverable, and reusable low-toxicity green biomaterials in natural environmental conditions.

(η8-Cyclooctatetraene)(η5-fluorenyl)titanium: a processable molecular spin qubit with optimized control of the molecule-substrate interface

Depositing single paramagnetic molecules on surfaces for sensing and quantum computing applications requires subtle topological control. To overcome issues that are often encountered with sandwich metal complexes, we exploit here the low symmetry architecture and suitable vaporability of mixed-sandwich [FluTi(cot)], Flu = fluorenyl, cot = cyclooctatetraene, to drive submonolayer coverage and select an adsorption configuration that preserves the spin of molecules deposited on Au(111). Electron paramagnetic resonance spectroscopy and ab initio quantum computation evidence a d z 2 ground state that protects the spin from phonon-induced relaxation. Additionally, computed and measured spin coherence times exceed 10 μs despite the molecules being rich in hydrogen. A thorough submonolayer investigation by scanning tunneling microscopy, X-ray photoelectron and absorption spectrocopies and X-ray magnetic circular dichroism measurements supported by DFT calculations reveals that the most stable configuration, with the fluorenyl in contact with the metal surface, prevents titanium(iii) oxidation and spin delocalization to the surface. This is a necessary condition for single molecular spin qubit addressing on surfaces.

Combined Multireference-Multiscale Approach to the Description of Photosynthetic Reaction Centers

A first-principles description of the primary photochemical processes that drive photosynthesis and sustain life on our planet remains one of the grand challenges of modern science. Recent research established that explicit incorporation of protein electrostatics in excited-state calculations of photosynthetic pigments, achieved for example with quantum-mechanics/molecular-mechanics (QM/MM) approaches, is essential for a meaningful description of the properties and function of pigment-protein complexes. Although time-dependent density functional theory has been used productively so far in QM/MM approaches for the study of such systems, this methodology has limitations. Here we pursue for the first time a QM/MM description of the reaction center in the principal enzyme of oxygenic photosynthesis, Photosystem II, using multireference wave function theory for the high-level QM region. We identify best practices and establish guidelines regarding the rational choice of active space and appropriate state-averaging for the efficient and reliable use of complete active space self-consistent field (CASSCF) and the N-electron valence state perturbation theory (NEVPT2) in the prediction of low-lying excited states of chlorophyll and pheophytin pigments. Given that the Gouterman orbitals are inadequate as a minimal active space, we define specific minimal and extended active spaces for the NEVPT2 description of electronic states that fall within the Q and B bands. Subsequently, we apply our multireference-QM/MM protocol to the description of all pigments in the reaction center of Photosystem II. The calculations reproduce the electrochromic shifts induced by the protein matrix and the ordering of site energies consistent with the identity of the primary donor (ChlD1) and the experimentally known asymmetric and directional electron transfer. The optimized protocol sets the stage for future multireference treatments of multiple pigments, and hence for multireference studies of charge separation, while it is transferable to the study of any photoactive embedded tetrapyrrole system.

A modular, composite framework for the utilization of reduced-scaling Coulomb and exchange construction algorithms: Design and implementation

Multiple algorithms exist for calculating Coulomb (J) or exchange (K) contributions to Fock-like matrices, and it is beneficial to develop a framework that allows the seamless integration and combination of different J and K construction algorithms. In Psi4, we have implemented the "CompositeJK" formalism for this purpose. CompositeJK allows for the combination of any J and K construction algorithms for any quantum chemistry method formulated in terms of J-like or K-like matrices (including, but not limited to, Hartree-Fock and density functional theory) in a highly modular and intuitive fashion, which is simple to utilize for both developers and users. Using the CompositeJK framework, Psi4 was interfaced to the sn-LinK implementation in the GauXC library, adding the first instance of noncommercial graphics processing unit (GPU) support for the construction of Fock matrix elements to Psi4. On systems with hundreds of atoms, the interface to the CPU sn-LinK implementation displays a higher performance than all the alternative JK construction methods available in Psi4, with up to x2.8 speedups compared to existing Psi4JK implementations. The GPU sn-LinK implementation, harnessing the power of GPUs, improves the observed performance gains to up to x7.0.

Quantum chemical package Jaguar: A survey of recent developments and unique features

This paper is dedicated to the quantum chemical package Jaguar, which is commercial software developed and distributed by Schrödinger, Inc. We discuss Jaguar's scientific features that are relevant to chemical research as well as describe those aspects of the program that are pertinent to the user interface, the organization of the computer code, and its maintenance and testing. Among the scientific topics that feature prominently in this paper are the quantum chemical methods grounded in the pseudospectral approach. A number of multistep workflows dependent on Jaguar are covered: prediction of protonation equilibria in aqueous solutions (particularly calculations of tautomeric stability and pKa), reactivity predictions based on automated transition state search, assembly of Boltzmann-averaged spectra such as vibrational and electronic circular dichroism, as well as nuclear magnetic resonance. Discussed also are quantum chemical calculations that are oriented toward materials science applications, in particular, prediction of properties of optoelectronic materials and organic semiconductors, and molecular catalyst design. The topic of treatment of conformations inevitably comes up in real world research projects and is considered as part of all the workflows mentioned above. In addition, we examine the role of machine learning methods in quantum chemical calculations performed by Jaguar, from auxiliary functions that return the approximate calculation runtime in a user interface, to prediction of actual molecular properties. The current work is second in a series of reviews of Jaguar, the first having been published more than ten years ago. Thus, this paper serves as a rare milestone on the path that is being traversed by Jaguar's development in more than thirty years of its existence.

Trapping an Elusive Fe(IV)-Superoxo Intermediate Inside a Self-Assembled Nanocage in Water at Room Temperature

Molecular cavities that mimic natural metalloenzymes have shown the potential to trap elusive reaction intermediates. Here, we demonstrate the formation of a rare yet stable Fe(IV)-superoxo intermediate at room temperature subsequent to dioxygen binding at the Fe(III) site of a (Et4N)2[FeIII(Cl)(bTAML)] complex confined inside the hydrophobic interior of a water-soluble Pd6L412+ nanocage. Using a combination of electron paramagnetic resonance, Mössbauer, Raman/IR vibrational, X-ray absorption, and emission spectroscopies, we demonstrate that the cage-encapsulated complex has a Fe(IV) oxidation state characterized by a stable S = 1/2 spin state and a short Fe-O bond distance of ∼1.70 Å. We find that the O2 reaction in confinement is reversible, while the formed Fe(IV)-superoxo complex readily reacts when presented with substrates having weak C-H bonds, highlighting the lability of the O-O bond. We envision that such optimally trapped high-valent superoxos can show new classes of reactivities catalyzing both oxygen atom transfer and C-H bond activation reactions.

Low-Coordinate Iron Hydride Chemistry at an N,N,C-Heteroscorpionate Platform

Locally high-spin iron hydrides are proposed to play a critical role as intermediates in iron-molybdenum cofactor (FeMoco)-catalyzed N2 fixation. Inspired by these biological systems, we report herein our initial investigations into low-coordinate iron hydride chemistry supported by our N,N,C-heteroscorpionate ligands. Those ligands with smaller steric profiles are unable to completely suppress the formation of a binuclear [Fe(μ2-H)]2 complex; however, the incorporation of more substantial steric bulk allows for the isolation of a rare example of a terminal, high-spin (S = 2) Fe2+ hydride. Fourier transform infrared spectroscopy suggests an unusually weak Fe-H bond in the latter molecule. Mössbauer spectroscopies, coupled with density functional theory calculations, highlights the substantial influence of the terminal hydride ligand on 57Fe isomer shift.

Unraveling the Geometry-Driven C═C Epoxidation and C-H Hydroxylation Reactivity of Tetra-Coordinated Nonheme Iron(IV)-Oxo Complexes

The electronic structure and reactivity of tetra-coordinated nonheme iron(IV)-oxo complexes have remained unexplored for years. The recent synthesis of a closed-shell iron(IV)-oxo complex [(quinisox)FeIV(O)]+ (1) has set up a platform to understand how such complexes compare with the celebrated open-shell iron-oxo chemistry. Herein, using density functional theory and ab initio calculations, we present an in-depth electronic structure investigation of the C═C epoxidation [oxygen atom transfer (OAT)] and C-H hydroxylation [hydrogen atom transfer (HAT)] reactivity of 1. Using a solvent-coordinated geometry of 1 (1') and other potential tetra-coordinated iron(IV)-oxo complexes bearing rigid ligands (2 and 3), we established the geometric origin of spin-state energetics and reactivity of 1. Complex 1 featuring a strong Fe-O bond exhibits OAT and HAT reactivity in its quintet state. The lowest quintet OAT pathway has a lower barrier by ∼4 kcal/mol than the quintet HAT pathway, corroborating the experimentally observed gas-phase OAT reactivity preference. A conventional HAT reactivity preference for 2 and a comparable OAT and HAT reactivity for 3 are observed. This further supports the geometry-driven reactivity preference for 1. Noncovalent interaction analyses reveal a pronounced π-π interaction between the substrate and ligand in the OAT transition state, rationalizing the origin of the observed reactivity preference for 1.

Structural, optical and magnetic properties of a new metal-organic CoII-based complex

A mononuclear cobalt(ii) complex [C5H8N3]2[CoCl4(C5H7N3)2] (I) was synthesized and structurally characterized. Single crystal X-ray diffraction analysis indicates that monometallic Co(ii) ions acted as coordination nodes in a distorted octahedral geometry, giving rise to a supramolecular architecture. The latter is made up of a ½ unit form composed of an anionic element [Co0.5Cl2(C5H7N3)]- and one 2-amino-4-methylpyrimidinium cation [C5H8N3]+. The crystalline arrangement of this compound adopts the sandwich form where inorganic parts are sandwiched between the organic sheets following the [100] direction. More information regarding the structure hierarchy has been supplied based on Hirshfeld surface analysis; the X⋯H (X = N, Cl) interactions play a crucial role in stabilizing the self-assembly process of I, complemented by the intervention of π⋯π electrostatic interaction created between organic entities. Thermal analyses were carried out to study the thermal behavior process. Static magnetic measurements and ab initio calculations of compound I revealed the easy-axis anisotropy character of the central Co(ii) ion. Two-channel field-induced slow-magnetic relaxation was observed; the high-frequency channel is characterized by underbarrier relaxation with U eff = 16.5 cm-1, and the low-frequency channel involves a direct relaxation process affected by the phonon-bottleneck effect.

Hey ho, where'd the proton go? Final deprotonation of O6 within the S3 state of photosystem II

The deprotonation of O6 within the S3 state marks the final deprotonation event before the formation of oxygen‑oxygen bond interactions and eventual production and release of dioxygen. Gaining a thorough understanding of this event, from the proton acceptors involved, to the exfiltration pathways available, is key in determining the nature of the resulting oxygen species, influencing the mechanism through which the first oxygen‑oxygen bond forms. Computational analysis, using BS-DFT methodologies, showed that proton abstraction by the local Glu189 residue provides consistent evidence against this being a viable mechanistic pathway due to the lack of a stable product structure. In contrast, abstraction via W3 shows an increasingly stable oxo-oxo product state between r[O5O6] = 2.1 Å & 1.9 Å. The resulting oxo-oxo state is stabilised through donation of β electron character from O6 to Mn1 and α electron character from O6 to O5. This donation from the O6 lone pair is shown to be a key factor in stabilising the oxo-oxo state, in addition to showing the initiation of first O5-O6 bond.

Oxygenolytic cleavage of 1,2-diols with dioxygen by a mononuclear nonheme iron complex: Mimicking the reaction of myo-inositol oxygenase

A mononuclear iron(II) complex, [(TpPh2)FeII(OTf)(CH3CN)] (1) (TpPh2 = hydrotris(3,5-diphenylpyrazol-1-yl)borate, OTf = triflate) has been isolated and its efficiency toward the aliphatic CC bond cleavage reaction of 1,2-diols with dioxygen has been investigated. Separate reactions between 1 and different 1,2-diolates form the corresponding iron(II)-diolate complexes in solution. While the iron(II) complex of the tetradentate TPA (tris(2-pyridylmethyl)amine) ligand is not efficient in affecting the CC cleavage of 1,2-diol with dioxygen, complex 1 displays catalytic activity to afford carboxylic acid and aldehyde. Isotope labeling studies with 18O2 reveal that one oxygen atom from dioxygen is incorporated into the carboxylic acid product. The oxygenative CC cleavage reactions occur on the 1,2-diols containing at least one α-H atom. The kinetic isotope effect value of 5.7 supports the abstraction of an α-H by an iron(III)-superoxo species to propagate the CC cleavage reactions. The oxidative cleavage of 1,2-diolates by the iron(II) complex mimics the reaction catalyzed by the nonheme diiron enzyme, myo-inositol oxygenase.

Rabbit muscle pyruvate kinase activators: Synthesis, molecular docking and theoretical studies of N-substituted sulfonamide derivatives

Pyruvate kinase (PK) activators have potential therapeutic applications in diseases such as sickle cell anemia. In this study, N-Substituted sulfonamide derivatives of 1,4-dihydropyridines were synthesized and evaluated as PK activators in vitro and using molecular docking studies. The compounds were synthesized by reacting dicarbonyl compounds with ammonium acetate, 5-nitrobenzaldehyde, and alumina sulfuric acid (ASA), followed by reduction and sulfonylation. The structures of the compounds were analyzed using spectroscopic techniques. DFT calculations provided insights into the electronic properties. Molecular docking of the compounds into the active site of PK showed favorable binding interactions. ADME evaluation indicated suitable solubility, BBB permeation, and lack of CYP450 inhibition. Overall, this study demonstrates the potential of new hybrid 1,4-dihydropyridine substituted sulfonamides as PK activators for further development. According to AC50 values, the compound (DTSF7, 0.97μM) is about 100-fold higher affective than the clinically used sulfonamide compound (AC50 = 90μM) for PK.

Ab initio multireference calculation of electronic spectra of the osmium complexes, [Os(bpy) 3 ] 2 + and [Os(phen) 3 ] 2 +

The spin-orbit coupling corrected absorption spectra of osmium complexes, [Os(bpy)   3 ]   2 + and [Os(phen)   3 ]   2 + , were calculated by using ab initio multireference perturbation method (NEVPT2) with relativistic effects taken into account throughout ZORA approximation and corresponding all-electron basis sets. For the same purpose, the time-dependent DFT techniques were used. A very good agreement between NEVPT2 and experimental spectra should be highlighted, especially for the MLCT transitions that occur in visible and near-UV regions ( 16 , 000 - 33 , 000 cm   - 1 ). Moreover, the present study offers description of excited states of titled osmium complexes and their spectra interpretation using molecular orbitals.

Efficient State-Specific Natural Orbital Based Equation of Motion Coupled Cluster Method for Core-Ionization Energies: Theory, Implementation, and Benchmark

We have implemented a reduced-cost partial triples correction scheme to the equation of motion coupled cluster method for core-ionization energy based on state-specific natural orbitals. The second-order Algebraic Diagrammatic Construction (ADC) method is used to generate the state-specific natural orbital, which provides quicker convergence of the core-IP value with respect to the size of the virtual space than that observed in standard MP2-based natural orbitals. The error due to truncation of the virtual orbital can be reduced by using a perturbative correction. The accuracy of the method can be controlled by a single threshold, and there is a black box to use. The inclusion of the partial triples correction in the natural orbital based EOM-CCSD method greatly improves the agreement of the results with the experiment. The efficiency of the present implementation is demonstrated by calculating the core-ionization energy of a molecule containing 60 atoms and more than 2000 basis functions.

Exerting pulling forces in fluids by directional disassembly of microcrystalline fibres

Biomolecular polymerization motors are biochemical systems that use supramolecular (de-)polymerization to convert chemical potential into useful mechanical work. With the intent to explore new chemomechanical transduction strategies, here we show a synthetic molecular system that can generate forces via the controlled disassembly of self-organized molecules in a crystal lattice, as they are freely suspended in a fluid. An amphiphilic monomer self-assembles into rigid, high-aspect-ratio microcrystalline fibres. The assembly process is regulated by a coumarin-based pH switching motif. The microfibre crystal morphology determines the monomer reactivity at the interface, resulting in anisotropic etching. This effect exerts a directional pulling force on microscopic beads adsorbed on the crystal surface through weak multivalent interactions. We use optical-tweezers-based force spectroscopy to extract mechanistic insights into this process, quantifying a stall force of 2.3 pN (±0.1 pN) exerted by the ratcheting mechanism produced by the disassembly of the microfibres.

Synthesis and Characterization of Bridging-Diazene Diiron Half-Sandwich Complexes: The Role of Sulfur Hydrogen Bonding

We report two bridging-diazene diiron complexes [Cp*Fe(8-quinolinethiolate)]2(μ-N2H2) (1-N2H2) and [Cp*Fe(1,2-Cy2PC6H4S)]2(μ-N2H2) (2-N2H2), synthesized by the reaction of hydrazine with the corresponding thiolate-based iron half-sandwich complex, [Cp*Fe(8-quinolinethiolate)]2 (1) and Cp*Fe(1,2-Cy2PC6H4S) (2). Crystallographic analysis reveals that the thiolate sites in 1-N2H2 and 2-N2H2 can engage in N-H···S hydrogen bonding with the diazene protons. 1-N2H2 is thermally stable in both solid and solution states, allowing for one-electron oxidation to afford a cationic diazene radical complex [1-N2H2]+ at room temperature. In contrast, 2-N2H2 tends to undergo N2H2/N2 transformation, leading to the formation of a Fe(III)-H species by the loss of N2. In addition to stabilizing HN=NH species through the hydrogen bonding, the thiolate-based ligands also seem to facilitate proton-coupled electron transfer, thereby promoting N-H cleavage.

Deciphering the doublet luminescence mechanism in neutral organic radicals: spin-exchange coupling, reversed-quartet mechanism, excited-state dynamics

Neutral organic radical molecules have recently attracted considerable attention as promising luminescent and quantum-information materials. However, the presence of a radical often shortens their excited-state lifetime and results in fluorescence quenching due to enhanced intersystem crossing (EISC). Recently, an experimental report introduced an efficient luminescent radical molecule, tris(2,4,6-trichlorophenyl)methyl-carbazole-anthracene (TTM-1Cz-An). In this study, we systematically performed quantum theoretical calculations combined with the path integral approach to quantitatively calculate the excited-state dynamics processes and spectral characteristics. Our theoretical findings suggest that the sing-doublet D1 state, originating from the anthracene excited singlet state, is quickly converted to the doublet (trip-doublet) state via EISC, facilitated by a significant nonequivalence exchange interaction, with ΔJ ST = 0.174 cm-1. The formation of the quartet state (Q1, trip-quartet) was predominantly dependent on the exchange coupling 3/2J TR = 0.086 cm-1 between the triplet spin electrons of anthracene and the TTM-1Cz radical. Direct spin-orbit coupling ISC to the Q1 state was minimal due to the nearly identical spatial wavefunctions of the and Q1 levels. The effective occurrence of reverse intersystem crossing (RISC) from the Q1 to D1 state is a critical step in controlling the luminescence of TTM-1Cz-An. The calculated RISC rate k RISC, including the Herzberg-Teller effect, was 3.64 × 105 s-1 at 298 K, significantly exceeding the phosphorescence and nonradiative rates of the Q1 state, thus enabling the D1 repopulation. Subsequently, a strong electronic coupling of 37.4 meV was observed between the D1 and D2 states, along with a dense manifold of doublet states near the D1 state energy, resulting in a larger reverse internal conversion rate k RIC of 9.26 × 1010 s-1. Distributed to the D2 state, the obtained emission rate of k f = 2.98-3.18 × 107 s-1 was in quite good agreement with the experimental value of 1.28 × 107 s-1, and its temperature effect was not remarkable. Our study not only provides strong support for the experimental findings but also offers valuable insights for the molecular design of high-efficiency radical emitters.

Assessment of the applicability of DFT methods to [Cp*Rh]-catalyzed hydrogen evolution processes

The present computational study provides a benchmark of density functional theory (DFT) methods in describing hydrogen evolution processes catalyzed by [Cp*Rh]-containing organometallic complexes. A test set was composed of 26 elementary reactions featuring chemical transformations and bonding situations essential for the field, including the emerging concept of non-innocent Cp* behavior. Reference values were obtained from a highly accurate 3/4 complete basis set and 6/7 complete PNO space extrapolated DLPNO-CCSD(T) energies. The performance of lower-level extrapolation procedures was also assessed. We considered 84 density functionals (DF) (including 13 generalized gradient approximations (GGA), nine meta-GGAs, 33 hybrids, and 29 double-hybrids) and three composite methods (HF-3c, PBEh-3c, and r2SCAN-3c), combined with different types of dispersion corrections (D3(0), D3BJ, D4, and VV10). The most accurate approach is the PBE0-DH-D3BJ (MAD of 1.36 kcal mol-1) followed by TPSS0-D3BJ (MAD of 1.60 kcal mol-1). Low-cost r2SCAN-3c composite provides a less accurate but much faster alternative (MAD of 2.39 kcal mol-1). The widely used Minnesota-family M06-L, M06, and M06-2X DFs should be avoided (MADs of 3.70, 3.94, and 4.01 kcal mol-1, respectively).

Divergent catalytic behaviors of assembled organogold(i) clusters derived from enyne cyclization

Homogeneous gold catalysis has attracted much recent attention due to diverse activation modes of gold(i) towards unsaturated organic groups. Because of attractive aurophilic interaction, structural transformations of metalated species into high nuclear clusters are often proposed in gold catalysis, while to date little is known about their assembly behaviors and catalytic activity. In this work, based on stoichiometric Au(i)-mediated enyne cyclization reactions, we achieve a discrete vicinal dicarbanion-centered Au4 intermediate and three assembled Au11, Au28, and Au14 clusters held together by several aryl dicarbanions. Spectral monitoring, kinetic and theoretical investigations confirm that these discrete and assembled intermediates display four different pathways upon catalyzing the cyclization reaction of the same 1,5-enyne substrate. The discrete Au4 cluster undergoes a full protodeauration process to generate active [Au(PPh3)]+ species for catalytic use. In contrast, the net-like Au11 cluster experiences a substrate-induced dissociation to generate a semi-stable Au10 unit and an active [alkyne-Au(PPh3)]+ fragment for further transformation. The dumbbell-like Au28 cluster is prone to cleavage of the central Au-Au linkage and each Au14 moiety exposes a coordination unsaturated site to activate a substrate molecule. However, the synthetic closed-Au14 cluster with full ligand protection is no longer catalytically active.

Square-planar imido complexes of cobalt: synthesis, reactivity and computational study

Treatment of [Co(N2)(tBuPNP)] (tBuPNP = anion of 2,5-bis(di-tert-butylphosphinomethyl)pyrrole) with one equivalent of an aryl azide generates the four-coordinate imido complexes [Co(NAr)(tBuPNP)] (Ar = mesityl, phenyl, or 4-tBu-phenyl). X-ray crystallographic analysis of the compounds shows an unusual square-planar geometry about cobalt with nearly linear imido units. In the presence of the hydrogen atom donor, TEMPOH, [Co(NPh)(tBuPNP)] undergoes addition of the H atom to the imido nitrogen to generate the corresponding amido complex, [Co(NHPh)(tBuPNP)], whose structure and composition were verified by independent synthesis. Despite the observation of H atom transfer reactivity with TEMPOH, the imido complexes do not show catalytic activity for C-H amination or aziridination for several substrates examined. In the case of [Co(NPh)(tBuPNP)], addition of excess azide produced the tetrazido complex, [Co(N4Ph2)(tBuPNP)], whose bond metrics were most consistent with an anionic Ph2N4 ligand. Density Functional Theory (DFT) investigations of the imido and tetrazido species suggest that they adopt a ground state best described as possessing a low-spin cobalt(II) ion ferromagnetically coupled to an iminyl radical.

Redox-Responsive Halogen Bonding as a Highly Selective Interaction for Electrochemical Separations

Leveraging specific noncovalent interactions can broaden the mechanims for selective electrochemical separations beyond solely electrostatic interactions. Here, we explore redox-responsive halogen bonding (XB) for selective electrosorption in nonaqueous media, by taking advantage of directional interactions of XB alongisde a cooperative and synergistic ferrocene redox-center. We designed and evaluated a new redox-active XB donor polymer, poly(5-iodo-4-ferrocenyl-1-(4-vinylbenzyl)-1H-1,2,3-triazole) (P(FcTS-I)), for the electrochemically switchable binding and release of target organic and inorganic ions at a heterogeneous interface. Under applied potential, the oxidized ferrocene amplifies the halogen binding site, leading to significantly enhanced uptake and selectivity towards key inorganic and organic species, including chloride, bisulfate, and benzenesulfonate, compared to the open-circuit potential or the hydrogen bonding donor analog. Density functional theory calculations, as well as spectroscopic analysis, offer mechanistic insight into the degree of amplification of σ-holes at a molecular level, with selectivity modulated by charge transfer and dispersion interactions. Our work highlights the potential of XB in selective electrosorption by uniquely leveraging noncovalent interactions for redox-mediated electrochemical separations.

Ultrafast photochemistry and electron diffraction for cyclobutanone in the S2 state: Surface hopping with time-dependent density functional theory

We simulate the photodynamics of gas-phase cyclobutanone excited to the S2 state using fewest switches surface hopping (FSSH) dynamics powered by time-dependent density functional theory (TDDFT). We predict a total photoproduct yield of 8%, with a C3:C2 product ratio of 0 trajectories to 8 trajectories. One primary S2 → S1 conical intersection is identified involving the compression of an α-carbon-carbon-hydrogen bond angle. Excited state lifetimes computed with respect to electronic state populations were found to be 3.96 ps (S2 → S1) and 498 fs (S1 → S0). We also generate time-resolved difference pair distribution functions (ΔPDFs) from our TDDFT-FSSH dynamics results in order to generate direct comparisons with ultrafast electron diffraction experiment observables. Global and target analysis of time-resolved ΔPDFs produced a distinct set of lifetimes: (i) a 0.548 ps decay and (ii) a 1.69 ps decay, both resembling the S2 minimum, as well as (iii) a long decay that resembles the S1 minimum geometry and the fully separated C2 products. Finally, we contextualize our results by considering the impact of the most likely sources of significant errors.

Beyond Strain Release: Delocalization-Enabled Organic Reactivity

The release of strain energy is a fundamental driving force for organic reactions. However, absolute strain energy alone is an insufficient predictor of reactivity, evidenced by the similar ring strain but disparate reactivity of cyclopropanes and cyclobutanes. In this work, we demonstrate that electronic delocalization is a key factor that operates alongside strain release to boost, or even dominate, reactivity. This delocalization principle extends across a wide range of molecules containing three-membered rings such as epoxides, aziridines, and propellanes and also applies to strain-driven cycloaddition reactions. Our findings lead to a "rule of thumb" for the accurate prediction of activation barriers in such systems, which can be easily applied to reactions involving many of the strained building blocks commonly encountered in organic synthesis, medicinal chemistry, polymer science, and bioconjugation. Given the significance of electronic delocalization in organic chemistry, for example in aromatic π-systems and hyperconjugation, we anticipate that this concept will serve as a versatile tool to understand and predict organic reactivity.

Orchestration of ferro- and anti-ferromagnetic ordering in gold nanoclusters

The unpaired electron in the gold clusters (Aun, n = no. of Au atoms) with an odd number of total electrons is solely responsible for the magnetic properties in the small-sized Au nano-clusters. However, no such unpaired electron is available due to pairing in the even number of atom gold clusters and behaving as a diamagnetic entity similar to bulk gold. In this work, we unveiled the spin-density distribution of odd Aun clusters with n = 1 to 19 that reveals that a single unpaired electron gets distributed non-uniformly among all Au-atoms depending on the cluster size and morphology. The delocalization of the unpaired electron leads to the spin dilution approaching a value of ∼1/n spin moments on each atom for the higher clusters. Interestingly, small odd-numbered gold clusters possess spin-magnetic moments similar to the delocalized spin moments as of organic radicals. Can cooperative magnetic properties be obtained by coupling these individual magnetic gold nanoparticles? In this work, by applying state-of-the-art computational methodologies, we have demonstrated ferromagnetic or anti-ferromagnetic couplings between such magnetic nanoclusters upon designing suitable organic spacers. These findings will open up a new avenue of nanoscale magnetic materials combining organic spacers and odd-electron nano-clusters.

Coordination of dissolved transition metals in pristine battery electrolyte solutions determined by NMR and EPR spectroscopy

The solvation of dissolved transition metal ions in lithium-ion battery electrolytes is not well-characterised experimentally, although it is important for battery degradation mechanisms governed by metal dissolution, deposition, and reactivity in solution. This work identifies the coordinating species in the Mn2+ and Ni2+ solvation spheres in LiPF6/LiTFSI-carbonate electrolyte solutions by examining the electron-nuclear spin interactions, which are probed by pulsed EPR and paramagnetic NMR spectroscopy. These techniques investigate solvation in frozen electrolytes and in the liquid state at ambient temperature, respectively, also probing the bound states and dynamics of the complexes involving the ions. Mn2+ and Ni2+ are shown to primarily coordinate to ethylene carbonate (EC) in the first coordination sphere, while PF6- is found primarily in the second coordination sphere, although a degree of contact ion pairing does appear to occur, particularly in electrolytes with low EC concentrations. NMR results suggest that Mn2+ coordinates more strongly to PF6- than to TFSI-, while the opposite is true for Ni2+. This work provides a framework to experimentally determine the coordination spheres of paramagnetic metals in battery electrolyte solutions.

The trials and triumphs of modelling X-ray absorption spectra of transition metal phthalocyanines

This study explores the electronic structure of Co, Fe and Mn phthalocyanines (TMPcs) as well as their perfluorinated counterparts through a series of electronic structure calculations utilizing multireference methods and by simulating their metal L-edge and ligand (nitrogen and fluorine) K-edge X-ray absorption spectra (XAS) in an angle-resolved manner. Simulations targeting different ground-state symmetries, where relevant, have been conducted to observe changes in the N K-edge lineshape. The applicability of the quasi-degenerate formulation of n-electron valence state perturbation theory (QD-NEVPT2) for L-edge X-ray absorption spectroscopy (XAS) is evaluated, alongside the use of a restricted active space (RAS) formalism to describe the final-state multiplets generated by L-shell X-ray processes. Our findings provide valuable insights into the electronic properties of TMPcs, in particular with respect to the effect of fluorination, and demonstrate the broad applicability of various formulations of NEVPT2 in spectral simulations. Moreover, this study highlights the utility of manual truncation of the configuration spaces in order to allow for large active orbital spaces in aforementioned calculations.

Molecular Flexibility in Solvated Crystals of the Dimer, Au2(μ-1,2-bis(diphenylphosphino)ethane)2I2, with Three-Coordinate Gold(I)

We report the ability to trap the dimer Au2(μ-dppe)2I2 (dppe is 1,2-bis(diphenylphosphino)ethane) with different separations between the three-coordinate gold ions in crystalline solvates. All of these solvates ((Au2(μ-dppe)2I2·4(CH2Cl2) (1), Au2(μ-dppe)2I2·2(CH2Cl2) (2), the polymorphs α-Au2(μ-dppe)2I2·2(HC(O)NMe2) (3) and β-Au2(μ-dppe)2I2·2(HC(O)NMe2) (4), and Au2(μ-dppe)2I2·4(CHCl3) (5)) along with polymeric {Au(μ-dppe)I}n·n(CHCl3) (6)) originated from the same reaction, only the solvent system used for crystallization differed. In the different solvates of Au2(μ-dppe)2I2, the Au···Au separation varied from 3.192(1) to 3.7866(3) Å. Computational studies undertaken to understand the flexible nature of these dimers indicated that the structural differences were primarily a result of crystal packing effects with aurophillic interactions having a minimal effect.

Self-association as a solubility limiting factor of riboflavin in aqueous media

Single crystal X-ray diffraction analysis of needle shaped riboflavin (RF) crystals revealed π-stacking of RF's isoalloxazine units (distance: 3.643-3.313 Å) with syn-orientated ribityl chains. In line with this, classical molecular dynamics (MD) (400 ns) using an isobaric-isothermal (NPT) ensemble of eight RF in a water box (〈V〉 ∼ 508.62 nm3, 〈p〉 = 1.11 bar) revealed anti-aligned aggregation of RF in water (COM-distance: 4 Å). Comparing umbrella sampling for the separation of two RF molecules to the separation of two lumichrome molecules, the similar mean potential force for the separation of RF and lumichrome (22.8 kJ mol-1; 24.4 kJ mol-1) proved dispersive interactions as the origin of RF's aggregation. Though stacking of RF is the major water-solubility limiting factor, the conformation of RF's ribityl chain may alter the solubility in water. Both, MD (in water) and COSMO-RS (in water continuum) predicted that conformations of RF with an extended ribityl chain are thermodynamically preferred over any conformations with internal hydrogen bonds between hydroxyl groups and nitrogen/oxygen atoms of the pyrimidine moiety of the flavin ring. Interestingly, COSMO-RS predicted the solubility of the extended conformation to be significantly lower than the latter leading to the very low average solubility of RF. Nuclear Overhauser effect measurements (NOESY) of the structurally related sodium riboflavin 5'-monophosphate (RF-PO4) in deuterium oxide confirmed π-stacking of the isoalloxazine rings. In conformity with the 350 times higher water-solubility of RF-PO4, NOESY also indicated a contorted conformation of the ribityl phosphate chain, whereas, for RF, indications for a contorted chain were not observed.

Lanthanide(III) SMMs with cationic and anionic complex fragments formed by a Schiff base: structure, luminescence, magnetic properties and ab initio calculations

The syntheses, structures, luminescence and magnetic properties of a new series of Ln(III) complexes of the formula [Ln(L)(H2O)2(DMF)2][Ln(L)2] (in which H2L is N,N'-ethylaminebis[1-phenyl-3-methyl-4-formylimino-2-pyrazoline-5-one]; Ln(III) - Gd (1), Tb (2), or Dy (3) ions). The crystal structures were determined by single-crystal X-ray diffraction measurements for all the above-mentioned complexes. The crystals of these compounds consist of cationic [Ln(L)(H2O)2(DMF)2]+ and anionic [Ln(L)2] moieties which form a 3D supramolecular architecture by the H-bonds and electrostatic forces. Luminescence emission in the visible range was observed for Tb(III) and Dy(III) compounds upon ligand sensitization, with moderate quantum yields of 3.2% for the Dy complex and 24.2% for the Tb analogue. Moreover the Tb(III) complex demonstrates triboluminescence activity. The dynamic magnetization studies revealed that 1 and 2 demonstrate field-induced magnetic relaxation with effective energy barriers, ΔE|kB = 24 K (for 1) and 85 K (for 2), while the Dy complex 3 exhibits slow relaxation of magnetization in zero field with an activation energy of 256 K.

Comprehensive Exploration of Optics and Magnetism in a Hydrothermally Synthesized Nickel(II) Complex

The hydrothermal reaction of 2-MeIm (2-MeIm: 2-methylimidazole) with nickel sulfate hexahydrate in methanol afforded a mononuclear complex formulated as [Ni(SO4)(2-MeIm)2(H2O)3]·CH3OH (1). The title compound was described by X-ray single-crystal diffraction, thermal assessment, IR, and UV-vis spectroscopy. The crystal structure of 1 is composed of segregated [Ni(SO4)(2-MeIm)2(H2O)3] neutral entities and a solvent methanol molecule. Two (2-MeIm) ligands, a sulfate group, and a water molecule reside in the equatorial positions of the vertices in this 6-fold coordination. Two aqua ligands lay in the apical positions, resulting in a subtly distorted octahedral framework, as was supported by spectroscopic analysis. The complex's self-assembly is firmly governed by robust O-H···O/N-H···O interactions. Further details on these bonds have been furnished via Hirshfeld surface scrutiny and 2D fingerprint plots. As proven by TGA/DSC analysis, raising the temperature of 1 above 60 °C instigates progressive decomposition stages, which culminates in the production of metal oxide as the ultimate product at 700 °C. The optical analysis suggests the dielectric nature of the material with large direct and indirect gap energies of 5.25 and 4.96 eV, respectively. The results of magnetic studies suggest that 1 undergoes a transition to a magnetically ordered state below 6 K.

Chemical fixation of atmospheric CO2 in tricopper(II)-carbonato complexes with tetradentate N-donor ligands: reactive intermediates, probable mechanisms, and catalytic and magneto-structural studies

In the present era, the fixation of atmospheric CO2 is of significant importance and plays a crucial role in maintaining the balance of carbon and energy flow within ecosystems. Generally, CO2 fixation is carried out by autotrophic organisms; however, the scientific community has paid substantial attention to execute this process in laboratory. In this report, we synthesized two carbonato-bridged trinuclear copper(II) complexes, [Cu3(L1)3(μ3-CO3)](ClO4)3 (1) and [Cu3(L2)3(μ3-CO3)](ClO4)3 (2) via atmospheric fixation of CO2 starting with Cu(ClO4)2·6H2O and easily accessible pyridine/pyrazine-based N4 donor Schiff base ligands L1 and L2, respectively. Under very similar reaction conditions, the ligand framework embedded with the phenolate moiety (HL3) fails to do so because of the reduction of the Lewis acidity of the metal center, inhibiting the formation of a reactive hydroxide bound copper(II) species, which is required for the fixation of atmospheric CO2. X-ray crystal structures display that carbonate-oxygen atoms bridge three copper(II) centers in μ3syn-anti disposition in 1 and 2, whereas [Cu(HL3)(ClO4)] (3) is a mononuclear complex. Interestingly, we also isolated an important intermediate of atmospheric CO2 fixation and structurally characterized it as an anti-anti μ2 carbonato-bridged dinuclear copper(II) complex, [Cu2(L2)2(μ2-CO3)](ClO4)2·MeOH (2-I), providing an in-depth understanding of CO2 fixation in these systems. Variable temperature magnetic susceptibility measurement suggests ferromagnetic interactions between the metal centers in both 1 and 2, and the results have been further supported by DFT calculations. The catalytic efficiency of our synthesized complexes 1-3 was checked by means of catechol oxidase and phenoxazinone synthase-like activities. While complexes 1 and 2 showed oxidase-like activity for aerobic oxidation of o-aminophenol and 3,5-di-tert-butylcatechol, complex 3 was found to be feebly active. ESI mass spectrometry revealed that the oxidation reaction proceeds through the formation of complex-substrate intermediations and was further substantiated by DFT calculations. Moreover, active catalysts 1 and 2 were effectively utilized for the base-free oxidation of benzylic alcohols in the presence of air as a green and sustainable oxidant and catalytic amount of TEMPO in acetonitrile. Various substituted benzylic alcohols smoothly converted to their corresponding aldehydes under very mild conditions and ambient temperature. The present catalytic protocol showcases its environmental sustainability by producing minimal waste.

Bartender: Martini 3 Bonded Terms via Quantum Mechanics-Based Molecular Dynamics

Coarse-grained (CG) molecular dynamics (MD) simulations have grown in applicability over the years. The recently released version of the Martini CG force field (Martini 3) has been successfully applied to simulate many processes, including protein-ligand binding. However, the current ligand parametrization scheme is manual and requires an a priori reference all-atom (AA) simulation for benchmarking. For systems with suboptimal AA parameters, which are often unknown, this translates into a CG model that does not reproduce the true dynamical behavior of the underlying molecule. Here, we present Bartender, a quantum mechanics (QM)/MD-based parametrization tool written in Go. Bartender harnesses the power of QM simulations and produces reasonable bonded terms for Martini 3 CG models of small molecules in an efficient and user-friendly manner. For small, ring-like molecules, Bartender generates models whose properties are indistinguishable from the human-made models. For more complex, drug-like ligands, it is able to fit functional forms beyond simple harmonic dihedrals and thus better captures their dynamical behavior. Bartender has the power to both increase the efficiency and the accuracy of Martini 3-based high-throughput applications by producing numerically stable and physically realistic CG models.

Cobalt-Mediated Photochemical C-H Arylation of Pyrroles

Precious metal complexes remain ubiquitous in photoredox catalysis (PRC) despite concerted efforts to find more earth-abundant catalysts and replacements based on 3d metals in particular. Most otherwise plausible 3d metal complexes are assumed to be unsuitable due to short-lived excited states, which has led researchers to prioritize the pursuit of longer excited-state lifetimes through careful molecular design. However, we report herein that the C-H arylation of pyrroles and related substrates (which are benchmark reactions for assessing the efficacy of photoredox catalysts) can be achieved using a simple and readily accessible octahedral bis(diiminopyridine) cobalt complex, [1-Co](PF6)2. Notably, [1-Co]2+ efficiently functionalizes both chloro- and bromoarene substrates despite the short excited-state lifetime of the key photoexcited intermediate *[1-Co]2+ (8 ps). We present herein the scope of this C-H arylation protocol and provide mechanistic insights derived from detailed spectroscopic and computational studies. These indicate that, despite its transient existence, reduction of *[1-Co]2+ is facilitated via pre-assembly with the NEt3 reductant, highlighting an alternative strategy for the future development of 3d metal-catalyzed PRC.

Directly-Fused Ni(II)Porphyrin Conjugated Polymers with Blocked meso-Positions: Impact on Electrocatalytic Properties

The oxidative coupling reaction of two Ni(II) porphyrins meso-substituted with three and four phenyl groups, Ni(II) 5,10,15-(triphenyl)porphyrin (NiPh3P) and Ni(II) 5,10,15,20-(tetraphenyl)porphyrin (NiPh4P) respectively, was investigated in a oxidative chemical vapor deposition (oCVD) process. Irrespective of the number of meso-substituents, high-resolution mass spectrometry evidences the formation of oligomeric species containing up to five porphyrin units. UV-Vis-NIR and XPS analyses of the oCVD films highlighted a strong dependence of the intermolecular coupling reaction with the substrate temperature. Specifically, higher substrate temperatures yield lowering of valence band maxima and reduction of the band gap. The formation of conjugated polymeric assemblies results in increased conductivities as compared to their sublimed counterparts. Yet, electrocatalytic measurements exhibit water oxidation onset overpotentials (308 mV for pNiPh3P and 343 mV for pNiPh4P) comparatively higher than the onset overpotential measured for the oCVD film from Ni(II) 5,15-(diphenyl)porphyrin (pNiPh2P), i. e. 283 mV. Although DFT and comparative oCVD studies suggest the formation of directly fused porphyrins involving 'phenyl-mediated' and β-β linkages when reacting tetra-meso-substituted porphyrins, the present findings highlight that multiple direct fusion (β-β/meso-meso/β-β or meso-β/β-meso) is essential for Ni(II) porphyrin-based conjugated polymers to enable a dinuclear radical oxo-coupling operating mechanism for water oxidation at low overpotential and durable catalytic activity.