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

The Decay of Dispersion Interaction and Its Remarkable Effects on the Kinetics of Activation Reactions Involving Alkyl Chains

The importance of dispersion interactions in many chemical processes is well recognized. It is known that the dispersion strength would decay with the increasing separation between the interacting groups; however, its effects on chemical reactivity have not been well understood. Here we reveal the decay law of dispersion interactions along the n-alkyl chain, its effective interaction ranges for common functional groups, and their remarkable effects on the kinetics of activation reactions involving alkyl chains. This is achieved by DLPNO-CCSD(T) calculations and the local energy decomposition analysis and is supported by experimental findings. In particular, our calculations indicate that the lifetime of alkyl-substituted cis-azobenzenes increases with the alkyl chain length but reaches a steady value when alkyl chains are longer than butyl groups, which is in satisfactory accordance with experimental measurements. We also propose a concise expression to describe the dispersion decay, which shows excellent agreement with our computed results.

Computational Tools for Handling Molecular Clusters: Configurational Sampling, Storage, Analysis, and Machine Learning

Computational modeling of atmospheric molecular clusters requires a comprehensive understanding of their complex configurational spaces, interaction patterns, stabilities against fragmentation, and even dynamic behaviors. To address these needs, we introduce the Jammy Key framework, a collection of automated scripts that facilitate and streamline molecular cluster modeling workflows. Jammy Key handles file manipulations between varieties of integrated third-party programs. The framework is divided into three main functionalities: (1) Jammy Key for configurational sampling (JKCS) to perform systematic configurational sampling of molecular clusters, (2) Jammy Key for quantum chemistry (JKQC) to analyze commonly used quantum chemistry output files and facilitate database construction, handling, and analysis, and (3) Jammy Key for machine learning (JKML) to manage machine learning methods in optimizing molecular cluster modeling. This automation and machine learning utilization significantly reduces manual labor, greatly speeds up the search for molecular cluster configurations, and thus increases the number of systems that can be studied. Following the example of the Atmospheric Cluster Database (ACDB) of Elm (ACS Omega, 4, 10965-10984, 2019), the molecular clusters modeled in our group using the Jammy Key framework have been stored in an improved online GitHub repository named ACDB 2.0. In this work, we present the Jammy Key package alongside its assorted applications, which underline its versatility. Using several illustrative examples, we discuss how to choose appropriate combinations of methodologies for treating particular cluster types, including reactive, multicomponent, charged, or radical clusters, as well as clusters containing flexible or multiconformer monomers or heavy atoms. Finally, we present a detailed example of using the tools for atmospheric acid-base clusters.

Triplet Excited States with Multilevel Coupled Cluster Theory

We extend the multilevel coupled cluster framework with triplet excitation energies at the singles and perturbative doubles (MLCC2) and singles and doubles (MLCCSD) levels of theory. In multilevel coupled cluster theory, we partition the orbitals and restrict the higher-order excitations in the cluster operator to a set of active orbitals. With an appropriate choice of these orbitals, the multilevel approach can give significant computational savings while maintaining the high accuracy of standard coupled cluster theory. In this work, we generated active orbitals from approximate correlated natural transition orbitals (CNTOs). The CNTOs form a compact orbital space specifically tailored to describe the triplet excited states of interest. We compare the performance of MLCCSD and MLCC2, in terms of cost and accuracy, to those of their standard coupled cluster counterparts (CC2 and CCSD) and finally show proof-of-concept calculations of the singlet-triplet gaps of molecules that are of interest for their potential use in organic light-emitting diodes.

Improved Configurational Sampling Protocol for Large Atmospheric Molecular Clusters

The nucleation process leading to the formation of new atmospheric particles plays a crucial role in aerosol research. Quantum chemical (QC) calculations can be used to model the early stages of aerosol formation, where atmospheric vapor molecules interact and form stable molecular clusters. However, QC calculations heavily depend on the chosen computational method, and when dealing with large systems, striking a balance between accuracy and computational cost becomes essential. We benchmarked the binding energies and structures and found the B97-3c method to be a good compromise between the accuracy and computational cost for studying large cluster systems. Further, we carefully assessed configurational sampling procedures for targeting large atmospheric molecular clusters containing up to 30 molecules (approximately 2 nm in diameter) and proposed a funneling approach with highly improved accuracy. We find that several parallel ABCluster explorations lead to better guesses for the cluster global energy minimum structures than one long exploration. This methodology allows us to bridge computational studies of molecular clusters, which typically reach only around 1 nm, with experimental studies that often measure particles larger than 2 nm. By employing this workflow, we searched for low-energy configurations of large sulfuric acid-ammonia and sulfuric acid-dimethylamine clusters. We find that the binding free energies of clusters containing dimethylamine are unequivocally more stable than those of the ammonia-containing clusters. Our improved configurational sampling protocol can in the future be applied to study the growth and dynamics of large clusters of arbitrary compositions.

Cluster-in-Molecule Approach with Explicitly Correlated Methods for Large Molecules

In this article, we present a series of explicitly correlated local correlation methods developed under the cluster-in-molecule (CIM) framework, including explicitly correlated second-order Møller-Plesset perturbation (MP2), coupled-cluster singles and doubles (CCSD), domain-based local pair natural orbital CCSD (DLPNO-CCSD), and DLPNO-CCSD with perturbative triples (DLPNO-CCSD(T)). In these methods, F12 correction is decomposed into contributions from each occupied local molecular orbital and then evaluated independently in a given cluster, which consists of a subset of localized orbitals. These newly developed methods allow F12 calculations of large molecules (up to 145 atoms for quasi-one-dimensional systems) on a single node. We use these methods to investigate the relative stability between extended and folded alkane C30H62, the relative stability of four secondary structures of a polyglycine Ace(Gly)10NH2, and the binding energies of two host-guest complexes. The results demonstrate that the combination of CIM with F12 methods is a promising way to investigate large molecules with small basis set errors.

Theoretical Insight into the Effect of Phosphorus Oxygenation on Nonradiative Decays: Comparative Analysis of P-Bridged Stilbene Analogs

Incorporation of the phosphorus element into a π-conjugated skeleton offers valuable prospects for adjusting the electronic structure of the resulting functional π-electron systems. Trivalent phosphorus has the potential to decrease the LUMO level through σ*-π* interaction, which is further enhanced by its oxygenation to the pentavalent P center. This study shows that utilizing our computational analysis to examine excited-state dynamics based on radiative/nonradiative rate constants and fluorescence quantum yield (ΦF) is effective for analyzing the photophysical properties of P-containing organic dyes. We theoretically investigate how the trivalent phosphanyl group and pentavalent phosphine oxide moieties affect radiative and nonradiative decay processes. We evaluate four variations of P-bridged stilbene analogs. Our analysis reveals that the primary decay pathway for photoexcited bis-phosphanyl-bridged stilbene is the intersystem crossing (ISC) to the triplet state and nonradiative. The oxidation of the phosphine moiety, however, suppresses the ISC due to the relative destabilization of the triplet states. The calculated rate constants match an increase in experimental ΦF from 0.07 to 0.98, as simulated from 0.23 to 0.94. The reduced HOMO-LUMO gap supports a red shift in the fluorescence spectra relative to the phosphine analog. The thiophene-fused variant with the nonoxidized trivalent P center exhibits intense emission with a high ΦF, 0.95. Our prediction indicates that the ISC transfer is obstructed owing to the relatively destabilized triplet state induced by the thiophene substitution. Conversely, the thiophene-fused analog with the phosphine oxide moieties triggers a high-rate internal conversion mediated by conical intersection, leading to a decreased ΦF.

Many-Body Methods for Surface Chemistry Come of Age: Achieving Consensus with Experiments

The adsorption energy of a molecule onto the surface of a material underpins a wide array of applications, spanning heterogeneous catalysis, gas storage, and many more. It is the key quantity where experimental measurements and theoretical calculations meet, with agreement being necessary for reliable predictions of chemical reaction rates and mechanisms. The prototypical molecule-surface system is CO adsorbed on MgO, but despite intense scrutiny from theory and experiment, there is still no consensus on its adsorption energy. In particular, the large cost of accurate many-body methods makes reaching converged theoretical estimates difficult, generating a wide range of values. In this work, we address this challenge, leveraging the latest advances in diffusion Monte Carlo (DMC) and coupled cluster with single, double, and perturbative triple excitations [CCSD(T)] to obtain accurate predictions for CO on MgO. These reliable theoretical estimates allow us to evaluate the inconsistencies in published temperature-programed desorption experiments, revealing that they arise from variations in employed pre-exponential factors. Utilizing this insight, we derive new experimental estimates of the (electronic) adsorption energy with a (more) precise pre-exponential factor. As a culmination of all of this effort, we are able to reach a consensus between multiple theoretical calculations and multiple experiments for the first time. In addition, we show that our recently developed cluster-based CCSD(T) approach provides a low-cost route toward achieving accurate adsorption energies. This sets the stage for affordable and reliable theoretical predictions of chemical reactions on surfaces to guide the realization of new catalysts and gas storage materials.

Structures and Anharmonic Analyses of the O-H Stretching Vibrations of Jet-Cooled Benzoic Acid (BA), (BA)(H2O)n, and (BA)2(H2O)n (n = 1, 2) Clusters, and Their Ring-Deuterated Isotopologues Measured with IR-VUV Spectroscopy─Unraveling the Complex Anharmonic Couplings in the Cyclic Structures

The IR spectra of benzoic acid (BA), (BA)(H2O)n and (BA)2(H2O)n (n = 1, 2) clusters, and their ring-deuterated isotopologues in the 2800-3750 cm-1 region were measured with IR-vacuum ultraviolet spectroscopy under the jet-cooled condition. For (BA)(H2O) and (BA)(H2O)2, only a single isomer was observed for each species, whereas for (BA)2(H2O) and (BA)2(H2O)2, more than one isomers were present. The observed IR spectra were very complex and showed similar structures between (BA)m(H2O)n and their ring-deuterated isotopologues (BA-d5)m(H2O)n for specific values of m and n. The anharmonic analysis based on the vibrational second-order perturbation theory indicated that the complexity of the IR spectra in these clusters was due to the appearance of many bands of (i) the overtone and combination modes involving the O-H bend of H2O and the in-plane C-O-H bends and the C═O stretch of BA, and (ii) the combination modes involving the hydrogen-bonded O-H stretch and low-frequency intermolecular vibrations, with considerable intensities.

Accelerating Coupled-Cluster Calculations with GPUs: An Implementation of the Density-Fitted CCSD(T) Approach for Heterogeneous Computing Architectures Using OpenMP Directives

An algorithm is presented for the coupled-cluster singles, doubles, and perturbative triples correction [CCSD(T)] method based on the density fitting or the resolution-of-the-identity (RI) approximation for performing calculations on heterogeneous computing platforms composed of multicore CPUs and graphics processing units (GPUs). The directive-based approach to GPU offloading offered by the OpenMP application programming interface has been employed to adapt the most compute-intensive terms in the RI-CCSD amplitude equations with computational costs scaling as O ( N O 2 N V 4 ) , O ( N O 3 N V 3 ) , and O ( N O 4 N V 2 ) (where NO and NV denote the numbers of correlated occupied and virtual orbitals, respectively) and the perturbative triples correction to execute on GPU architectures. The pertinent tensor contractions are performed using an accelerated math library such as cuBLAS or hipBLAS. Optimal strategies are discussed for splitting large data arrays into tiles to fit them into the relatively small memory space of the GPUs, while also minimizing the low-bandwidth CPU-GPU data transfers. The performance of the hybrid CPU-GPU RI-CCSD(T) code is demonstrated on pre-exascale supercomputers composed of heterogeneous nodes equipped with NVIDIA Tesla V100 and A100 GPUs and on the world's first exascale supercomputer named "Frontier", the nodes of which consist of AMD MI250X GPUs. Speedups within the range 4-8× relative to the recently reported CPU-only algorithm are obtained for the GPU-offloaded terms in the RI-CCSD amplitude equations. Applications to polycyclic aromatic hydrocarbons containing 16-66 carbon atoms demonstrate that the acceleration of the hybrid CPU-GPU code for the perturbative triples correction relative to the CPU-only code increases with the molecule size, attaining a speedup of 5.7× for the largest circumovalene molecule (C66H20). The GPU-offloaded code enables the computation of the perturbative triples correction for the C60 molecule using the cc-pVDZ/aug-cc-pVTZ-RI basis sets in 7 min on Frontier when using 12,288 AMD GPUs with a parallel efficiency of 83.1%.

Assessment of DLPNO-MP2 Approximations in Double-Hybrid DFT

The unfavorable scaling (N5) of the conventional second-order Møller-Plesset theory (MP2) typically prevents the application of double-hybrid (DH) density functionals to large systems with more than 100 atoms. A prominent approach to reduce the computational demand of electron correlation methods is the domain-based local pair natural orbital (DLPNO) approximation that is successfully used in the framework of DLPNO-CCSD(T). Its extension to MP2 [Pinski P.; Riplinger, C.; Valeev, E. F.; Neese, F. J. Chem. Phys. 2015, 143, 034108.] paved the way for DLPNO-based DH (DLPNO-DH) methods. In this work, we assess the accuracy of the DLPNO-DH approximation compared to conventional DHs on a large number of 7925 data points for thermochemistry and 239 data points for structural features, including main-group and transition-metal systems. It is shown that DLPNO-DH-DFT can be applied successfully to perform energy calculations and geometry optimizations for large molecules at a drastically reduced computational cost. Furthermore, PNO space extrapolation is shown to be applicable, similar to its DLPNO-CCSD(T) counterpart, to reduce the remaining error.

Unravelling strong temperature-dependence of JHD in transition metal hydrides: solvation and non-covalent interactions versus temperature-elastic H-H bonds

A number of transition metal hydrides reveal intriguing temperature-dependent JHD in their deuterated derivatives and possibly the temperature dependent hydrogen-hydrogen distance (r(H-H)) as well. Previously, theoretical studies rationalized JHD and r(H-H) changes in such compounds through a "temperature-elastic" structure model with a significant population of vibrational states in an anharmonic potential. Based on the first variable temperature neutron diffraction study of a relevant complex, (p-H-POCOP)IrH2, observation of its elusive counterpart with longer r(H-H), crystallized as an adduct with C6F5I, and thorough spectroscopic and computational study, we argue that the model involving isomeric species in solution at least in some cases is more relevant. The existence of such isomers is enabled or enhanced by solvation and weak non-covalent interactions with solvent, such as halogen or dihydrogen bonds. "Non-classical" hydrides with r(H-H) ≈ 1.0-1.6 Å are especially sensitive to the above-mentioned factors.

Reaction Center Excitation in Photosystem II: From Multiscale Modeling to Functional Principles

Oxygenic photosynthesis is the fundamental energy-converting process that utilizes sunlight to generate molecular oxygen and the organic compounds that sustain life. Protein-pigment complexes harvest light and transfer excitation energy to specialized pigment assemblies, reaction centers (RC), where electron transfer cascades are initiated. A molecular-level understanding of the primary events is indispensable for elucidating the principles of natural photosynthesis and enabling development of bioinspired technologies. The primary enzyme in oxygenic photosynthesis is Photosystem II (PSII), a membrane-embedded multisubunit complex, that catalyzes the light-driven oxidation of water. The RC of PSII consists of four chlorophyll a and two pheophytin a pigments symmetrically arranged along two core polypeptides; only one branch participates in electron transfer. Despite decades of research, fundamental questions remain, including the origin of this functional asymmetry, the nature of primary charge-transfer states and the identity of the initial electron donor, the origin of the capability of PSII to enact charge separation with far-red photons, i.e., beyond the "red limit" where individual chlorophylls absorb, and the role of protein conformational dynamics in modulating charge-separation pathways.In this Account, we highlight developments in quantum-chemistry based excited-state computations for multipigment assemblies and the refinement of protocols for computing protein-induced electrochromic shifts and charge-transfer excitations calibrated with modern local correlation coupled cluster methods. We emphasize the importance of multiscale atomistic quantum-mechanics/molecular-mechanics and large-scale molecular dynamics simulations, which enabled direct and accurate modeling of primary processes in RC excitation at the quantum mechanical level.Our findings show how differential protein electrostatics enable spectral tuning of RC pigments and generate functional asymmetry in PSII. A chlorophyll pigment on the active branch (ChlD1) has the lowest site energy in PSII and is the primary electron donor. The complete absence of low-lying charge-transfer states within the central pair of chlorophylls excludes a long-held assumption about the initial charge separation. Instead, we identify two primary charge separation pathways, both with the same pheophytin acceptor (PheoD1): a fast pathway with ChlD1 as the primary electron donor (short-range charge-separation) and a slow pathway with PD1PD2 as the initial donor (long-range charge separation). The low-energy spectrum is dominated by two states with significant charge-transfer character, ChlD1δ+PheoD1δ- and PD1δ+PheoD1δ-. The conformational dynamics of PSII allows these charge-transfer states to span wide energy ranges, pushing oxygenic photosynthesis beyond the "red limit". These results provide a quantum mechanical picture of the primary events in the RC of oxygenic photosynthesis, forming a solid basis for interpreting experimental observations and for extending photosynthesis research in new directions.

Coupled-Cluster Density-Based Many-Body Expansion

While CCSD(T) is often considered the "gold standard" of computational chemistry, the scaling of its computational cost as N7 limits its applicability for large and complex molecular systems. In this work, we apply the density-based many-body expansion [ Int. J. Quantum Chem. 2020, 120, e26228] in combination with CCSD(T). The accuracy of this approach is assessed for neutral, protonated, and deprotonated water hexamers, as well as (H2O)16 and (H2O)17 clusters. For the neutral water clusters, we find that already with a density-based two-body expansion, we are able to approximate the supermolecular CCSD(T) energies within chemical accuracy (4 kJ/mol). This surpasses the accuracy that is achieved with a conventional, energy-based three-body expansion. We show that this accuracy can be maintained even when approximating the electron densities using Hartree-Fock instead of using coupled-cluster densities. The density-based many-body expansion thus offers a simple, resource-efficient, and highly parallelizable approach that makes CCSD(T)-quality calculations feasible where they would otherwise be prohibitively expensive.

The electrophilic aromatic bromination of benzenes: mechanistic and regioselective insights from density functional theory

The HBr-assisted electrophilic aromatic bromination of benzene, anisole and nitrobenzene was investigated using static DFT calculations in gas phase and implicit apolar (CCl4) and polar (acetonitrile) solvent models at the ωB97X-D/cc-pVTZ level of theory. The reaction profiles corresponding to either a direct substitution reaction or an addition-elimination process were constructed and insight into the preferred regioselectivity was provided using a combination of conceptual DFT reactivity indices, aromaticity indices, Wiberg bond indices and the non-covalent interaction index. Our results show that under the considered reaction conditions the bromination reaction preferentially occurs through an addition-elimination mechanism and without formation of a stable charged Wheland intermediate. The ortho/para directing effect of the electron-donating methoxy-group in anisole was ascribed to a synergy between strong electron delocalisation and attractive interactions. In contrast, the preferred meta-addition on nitrobenzene could not be traced back to any of these effects, nor to the intrinsic reactivity property of the reactant. In this case, an electrostatic clash between the ipso-carbon of the ring and the nitrogen atom resulting from the later nature of the rate-determining step, with respect to anisole, appeared to play a crucial role.

Prebiotic dimer and trimer peptide formation in gas-phase atmospheric nanoclusters of water

Insight into the origin of prebiotic molecules is key to our understanding of how living systems evolved into the complex network of biological processes on Earth. By modelling diglycine and triglycine peptide formation in the prebiotic atmosphere, we provide a plausible pathway for peptide growth. By examining different transition states (TSs), we conclude that the formation of diglycine and triglycine in atmospheric nanoclusters of water in the prebiotic atmosphere kinetically favors peptide growth by an N-to-C synthesis of glycines through a trans conformation. Addition of water stabilizes the TS structures and lowers the Gibbs free activation energies. At temperatures that model the prebiotic atmosphere, the free energies of activation with a six water nanocluster as part of the TS are predicted to be 16 kcal mol-1 relative to the prereactive complex. Examination of the trans vs. cis six water transition states reveals that a homodromic water network that maximizes the acceptor/donor nature of the six waters is responsible for enhanced kinetic favorability of the trans N-to-C pathway. Compared to the non-hydrated trans TS, the trans six-water TS accelerates the reaction of diglycine and glycine to form triglycine by 13 orders of magnitude at 217 K. Nature uses the trans N-to-C pathway to synthesize proteins in the ribosome, and we note the similarities in hydrogen bond stabilization between the transition state for peptide synthesis in the ribosome and the transition states formed in nanoclusters of water in the same pathway. These results support the hypothesis that small oligomers formed in the prebiotic atmosphere and rained onto earth's surface.

Accurate Calculation of Isomerization and Conformational Energies of Larger Molecules Using Explicitly Correlated Local Coupled Cluster Methods in Molpro and ORCA

An overview of the approximations in the explicitly correlated local coupled cluster methods PNO-LCCSD(T)-F12 in Molpro and DLPNO-CCSD(T)F12 in ORCA is given. Options to select the domains of projected atomic orbitals (PAOs), pair natural orbitals (PNOs), and triples natural orbitals (TNOs) in both programs are described and compared in detail. The two programs are applied to compute isomerization and conformational energies of the ISOL24 and ACONFL test sets, where the former is part of the GMTKN55 benchmark suite. Thorough studies of basis set effects are presented for selected systems. These revealed large intramolecular basis set superposition effects that make it practically impossible to reliably determine the complete basis set (CBS) limits without including explicitly correlated terms. The latter strongly reduce the basis set dependence and at the same time also errors caused by the local domain approximations. On the basis of these studies, the PNO-LCCSD(T)-F12 method is applied to determine new reference energies for the above-mentioned benchmark sets. We are confident that our results should agree within a few tenths of a kcal mol-1 with the (unknown) CCSD(T)/CBS values, which therefore allowed us to define computational settings for accurate explicitly correlated local coupled cluster methods with moderate computational effort. With these protocols, especially PNO-LCCSD(T)-F12b/AVTZ', reliable reference values for comprehensive benchmark sets can be generated efficiently. This can significantly advance the development and evaluation of the performance of approximate electronic structure methods, especially improved density functional approximations or machine learning approaches.

A Perspective on Sustainable Computational Chemistry Software Development and Integration

The power of quantum chemistry to predict the ground and excited state properties of complex chemical systems has driven the development of computational quantum chemistry software, integrating advances in theory, applied mathematics, and computer science. The emergence of new computational paradigms associated with exascale technologies also poses significant challenges that require a flexible forward strategy to take full advantage of existing and forthcoming computational resources. In this context, the sustainability and interoperability of computational chemistry software development are among the most pressing issues. In this perspective, we discuss software infrastructure needs and investments with an eye to fully utilize exascale resources and provide unique computational tools for next-generation science problems and scientific discoveries.

Adamantylidenecarbene: Photochemical Generation, Trapping, and Theoretical Studies

Photolysis of 1-(2-adamantylidene)-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene in benzene (or benzene-d6) at ambient temperature produces adamantylidenecarbene. The carbene undergoes dimerization to a cumulene and may also be trapped in a stereospecific fashion by cis- and trans-4-methyl-2-pentene. No products attributable to 4-homoadamantyne, resulting from ring expansion of the carbene, could be detected. Coupled cluster/density functional theory calculations place the singlet carbene ∼49 kcal/mol below the triplet and show that the former must overcome a barrier of ∼13.5 kcal/mol to rearrange into 4-homoadamantyne.

Activation of the GaI Cation for Bond Activation: from Oxidative Additions into C-Cl and H-P Bonds to Reversible Insertion into P4

Although the discovery of the GaI complex salt [Ga(PhF)2-3 ][Al(ORF )4 ] (RF =C(CF3 )3 , PhF=C6 H5 F) invoked the preparation of a diverse library of cationic Ga(I) coordination complexes and clusters, studies on small molecule activation with low-valent GaI cations are scarce. Herein, a first experimental study on the reactivity of a monomeric Ga(I) cation activated with a pyridine-diimine pincer ligand (in [Ga(PDIdipp )][Al(ORF )4 ]) towards small-molecules is reported. First controlled oxidative additions of the GaI cation into C-Cl, H-P and P-P bonds are presented. Moreover, the [4+1]cycloaddition to butadienes was achieved. Intriguingly, the isolated, blue insertion product into the P-P bond of P4 allows for the quantitative release of the P4 molecule upon reaction with AlEt3 and butadienes. Reversible P4 insertion of main-group metals has previously been reported for Ge and Sn, respectively. The experimental study is supported by high-level computational analysis of the in-part reversible oxidative additions at the DLPNO-CCSD(T)/def2-TZVPP//PBEh-3c/def2-mSVP level of theory with COSMO-RS solvation in 1,2-difluorobenzene.

A 2,2-Difluoroimidazolidine Derivative for Deoxyfluorination Reactions: Mechanistic Insights by Experimental and Computational Studies

A N-heterocyclic deoxyfluorinating agent SIMesF2 was synthesized by nucleophilic fluorination of N,N-1,3-dimesityl-2-chloroimidazolidinium chloride (3) at room temperature. SIMesF2 was applied to deoxyfluorinate carboxylic acids and alcohols and convert benzaldehyde into difluorotoluene. Mechanistic studies by NMR spectroscopy suggest reaction pathways of the carboxylic acid to acyl fluoride via outer-sphere fluorinations at an imidazolidinium ion by polyfluoride. DFT studies give further insight by exploring mechanistic details which distinguish the fluorination of aldehydes from that of carboxylic acids. Furthermore, a consecutive reaction sequence for the oxidation of an aldehyde followed by in situ fluorination of the generated carboxylic acid was developed.

Reduced Scaling Real-Time Coupled Cluster Theory

Real-time coupled cluster (CC) methods have several advantages over their frequency-domain counterparts, namely, response and equation of motion CC theories. Broadband spectra, strong fields, and pulse manipulation allow for the simulation of complex spectroscopies that are unreachable using frequency-domain approaches. Due to the high-order polynomial scaling, the required numerical time propagation of the CC residual expressions is a computationally demanding process. This scaling may be reduced by local correlation schemes, which aim to reduce the size of the (virtual) orbital space by truncation according to user-defined parameters. We present the first application of local correlation to real-time CC. As in previous studies of locally correlated frequency-domain CC, traditional local correlation schemes are of limited utility for field-dependent properties; however, a perturbation-aware scheme proves promising. A detailed analysis of the amplitude dynamics suggests that the main challenge is a strong time dependence of the wave function sparsity.

Exploring π-π interactions and electron transport in complexes involving a hexacationic host and PAH guest: a promising avenue for molecular devices

Single isolated molecules and supramolecular host-guest systems, which consist of π-π stacking interactions, are emerging as promising building blocks for creating molecular electronic devices. In this article, we have investigated the noncovalent π-π interaction and intermolecular electron charge transport involved in a series of host-guest complexes formed between a cage-like host (H6+) and polycyclic aromatic hydrocarbon (PAH) guests (G1-G7) using different quantum chemical approaches. The host (H6+) consists of two triscationic π-electron-deficient trispyridiniumtriazine (TPZ3+) units that are bridged face-to-face by three ethylene-triazole-ethylene. Our theoretical calculations show that the perylene and naphthalene inclusion complexes G7⊂H and G1⊂H have the highest and lowest interaction energies, respectively. In addition, energy decomposition analysis (EDA) indicated that the dispersion interaction term, ΔEdisp, significantly contributes to the host-guest interaction and is correlated with the existence of π-π van der Waals interaction. Using the nonequilibrium Greens function (NEGF) method in combination with density functional theory (DFT), the current-voltage (I-V) curves of the complexes were estimated. The conductance values increased when the guests were embedded inside the host cavity. Notably, the complex G7⊂H has the maximum conductance value. Overall, this study provided the electron transport of the PAH inclusion host-guest complex through π-π interaction and provided a direction for the fabrication of future supramolecular molecular devices.

Metastable Charged Dimers in Organometallic Species: A Look into Hydrogen Bonding between Metallocene Derivatives

Multiply charged complexes bound by noncovalent interactions have been previously described in the literature, although they were mostly focused on organic and main group inorganic systems. In this work, we show that similar complexes can also be found for organometallic systems containing transition metals and deepen in the reasons behind the existence of these species. We have studied the structures, binding energies, and dissociation profiles in the gas phase of a series of charged hydrogen-bonded dimers of metallocene (Ru, Co, Rh, and Mn) derivatives isoelectronic with the ferrocene dimer. Our results indicate that the carboxylic acid-containing dimers are more strongly bonded and present larger barriers to dissociation than the amide ones and that the cationic complexes tend to be more stable than the anionic ones. Additionally, we describe for the first time the symmetric proton transfer that can occur while in the metastable phase. Finally, we use a density-based energy decomposition analysis to shine light on the nature of the interaction between the dimers.

CO2 on Graphene: Benchmarking Computational Approaches to Noncovalent Interactions

Designing and optimizing graphene-based gas sensors in silico entail constructing appropriate atomistic representations for the physisorption complex of an analyte on an infinite graphene sheet, then selecting accurate yet affordable methods for geometry optimizations and energy computations. In this work, diverse density functionals (DFs), coupled cluster theory, and symmetry-adapted perturbation theory (SAPT) in conjunction with a range of finite and periodic surface models of bare and supported graphene were tested for their ability to reproduce the experimental adsorption energies of CO2 on graphene in a low-coverage regime. Periodic results are accurately reproduced by the interaction energies extrapolated from finite clusters to infinity. This simple yet powerful scheme effectively removes size dependence from the data obtained using finite models, and the latter can be treated at more sophisticated levels of theory relative to periodic systems. While for small models inexpensive DFs such as PBE-D3 afford surprisingly good agreement with the gold standard of quantum chemistry, CCSD(T), interaction energies closest to experiment are obtained by extrapolating the SAPT results and with nonlocal van der Waals functionals in the periodic setting. Finally, none of the methods and models reproduce the experimentally observed CO2 tilted adsorption geometry on the Pt(111) support, calling for either even more elaborate theoretical approaches or a revision of the experiment.

Complexes of resorcin[4]arene with secondary amines: synthesis, solvent influence on "in-out" structure, and theoretical calculations of non-covalent interactions

Resorcin[4]arenes (R[4]A) are macrocyclic compounds with a cavity structure. Despite a relatively small cavity, these compounds are capable of forming complexes with small organic molecules. The current paper focuses on the synthesis of complexes between R[4]A and secondary aliphatic amines (sec-amines). Through NMR spectroscopy, it was observed that "in-out" complexes are formed depending on the solvent. It was also found that the stoichiometry of the formed complexes depends on the size of the amine molecule. The automated interaction sites screening (aISS) made it possible to generate molecular ensembles of complexes. The geometry of the ensembles was first optimized with the r2scan-3c functional and, finally, the structure with the lowest energy, with the functional PBE0-D4/mTZVPP/CPCM. The Hartree-Fock plus London dispersion (HFLD) method was used for the study of non-covalent interactions (NCI). The calculations lead to the conclusion that a reduction in electrostatic interactions and an increase in exchange and dispersion interactions in CHCl3 in relation to DMSO are the driving forces behind the placement of sec-amine molecules into the R[4]A cavity and the formation of "in" type complexes.

High-Level Quantum Chemical Prediction of C-F Bond Dissociation Energies of Perfluoroalkyl Substances

In this study, 550 C-F bond dissociation energies (BDEs) of a variety of per- and polyfluoroalkyl substances (PFASs) obtained from high-level DLPNO-CCSD(T)/CBS calculations were used to assess the accuracy of contemporary density functional theory (DFT) and semiempirical methods. DLPNO-CCSD(T)/CBS gas phase C-F BDEs fall between 404.9-550.7 kJ mol-1 and M06-2X and ωB97M-V in conjunction with the aug-cc-pVTZ basis set predicted BDEs closest to the benchmark level with a mean absolute deviation (MAD) of 7.3 and 8.3 kJ mol-1, respectively. It was observed that DFT prediction errors increase with the degree of fluorination and system size. As such, previous model chemistry recommendations based on smaller nonfluorinated systems may not be carried over to modeling the energetics of PFASs and related systems.

Fragment-based models for dissociation of strong acids in water: Electrostatic embedding minimizes the dependence on the fragmentation schemes

Fragmentation methods such as MIM (Molecules-in-Molecules) provide a route to accurately model large systems and have been successful in predicting their structures, energies, and spectroscopic properties. However, their use is often limited to systems at equilibrium due to the inherent complications in the choice of fragments in systems away from equilibrium. Furthermore, the presence of charges resulting from any heterolytic bond breaking may increase the fragmentation error. We have previously suggested EE-MIM (Electrostatically Embedded Molecules-In-Molecules) as a method to mitigate the errors resulting from the missing long-range interactions in molecular clusters in equilibrium. Here, we show that the same method can be applied to improve the performance of MIM to solve the longstanding problem of dependency of the fragmentation energy error on the choice of the fragmentation scheme. We chose four widely used acid dissociation reactions (HCl, HClO4, HNO3, and H2SO4) as test cases due to their importance in chemical processes and complex reaction potential energy surfaces. Electrostatic embedding improves the performance at both one and two-layer MIM as shown by lower EE-MIM1 and EE-MIM2 errors. The EE-MIM errors are also demonstrated to be less dependent on the choice of the fragmentation scheme by analyzing the variation in fragmentation energy at the points with more than one possible fragmentation scheme (points where the fragmentation scheme changes). EE-MIM2 with M06-2X as the low-level resulted in a variation of less than 1 kcal/mol for all the cases and 1 kJ/mol for all but three cases, rendering our method fragmentation scheme-independent for acid dissociation processes.

Toward Modeling the Growth of Large Atmospheric Sulfuric Acid-Ammonia Clusters

Studying large atmospheric molecular clusters is needed to understand the transition between clusters and aerosol particles. In this work, we studied the (SA)n(AM)n clusters with n up to 30 and the (SA)m(AM)m±2 clusters, with m = 6-20. The cluster configurations are sampled using the ABCluster program, and the cluster geometries and thermochemical parameters are calculated using GFN1-xTB. The cluster binding energies are calculated using B97-3c. We find that the addition of sulfuric acid is preferred to the addition of ammonia. The addition free energies were found to have large uncertainties, which could potentially be attributed to errors in the applied level of theory. Based on DLPNO-CCSD(T0)/aug-cc-pVTZ benchmarks of the binding energies of the large (SA)8-9(AM)10 and (SA)10(AM)10-11 clusters, we find that ωB97X-D3BJ with a large basis set is required to yield accurate binding and addition energies. However, based on recalculations of the single-point energy at r2SCAN-3c and ωB97X-D3BJ/6-311++G(3df,3pd), we show that the single-point energy contribution is not the primary source of error. We hypothesize that a larger source of error might be present in the form of insufficient configurational sampling. Finally, we train Δ machine learning model on (SA)n(AM)n clusters with n up to 5 and show that we can predict the binding energies of clusters up to sizes of (SA)30(AM)30 with a binding energy error below 0.6 %. This is an encouraging approach for accurately modeling the binding energies of large acid-base clusters in the future.

Stereochemistry in All Its Shapes and Forms: The 56th Bürgenstock Conference

Acknowledging the crucial role of stereochemistry in fields as diverse as total synthesis, synthetic methodology, spectroscopy, and the study of the origin of life, the 56th SCS Conference on Stereochemistry, better known as the BÃ1/4rgenstock Conference, brought together a diverse range of chemistry expertise in Brunnen, Switzerland.

Theoretical investigation of anion perfluorocubane

CONTEXT

In this work, we did a theoretical exploration of C8F8 (Ib) and its anion radical analogue (IIb) in this work. By investigating the thermochemistry of electron capture, we find that the free energy associated with the conversion of C8H8 (Ia) into its anion radical analogue IIa is of the order of + 92.83 kcal.mol-1, while the conversion of Ib into IIb is - 6.42 kcal.mol-1. Therefore, species IIb is thermodynamically more stable than its neutral analogue. Natural bond orbitals (NBO) analyses revealed that compound Ib exhibits a relative electronic stability as a function of intramolecular delocalisations of the type [Formula: see text] of the order of 2.70 kcal.mol-1. Similar delocalizations for Ia are energetically lower (1.45 kcal.mol-1). Topological analyses of compounds Ib and IIb indicate that the addition of an electron to Ib enhances the covalency of the C-C bond, as can be seen by the reduction in the ellipticity of the C-C bond. The opposite is observed for Ia, whose addition of the electron (leading to IIa) reduces the covalency of the C-C bond. By comparing the free and packaged forms of the species, it is found that, in the crystalline form, the system will present greater relative stability due to the dispersive interactions involved, as evidenced by non-covalent interactions (NCI) analysis. Finally, it was possible to verify that the manifestation of the current density with a lower paratropic and less antiaromatic character in Ib and IIb point to C8F8 as a strong candidate for electron capture.

METHODS

Geometry optimization calculations were carried out, for all monomer structures using the hybrid functional B3LYP-D3 and the 6-31+G(d,p) basis set. To determine the formation thermochemistry of the ions, electronic energy corrections was performed using the DLPNO-CCSD(T)/aug-cc-pVTZ/C method. Starting from the optimised forms, shielding, nuclear magnetic resonance (NMR) spectra employing gauge-independent atomic orbital (GIAO), and NBO calculations were performed for these monomers, using the PBE0 functional and the pCSseg-2 atomic basis set. The magnetochemical analysis of ring currents was performed using the GIMIC formalism. For the topological analysis, it was applied the combination DLPNO-CCSD(T)/aug-cc-pVTZ/C, previously used for correcting the electronic energy.

Anion Photoelectron Spectroscopy and Theoretical Studies of Ge3n+1O (n = 1-3) Clusters with the C3v Symmetric Ge3 Structural Unit

We investigate Ge3n+1O-/0 (n = 1-3) clusters using anion photoelectron spectroscopy and theoretical calculations. The results show that the lowest energy structure of Ge4O- is a bent Cs symmetric trigonal bipyramidal structure, while Ge4O has a C3v symmetric trigonal bipyramidal structure. Ge7O- has two coexisting low-lying isomers, the first one can be viewed as a Ge2O unit interacting with a Ge5 trigonal bipyramid, the second one can be regarded as an O atom interacting with a Ge7 pentagonal bipyramid; whereas Ge7O has a C3v symmetric structure with a Ge atom and an O atom capping a Ge6 trigonal antiprism from the bottom and top, respectively. The structures of Ge10O- and Ge10O can be obtained by adding an O atom to different binding sites of a C3v symmetric Ge10. Chemical bonding analyses of Ge3n+1O (n = 1-3) reveal that the O atom interacts with its neighboring three Ge atoms forming one 4c-2e σ bond and two 4c-2e π bonds in the top Ge3O trigonal pyramid, while the terminal Ge atom forms one 4c-2e σ bond in the bottom Ge4 trigonal pyramid. The large HOMO-LUMO gaps of Ge3n+1O (n = 1-3) indicate that they have good stabilities. Ab initio molecular dynamics simulations suggest that both Ge7O and Ge10O are dynamically stable in general at 300 and 500 K. The current work suggests that the C3v symmetric Ge3 units and the insertion growth pattern may be viable for constructing 1D germanium oxide nanostructures with the chemical formula of Ge3n+1O.

Actinide-Actinide Bonding: Electron Delocalisation and σ-Aromaticity in the Tri-Thorium Cluster [{Th(η8 -C8 H8 )(μ-Cl)2 }3 K2 ]

The tri-thorium cluster [{Th(η8 -C8 H8 )(μ3 -Cl)2 }3 {K(THF)2 }2 ]∞ (Nature 2021, 598, 72-75) was reported to feature intriguing σ-aromatic bonding between the thorium atoms, a mode of metal-metal bonding unique in the actinide series. However, the presence of this bonding motif has since been challenged by others. Here, we computationally explore electron delocalisation in a molecular cluster fragment of [{Th(η8 -C8 H8 )(μ3 -Cl)2 }3 {K(THF)2 }2 ]∞ and examine its responses to an applied magnetic field using a variety of methods. We also discuss the importance of the choice of basis set for the Th atoms and issues regarding locating QTAIM bond critical points. When taken together, the computed data consistently suggest the presence of delocalised Th-Th bonding and Th3 σ-aromaticity.

Insights into the catalytic activity of boron-doped thiazoles in the Diels-Alder reaction

The role of boron-doped thiazoles as a Lewis acid catalyst in [4+2] cycloaddition reaction between 1,3-butadiene and acrolein has been addressed. Three different organic heterocycles were designed to study their catalytic activity. It has been observed that these heterocycles efficiently work as catalysts than the well-known Lewis acid BF3. All the reactions follow the normal electron demand process and are exothermic. Different conceptual DFT-based reactivity descriptors and electronic structure principles such as maximum hardness and minimum electrophilicity lend additional support to the feasibility of the reaction mechanism. The reaction force (RF), reaction electronic flux (REF), and its different components exhibit a detailed electronic activity throughout the reaction.

Correlation Consistent Basis Sets for Explicitly Correlated Theory: The Transition Metals

We present correlation consistent basis sets for explicitly correlated (F12) calculations, denoted VnZ(-PP)-F12-wis (n = D,T), for the d-block elements. The cc-pVDZ-F12-wis basis set is contracted to [8s7p5d2f] for the 3d-block, while its ECP counterpart for the 4d and 5d-blocks, cc-pVDZ-PP-F12-wis, is contracted to [6s6p5d2f]. The corresponding contracted sizes for cc-pVTZ(-PP)-F12-wis are [9s8p6d3f2g] for the 3d-block elements and [7s7p6d3f2g] for the 4d and 5d-block elements. Our VnZ(-PP)-F12-wis basis sets are evaluated on challenging test sets for metal-organic barrier heights (MOBH35) and group-11 metal clusters (CUAGAU-2). In F12 calculations, they are found to be about as close to the complete basis set limit as the combination of standard cc-pVnZ-F12 on main-group elements with the standard aug-cc-pV(n+1)Z(-PP) basis sets on the transition metal(s). While our basis sets are somewhat more compact than aug-cc-pV(n+1)Z(-PP), the CPU time benefit is negligible for catalytic complexes that contain only one or two transition metals among dozens of main-group elements; however, it is somewhat more significant for metal clusters.

Production of Dihydrogen Using Ammonia Borane as Reagent and Pyrazole as Catalyst

Theoretical chemistry (DLPNO-CCSD(T)/def2-TZVP//M06-2x/aug-cc-pVDZ) was used to design a system based on ammonia boranes catalyzed by pyrazoles with the aim of producing dihydrogen, nowadays of high interest as clean fuel. The reactivity of ammonia borane and cyclotriborazane were investigated, including catalytic activation through 1H-pyrazole, 4-methoxy-1H-pyrazole, and 4-nitro-1H-pyrazole. The results point toward a catalytic cycle by which, at the same time, ammonia borane can initially store and then, through catalysis, produce dihydrogen and amino borane. Subsequently, amino borane can trimerize to form cyclotriborazane that, in presence of the same catalyst, can also produce dihydrogen. This study proposes therefore a consistent progress in using environmentally sustainable (metal free) catalysts to efficiently extract dihydrogen from small B-N bonded molecules.

Phosphorescent Properties of Heteroleptic Ir(III) Complexes: Uncovering Their Emissive Species

In this contribution, we assess the computational machinery to calculate the phosphorescence properties of a large pool of heteroleptic [Ir(C^N)2(N^N)]+ complexes (where N^N is an ancillary ligand and C^N is a cyclometalating ligand) including their phosphorescent rates and their emission spectra. Efficient computational protocols are next proposed. Specifically, different flavors of DFT functionals were benchmarked against DLPNO-CCSD(T) for the phosphorescence energies. The transition density matrix and decomposition analysis of the emitting triplet excited state enable us to categorize the studied complexes into different cases, from predominant triplet ligand-centered (3LC) character to predominant charge-transfer (3CT) character, either of metal-to-ligand charge transfer (3MLCT), ligand-to-ligand charge transfer (3LLCT), or a combination of the two. We have also calculated the vibronically resolved phosphorescent spectra and rates. Ir(III) complexes with predominant 3CT character are characterized by less vibronically resolved bands as compared to those with predominant 3LC character. Furthermore, some of the complexes are characterized by close-lying triplet excited states so that the calculation of their phosphorescence properties poses additional challenges. In these scenarios, it is necessary to perform geometry optimizations of higher-lying triplet excited states (i.e., Tn). We demonstrate that in the latter scenarios all of the close-lying triplet species must be considered to recover the shape of the experimental emission spectra. The global analysis of computed emission energies, shape of the computed emission spectra, computed rates, etc. enable us to unambiguously pinpoint for the first time the triplet states involved in the emission process and to provide a general classification of Ir(III) complexes with regard to their phosphorescence properties.

On the Intermolecular Interactions in Thiophene-Cored Single-Stacking Junctions

There have been attempts, both experimental and based on density-functional theory (DFT) modeling, at understanding the factors that govern the electronic conductance behavior of single-stacking junctions formed by pi-conjugated materials in nanogaps. Here, a reliable description of relevant stacked configurations of some thiophene-cored systems is provided by means of high-level quantum chemical approaches. The minimal structures of these configurations, which are found using the dispersion-corrected DFT approach, are employed in calculations that apply the coupled cluster method with singles, doubles and perturbative triples [CCSD(T)] and extrapolations to the complete basis set (CBS) limit in order to reliably quantify the strength of intermolecular binding, while their physical origin is investigated using the DFT-based symmetry-adapted perturbation theory (SAPT) of intermolecular interactions. In particular, for symmetrized S-Tn dimers (where "S" and "T" denote a thiomethyl-containing anchor group and a thiophene segment comprising "n" units, respectively), the CCSD(T)/CBS interaction energies are found to increase linearly with n ≤ 6, and significant conformational differences between the flanking 2-thiophene group in S-T1 and S-T2 are described by the CCSD(T)/CBS and SAPT/CBS computations. These results are put into the context of previous work on charge transport properties of S-Tn and other types of supramolecular junctions.

Caracal: A Versatile Ring Polymer Molecular Dynamics Simulation Package

A new open-source program package named Caracal covering simulations of molecular systems with ring polymer molecular dynamics (RPMD) is presented. It combines a powerful RPMD implementation including chemical reaction rate calculations and biased periodic and nonperiodic samplings with a collection of easy to set up potential energy surface (PES) methodologies, thus delivering an all-inclusive approach. Most implemented PESs are based on the QMDFF and EVB-QMDFF methods. Where the quantum mechanically derived force field (QMDFF) can be set up for an arbitrary molecular system in a black-box fashion, the empirical valence bond (EVB)-QMDFF connects two QMDFFs and is able to represent the PES of a chemical reaction. With our previously published flavors of this composite method, PESs for almost arbitrary gas phase thermal ground state reactions can be set up. Given an optimized reaction path, the mechanism of the reaction can be classified and RPMD rate constants can be obtained via umbrella sampling and recrossing calculations on an EVB-QMDFF PES. Further, QMDFFs can be polymerized for the description of liquid systems. In this paper, the internal structure as well as the handling philosophy of Caracal are outlined. Further, examples of the different possible kinds of calculations are given.

Experimental and Theoretical Exploration of the Kinetics and Thermodynamics of the Nucleophile-Induced Fragmentation of Ylidenenorbornadiene Carboxylates

Ylidenenorbornadienes (YNDs), prepared by [4 + 2] cycloadditions between fulvenes and acetylene carboxylates, react with thiol nucleophiles to yield mixtures of four to eight diastereomers depending on the symmetry of the YND substrate. The mixtures of diastereomers fragment via a retro-[4 + 2] cycloaddition with a large variation in rate, with half-lives ranging from 16 to 11,000 min at 80 °C. The diastereomer-enriched samples of propane thiol adducts [YND-propanethiol (PTs)] were isolated and identified by nuclear Overhauser effect spectroscopy (NOESY) correlations. Simulated kinetics were used to extrapolate the rate constants of individual diastereomers from the observed rate data, and it correlated well with rate constants measured directly and from isolated diastereomer-enriched samples. The individual diastereomers of a model system fragment at differing rates with half-lives ranging from 5 to 44 min in CDCl3. Density functional theory calculations were performed to investigate the mechanism of fragmentation and support an asynchronous retro-[4 + 2] cycloaddition transition state. The computations generally correlated well with the observed free energies of activation for four diastereomers of the model system as a whole, within 2.6 kcal/mol. However, the observed order of the fragmentation rates across the set of diastereomers deviated from the computational results. YNDs display wide variability in the rate of fragmentation, dependent on the stereoelectronics of the ylidene substituents. A Hammett study showed that the electron-rich aromatic rings attached to the ylidene bridge increase the fragmentation rate, while electron-deficient systems slow fragmentation rates.

Accurate Evaluation of Dispersion Energies at Coupled Cluster Level to Understand the Substituent Effects in Am(III) and Eu(III) Complexes

The effect of cyclic and aromatic substituents on the complexation behavior of phosphine oxide ligands with Am(III) and Eu(III) was investigated at density functional theory (DFT) and domain-based local pair natural orbital coupled-cluster (DLPNO-CC) levels. Combining DFT with accurate coupled cluster methods, we have evaluated the dispersion energy contributions to the complexation energies for trivalent Am and Eu complexes for the first time. Irrespective of the nature of substituents on the P atom, the electronic structure of the P═O group remains identical in all of the ligands. The study reveals the importance of dispersion interactions during complexation and is estimated to be more significant for Am(III) than for Eu(III) complexes.

Corrigendum: Coupled cluster theory on modern heterogeneous supercomputers

[This corrects the article DOI: 10.3389/fchem.2023.1154526.].

Rh-Catalyzed [4 + 1] Reaction of Cyclopropyl-Capped Dienes (but not Common Dienes) and Carbon Monoxide: Reaction Development and Mechanistic Study

Transition-metal-catalyzed [4 + 1] reaction of dienes and carbon monoxide (CO) is the most straightforward and easily envisioned cyclization for the synthesis of five-membered carbocycles, which are ubiquitously found in natural products and functional molecules. Unfortunately, no test of this reaction was reported, and consequently, chemists do not know whether such kind of reaction works or not. Herein, we report that the [4 + 1] reaction of common dienes and CO cannot work, at least under the catalysis of [Rh(cod)Cl]2. However, using cyclopropyl-capped dienes (also named allylidenecyclopropanes) as substrates, the corresponding [4 + 1] reaction with CO proceeds smoothly in the presence of [Rh(cod)Cl]2. This [4 + 1] reaction, with a broad scope, provides efficient access to five-membered carbocyclic compounds of spiro[2.4]hept-6-en-4-ones. The [4 + 1] cycloadducts can be further transformed into other molecules by using the unique chemistry of cyclopropyl groups present in these molecules. The mechanism of this [4 + 1] reaction has been investigated by quantum chemical calculations, uncovering that cyclopropyl-capped dienes are strained dienes and the oxidative cyclization step in the [4 + 1] catalytic cycle can release this (angular) strain both kinetically and thermodynamically. The strain release in this step then propagates to all followed CO coordination/CO insertion/reductive elimination steps in the [4 + 1] catalytic cycle, helping the realization of this cycloaddition reaction. In contrast, common dienes (including cyclobutyl-capped dienes) do not have such advantages and their [4 + 1] reaction suffers from energy penalty in all steps involved in the [4 + 1] catalytic cycle. The reactivity of ene-allenes for the [4 + 1] reaction with CO is also discussed.

Mechanistic Analysis of Alkyne Haloboration: A DFT, MP2, and DLPNO-CCSD(T) Study

Stereocontrol of the alkyne haloboration reaction has received attention in many experimental but few theoretical studies. Here we present a detailed quantum-chemical study of mechanisms leading to Z versus E isomers of haloboration products, considering acetylene and propyne combined with BCl3, BBr3, and BI3. Calculations using B3LYP-D3, MP2, and DLPNO-CCSD(T) methods are used to study polar reactions between the alkyne and BX3 in the absence and presence of an additional halide anion whose content in the reaction mixture can be controlled experimentally. The formation of anti-haloboration products via radical mechanisms is also explored, namely, by adding BX3 to (Z)-halovinyl radical. For the anti-haloboration of propyne, the radical route is prohibited by the regiochemistry of the initiating halopropenyl radical, while the polar route is unlikely due to a competitive allene generation. In contrast, energetically accessible routes exist for both syn- and anti-bromoboration of acetylene; hence, careful control of reaction conditions is necessary to steer the stereochemical outcome. Methodologically, MP2 results correspond better to the DLPNO-CCSD(T) energies than the B3LYP-D3 results in terms of both reaction barrier heights and relative ordering of energetically close stationary points.

On the Reactivity of Mes*P(PMe3 ) towards Aluminum(I) Compounds - Evidence for the Intermediate Formation of Phosphaalumenes

Phosphaalumenes are the heavier isoelectronic analogs of alkynes and have eluded facile synthesis until recently. We have reported that the combination of a phosphinidene transfer agent, Ar TerP(PMe3 ) (Ar Ter=2,6-Ar2 -C6 H3 ), with (Cp*Al)4 (Cp*=C5 (CH3 )5 ) afforded the phosphaalumenes Ar TerPAlCp* as isolable, violet, thermally stable compounds. In here we describe attempts to utilize Mes*P(PMe3 ) (Mes*=2,4,6-tBu3 -C6 H2 ) as a phosphinidene source in combination with different Al(I) precursors, namely Dip NacnacAl (Dip Nacnac=HC[C(Me)NDip]2 , Dip=2,6-iPr2 -C6 H3 ), (Cp*Al)4 and Cp3t Al (Cp3t =1,2,4-tBu3 -C5 H2 ). In all cases the formation of phosphaalumenes was not observed, however, their intermediate formation is indicated by formation of the dimer [Cp*Al(μ-PMes*)]2 (2) and C-H-bond activation products along the putative P=Al bond, giving unusual 1,2-P,Al-tetrahydronaphtalene derivatives 1 and 4, clearly underlining the role the sterically demanding group on phosphorus plays in these transformations. The reactivity studies are supported by theoretical studies, demonstrating a thermodynamic preference for the C-H activation products. Additionally, we show that there are potential pitfalls in the synthesis of Cp*2 AlH, the precursor to make (Cp*Al)4 and give recommendations how to circumvent these.

An Efficient Reactive Force Field without Explicit Coordination Dependence for Studying Caustic Aluminum Chemistry

Reactive force fields (RFFs) are an expedient approach to sample chemical reaction paths in complex systems, relative to density functional theory. However, there is continued need to improve efficiencies, specifically in systems that have slow transverse degrees of freedom, as in highly viscous and superconcentrated solutions. Here, we present an RFF that is differentiated from current models (e.g., ReaxFF) by omitting explicit dependence on the atom coordination and employing a small parameter set based on Lennard-Jones, Gaussian, and Stillinger-Weber potentials. The model was parametrized from AIMD simulation data and is used to model aluminate reactivity in sodium hydroxide solutions with extensive validation against experimental radial distribution functions, computed free energy profiles for oligomerization, and formation energies. The model enables simulation of early stage Al(OH)₃ nucleation which has significant relevance to industrial processing of aluminum and has a computational cost that is reduced by 1 order of magnitude relative to ReaxFF.

Metal-free catalytic hydroboration of imine with pinacolborane using a pincer-type phosphorus compound: mechanistic insight and improvement of the reaction

A mechanistic study of the catalytic hydroboration of imine using a pincer-type phosphorus compound 1NP was performed through the combination of DFT and DLPNO-CCSD(T) calculations. The reaction proceeds through a phosphorus-ligand cooperative catalytic cycle, where the phosphorus center and triamide ligand work in a synergistic manner. First, the pinB-H bond activation by 1NP occurs through the cooperative functions of the phosphorus center and the triamide ligand, leading to a phosphorus-hydride intermediate 2NP. This is the rate-determining step, with the Gibbs energy barrier and Gibbs reaction energy of 25.3 and -17.0 kcal mol^-1, respectively. Subsequently, the hydroboration of phenylmethanimine takes place through a concerted transition state through the cooperative function of the phosphorus center and the triamide ligand. It leads to the final hydroborated product 4 with the regeneration of 1NP. Our computational results reveal that the experimentally isolated intermediate 3NP is a resting state of the reaction. It is formed through the B-N bond activation of 4 by 1NP, rather than via the insertion of the CN double bond of phenylmethanimine into the P-H bond of 2NP. However, this side reaction can be suppressed by utilizing a planar phosphorus compound AcrDipp-1NP as the catalyst, which features steric-demanding substituents on the chelated N atom of the ligand.

Low-energy electron interaction with 2-(trifluoromethyl)acrylic acid, a potential component for EUVL resist material

Motivated by the current introduction of extreme ultraviolet lithography (EUVL) into chip manufacturing processes, and thus the transition to electron-induced chemistry within the respective resist materials, we have studied low energy electron-induced fragmentation of 2-(trifluoromethyl)acrylic acid (TFMAA). This compound is chosen as a potential resist component, whereby fluorination enhances the EUV adsorption and may at the same time promote electron-induced dissociation. Dissociative ionization and dissociative electron attachment are studied, and to aid the interpretation of the observed fragmentation channels, the respective threshold values are calculated at the DFT and coupled cluster level of theory. Not surprisingly, we find significantly more extensive fragmentation in DI than in DEA and in fact, the only significant DEA fragmentation channel is the cleavage of HF from the parent molecule upon electron attachment. Rearrangement and new bond formation are substantial in DI and are, in fact, similar to DEA, mainly associated with HF formation. The observed fragmentation reactions are discussed in relation to the underlying reactions and potential implications for the suitability of TFMAA as a component of EUVL resist materials.

Repartitioned Brillouin-Wigner perturbation theory with a size-consistent second-order correlation energy

Second-order Møller-Plesset perturbation theory (MP2) often breaks down catastrophically in small-gap systems, leaving much to be desired in its performance for myriad chemical applications such as noncovalent interactions, thermochemistry, and dative bonding in transition metal complexes. This divergence problem has reignited interest in Brillouin-Wigner perturbation theory (BWPT), which is regular at all orders but lacks size consistency and extensivity, severely limiting its application to chemistry. In this work, we propose an alternative partitioning of the Hamiltonian that leads to a regular BWPT perturbation series that, through the second order, is size-extensive, size-consistent (provided its Hartree-Fock reference is also), and orbital invariant. Our second-order size-consistent Brillouin-Wigner (BW-s2) approach can describe the exact dissociation limit of H2 in a minimal basis set, regardless of the spin polarization of the reference orbitals. More broadly, we find that BW-s2 offers improvements relative to MP2 for covalent bond breaking, noncovalent interaction energies, and metal/organic reaction energies, although rivaling coupled-cluster with single and double substitutions for thermochemical properties.

MP2-Based Correction Scheme to Approach the Limit of a Complete Pair Natural Orbitals Space in DLPNO-CCSD(T) Calculations

The domain-based local pair natural orbital (PNO) coupled-cluster DLPNO-CCSD(T) method has been proven to provide accurate single-point energies at a fraction of the cost of canonical CCSD(T) calculations. However, the desired "chemical accuracy" can only be obtained with a large PNO space and extended basis set. We present a simple yet accurate and efficient correction scheme based on a perturbative approach. Here, in addition to DLPNO-CCSD(T) energy, one calculates DLPNO-MP2 correlation energy with the same settings as in the preceding coupled-cluster calculation. In the next step, the canonical MP2 correlation energy is obtained in the same orbital basis. This can be efficiently performed for essentially all molecule sizes accessible with the DLPNO-CCSD(T) method. By taking the difference between the canonical MP2 and DLPNO-MP2 energies, we obtain a correction term that can be added to the DLPNO-CCSD(T) correlation energy. This way, one can obtain the total correlation energy close to the limit of the complete PNO space (cPNO). The presented approach allows us to significantly increase the accuracy of the DLPNO-CCSD(T) method for both closed- and open-shell systems. The latter are known to be especially challenging for locally correlated methods. Unlike the previously developed PNO extrapolation procedure by Altun, Neese, and Bistoni ( J. Chem. Theory Comput. 2020, 16, 6142-6149), this strategy allows us to get the DLPNO-CCSD(T) correlation energy at the cPNO limit in a cost-efficient way, resulting in a minimal overall increase in calculation time as compared to the uncorrected method.

Coupled cluster theory on modern heterogeneous supercomputers

This study examines the computational challenges in elucidating intricate chemical systems, particularly through ab-initio methodologies. This work highlights the Divide-Expand-Consolidate (DEC) approach for coupled cluster (CC) theory-a linear-scaling, massively parallel framework-as a viable solution. Detailed scrutiny of the DEC framework reveals its extensive applicability for large chemical systems, yet it also acknowledges inherent limitations. To mitigate these constraints, the cluster perturbation theory is presented as an effective remedy. Attention is then directed towards the CPS (D-3) model, explicitly derived from a CC singles parent and a doubles auxiliary excitation space, for computing excitation energies. The reviewed new algorithms for the CPS (D-3) method efficiently capitalize on multiple nodes and graphical processing units, expediting heavy tensor contractions. As a result, CPS (D-3) emerges as a scalable, rapid, and precise solution for computing molecular properties in large molecular systems, marking it an efficient contender to conventional CC models.