Transferable Water Potentials Using Equivariant Neural Networks.

Machine learning interatomic potentials (MLIPs) have emerged as a technique that promises quantum theory accuracy for reduced cost. It has been proposed [J. Chem. Phys. 2023, 158, 084111] that MLIPs trained on solely liquid water data cannot accurately transfer to the vapor-liquid equilibrium while recovering the many-body decomposition (MBD) analysis of gas-phase water clusters. This suggests that MLIPs do not directly learn the physically correct interactions of water molecules, limiting transferability. In this work, we show that MLIPs using equivariant architecture and trained on 3200 liquid water structures reproduces liquid-phase water properties (e.g., density within 0.003 g/cm3 between 230 and 365 K), vapor-liquid equilibrium properties up to 550 K, the MBD analysis of gas-phase water cluster up to six-body interactions, and the relative energy and the vibrational density of states of ice phases. We show that potentials developed using equivariant MLIPs allow transferability for arbitrary phases of water that remain stable in nanosecond long simulations.

Probe the Dynamic Adsorption and Phase Transition of Underpotential Deposition Processes at Electrode-Electrolyte Interfaces.

Electrochemical scanning tunneling microscopy (EC-STM) and electrochemical quartz crystal microbalance (E-QCM) techniques in combination with DFT calculations have been applied to reveal the static phase and the phase transition of copper underpotential deposition (UPD) on a gold electrode surface. EC-STM demonstrated, for the first time, the direct visualization of the disintegration of (√3 × âˆš3)R30° copper UPD adlayer with coadsorbed SO42- while changing sample potential (ES) toward the redox Pa2/Pc2 peaks, which are associated with the phase transition between the Cu UPD (√3 × âˆš3)R30° phase II and disordered randomly adsorbed phase III. DFT calculations show that SO42- binds via three oxygens to the bridge sites of the copper with sulfate being located directly above the copper vacancy in the (√3 × âˆš3)R30° adlayer, whereas the remaining oxygen of the sulfate points away from the surface. E-QCM measurement of the change of the electric charge due to Cu UPD Faradaic processes, the change of the interfacial mass due to the adsorption and desorption of Cu(II) and SO42-, and the formation and stripping of UPD copper on the gold surface provide complementary information that validates the EC-STM and DFT results. This work demonstrated the advantage of using complementary in situ experimental techniques (E-QCM and EC-STM) combined with simulations to obtain an accurate and complete picture of the dynamic interfacial adsorption and UPD processes at the electrode/electrolyte interface.

Zeolite encapsulated organometallic complexes as model catalysts.

Heterogeneities in the structure of active centers in metal-containing porous materials are unavoidable and complicate the description of chemical events occurring along reaction coordinates at the atomic level. Metal containing zeolites include sites of varied local coordination and secondary confining environments, requiring careful titration protocols to quantify the predominant active sites. Hybrid organometallic-zeolite catalysts are useful well-defined platform materials for spectroscopic, kinetic, and computational studies of heterogeneous catalysis that avoid the complications of conventional metal-containing porous materials. Such materials have been synthesized and studied previously, but catalytic applications were mostly limited to liquid-phase oxidation and electrochemical reactions. The hydrothermal stability, time-on-stream stability, and utility of these materials in gas-phase oxidation reactions are under-studied. The potential applications for single-site heterogeneous catalysts in fundamental research are abundant and motivate future synthetic, spectroscopic, kinetic, and computational studies.

Cationic Tetrylene-Iron(0) Complexes: Access Points for Cooperative, Reversible Bond Activation and Open-Shell Iron(-I) Ferrato-Tetrylenes.

The open-shell cationic stannylene-iron(0) complex 4 (4=[PhiP DippSn⋅Fe⋅IPr]+ ; PhiP Dipp={[Ph2 PCH2 Si(i Pr)2 ](Dipp)N}; Dipp=2,6-i Pr2 C6 H3 ; IPr=[(Dipp)NC(H)]2 C:) cooperatively and reversibly cleaves dihydrogen at the Sn-Fe interface under mild conditions (1.5 bar, 298 K), in forming bridging hydrido-complex 6. The One-electron oreduction of the related GeII -Fe0 complex 3 leads to oxidative addition of one C-P linkage of the PhiP Dipp ligand in an intermediary Fe-I complex, leading to FeI phosphide species 7. One-electron reduction reaction of 4 gives access to the iron(-I) ferrato-stannylene, 8, giving evidence for the transient formation of such a species in the reduction of 3. The covalently bound tin(II)-iron(-I) compound 8 has been characterised through EPR spectroscopy, SQUID magnetometry, and supporting computational analysis, which strongly indicate a high localization of electron spin density at Fe-I in this unique d9 -iron complex.

Toward Large-Scale Restricted Active Space Calculations Inspired by the Schmidt Decomposition.

We present an alternative, memory-efficient, Schmidt decomposition-based description of the inherently bipartite restricted active space (RAS) scheme, which can be implemented effortlessly within the density matrix renormalization group (DMRG) method via the dynamically extended active space procedure. Benchmark calculations are compared against state-of-the-art results of C2 and Cr2, which are notorious for their multireference character. Our results for ground and excited states together with spectroscopic constants demonstrate that the proposed novel approach, dubbed as DMRG-RAS, which is variational and free of uncontrolled method errors, has the potential to outperfom conventional methods for strongly correlated molecules.

Sensitivity of coupled cluster electronic properties on the reference determinant: Can Kohn-Sham orbitals be more beneficial than Hartree-Fock orbitals?

Coupled cluster calculations are traditionally performed over Hartree-Fock reference orbitals (HF-CC methodology). However, in the literature it has been repeatedly raised whether the use of a Kohn-Sham reference (KS-CC methodology) might result in improved performance relative to HF-CC. In the present study, we re-examine the relation of HF-CC and KS-CC methods by comparing the results of widely applied truncated CC calculations (CCSD, CCSD(T), CCSDT) to the limit of full configuration interaction (FCI), which serves as an undebatable reference point of accuracy. Based on a series of CC calculations on diatoms and transition metal complexes, we demonstrate that no systematic improvement of coupled cluster electronic energies, densities and chemical reaction energies is expected when changing from HF to a KS reference. Nevertheless, fortuitous error cancellations might occasionally result in illusory improvement compared to HF-CC. Altogether, the application of KS-CC is not advantageous over HF-CC, but it is also not unreasonable as the choice of reference has negligible influence on the results at sufficiently high CC levels. In addition, KS-CC can be a particularly useful alternative if difficulties are encountered in HF or HF-CC convergence. It is also notable that KS-CC results are found to be practically independent of the chosen density functional, which implies that almost any KS-CC method can be used in place of HF-CC.

Isolation and Reactivity of Tetrylene-Tetrylone-Iron Complexes Supported by Bis(N-Heterocyclic Imine) Ligands.

The germanium iron carbonyl complex 3 was prepared by the reaction of dimeric chloro(imino)germylene [IPrNGeCl]2 (IPrN=bis(2,6-diisopropylphenyl)imidazolin-2-iminato) with one equivalent of Collman's reagent (Na2 Fe(CO)4 ) at room temperature. Similarly, the reaction of chloro(imino)stannylene [IPrNSnCl]2 with Na2 Fe(CO)4 (1 equiv) resulted in the Fe(CO)4 -bridged bis(stannylene) complex 4. We observed reversible formation of bis(tetrylene) and tetrylene-tetrylone character in complexes 3 vs. 5 and 4 vs. 6, which was supported by DFT calculations. Moreover, the Li/Sn/Fe trimetallic complex 12 has been isolated from the reaction of [IPrNSnCl]2 with cyclopentadienyl iron dicarbonyl anion. The computational analysis further rationalizes the reduction pathway from these chlorotetrylenes to the corresponding complexes.

Benchmarking Semiempirical QM Methods for Calculating the Dipole Moment of Organic Molecules.

The dipole moment is a simple descriptor of the charge distribution and polarity and is important for understanding and predicting various molecular properties. Semiempirical (SE) methods offer a cost-effective way to calculate dipole moment that can be used in high-throughput screening applications although the accuracy of the methods is still in question. Therefore, we have evaluated AM1, GFN0-xTB, GFN1-xTB, GFN2-xTB, PM3, PM6, PM7, B97-3c, HF-3c, and PBEh-3c SE methods, which cover a variety of SE approximations, to directly assess the performance of SE methods in predicting molecular dipole moments and their directions using 7211 organic molecules contained in the QM7b database. We find that B97-3c and PBEh-3c perform best against coupled-cluster reference values yielding dipole moments with a mean absolute error (MAE) of 0.10 and 0.11 D, respectively, which is similar to the MAE of density functional theory (DFT) methods (∼0.1 D) reported earlier. Analysis of the atomic composition shows that all SE methods show good performance for hydrocarbons for which the spread of error was within 1 D of the reference data, while the worst performances are for sulfur-containing compounds for which only B97-3c and PBEh-3c show acceptable performance. We also evaluate the effect of SE optimized geometry, instead of the benchmark DFT geometry, that shows a dramatic drop in the performance of AM1 and PM3 for which the range of error tripled. Based on our overall findings, we highlight that there is an optimal compromise between accuracy and computational cost using GFN2-xTB (MAE: 0.25 D) that is 3 orders of magnitude faster than B97-3c and PBEh-3c. Thus, we recommend using GFN2-xTB for cost-efficient calculation of the dipole moment of organic molecules containing C, H, O, and N atoms, whereas, for sulfur-containing organic molecules, we suggest PBEh-3c.

Accessing the main-group metal formyl scaffold through CO-activation in beryllium hydride complexes.

Carbon monoxide (CO) is an indispensable C1 building block. For decades this abundant gas has been employed in hydroformylation and Pausen-Khand catalysis, amongst many related chemistries, where a single, non-coupled CO fragment is delivered to an organic molecule. Despite this, organometallic species which react with CO to yield C1 products remain rare, and are elusive for main group metal complexes. Here, we describe a range of amido-beryllium hydride complexes, and demonstrate their reactivity towards CO, in its mono-insertion into the Be-H bonds of these species. The small radius of the Be2+ ion in conjunction with the non-innocent pendant phosphine moiety of the developed ligands leads to a unique beryllium formyl complex with an ylidic P-COC fragment, whereby the carbon centre, remarkably, datively binds Be. This, alongside reactivity toward carbon dioxide, sheds light on the insertion chemistry of the Be-H bond, complimenting the long-known chemistry of the heavier Alkaline Earth hydrides.

Coupling the chemical reactivity of bimetallic surfaces to the orientations of liquid crystals.

The development of responsive soft materials with tailored functional properties based on the chemical reactivity of atomically precise inorganic interfaces has not been widely explored. In this communication, guided by first-principles calculations, we design bimetallic surfaces comprised of atomically thin Pd layers deposited onto Au that anchor nematic liquid crystalline phases of 4'-n-pentyl-4-biphenylcarbonitrile (5CB) and demonstrate that the chemical reactivity of these bimetallic surfaces towards Cl2 gas can be tuned by specification of the composition of the surface alloy. Specifically, we use underpotential deposition to prepare submonolayer to multilayers of Pd on Au and employ X-ray photoelectron and infrared spectroscopy to validate computational predictions that binding of 5CB depends strongly on the Pd coverage, with ∼0.1 monolayer (ML) of Pd sufficient to cause the liquid crystal (LC) to adopt a perpendicular binding mode. Computed heats of dissociative adsorption of Cl2 on PdAu alloy surfaces predict displacement of 5CB from these surfaces, a result that is also confirmed by experiments revealing that 1 ppm Cl2 triggers orientational transitions of 5CB. By decreasing the coverage of Pd on Au from 1.8 ± 0.2 ML to 0.09 ± 0.02 ML, the dynamic response of 5CB to 1 ppm Cl2 is accelerated 3X. Overall, these results demonstrate the promise of hybrid designs of responsive materials based on atomically precise interfaces formed between hard bimetallic surfaces and soft matter.

An Isolable Three-Coordinate Germanone and Its Reactivity.

A rare three-coordinate germanone [IPrN]2 Ge=O (IPrN=bis(2,6-diisopropylphenyl)imidazolin-2-imino) was successfully isolated. The germanone has a rather high thermal stability in arene solvent, and no detectable change was observed at 80 °C for at least one week. However, high thermal stability of [IPrN]2 Ge=O does not prevent its reactivity toward small molecules. Structural analysis and initial reactivity studies revealed the highly polarized nature of the terminal Ge=O bond. Besides, the addition of phenylacetylene, as well as O-atom transfer with 2,6-dimethylphenyl isocyanide make it a mimic of nucleophilic transition-metal oxides. Mechanism for O-atom transfer reaction was investigated via DFT calculations, which revealed that the reaction proceeds via a [2+2] cycloaddition intermediate.

Trends in computational molecular catalyst design.

Computational methods have emerged as a powerful tool to augment traditional experimental molecular catalyst design by providing useful predictions of catalyst performance and decreasing the time needed for catalyst screening. In this perspective, we discuss three approaches for computational molecular catalyst design: (i) the reaction mechanism-based approach that calculates all relevant elementary steps, finds the rate and selectivity determining steps, and ultimately makes predictions on catalyst performance based on kinetic analysis, (ii) the descriptor-based approach where physical/chemical considerations are used to find molecular properties as predictors of catalyst performance, and (iii) the data-driven approach where statistical analysis as well as machine learning (ML) methods are used to obtain relationships between available data/features and catalyst performance. Following an introduction to these approaches, we cover their strengths and weaknesses and highlight some recent key applications. Furthermore, we present an outlook on how the currently applied approaches may evolve in the near future by addressing how recent developments in building automated computational workflows and implementing advanced ML models hold promise for reducing human workload, eliminating human bias, and speeding up computational catalyst design at the same time. Finally, we provide our viewpoint on how some of the challenges associated with the up-and-coming approaches driven by automation and ML may be resolved.

Reversible metathesis of ammonia in an acyclic germylene-Ni0 complex.

Carbenes, a class of low-valent group 14 ligand, have shifted the paradigm in our understanding of the effects of supporting ligands in transition-metal reactivity and catalysis. We now seek to move towards utilizing the heavier group 14 elements in effective ligand systems, which can potentially surpass carbon in their ability to operate via 'non-innocent' bond activation processes. Herein we describe our initial results towards the development of scalable acyclic chelating germylene ligands (viz. 1a/b), and their utilization in the stabilization of Ni0 complexes (viz. 4a/b), which can readily and reversibly undergo metathesis with ammonia with no net change of oxidation state at the GeII and Ni0 centres, through ammonia bonding at the germylene ligand as opposed to the Ni0 centre. The DFT-derived metathesis mechanism, which surprisingly demonstrates the need for three molecules of ammonia to achieve N-H bond activation, supports reversible ammonia binding at GeII, as well as the observed reversibility in the overall reaction.

Design of Chemoresponsive Soft Matter Using Hydrogen-Bonded Liquid Crystals.

Soft matter that undergoes programmed macroscopic responses to molecular analytes has potential utility in a range of health and safety-related contexts. In this study, we report the design of a nematic liquid crystal (LC) composition that forms through dimerization of carboxylic acids and responds to the presence of vapors of organoamines by undergoing a visually distinct phase transition to an isotropic phase. Specifically, we screened mixtures of two carboxylic acids, 4-butylbenzoic acid and trans-4-pentylcyclohexanecarboxylic acid, and found select compositions that exhibited a nematic phase from 30.6 to 111.7 °C during heating and 110.6 to 3.1 °C during cooling. The metastable nematic phase formed at ambient temperatures was found to be long-lived (>5 days), thus enabling the use of the LC as a chemoresponsive optical material. By comparing experimental infrared (IR) spectra of the LC phase with vibrational frequencies calculated using density functional theory (DFT), we show that it is possible to distinguish between the presence of monomers, homodimers and heterodimers in the mixture, leading us to conclude that a one-to-one heterodimer is the dominant species within this LC composition. Further support for this conclusion is obtained by using differential scanning calorimetry. Exposure of the LC to 12 ppm triethylamine (TEA) triggers a phase transition to an isotropic phase, which we show by IR spectroscopy to be driven by an acid-base reaction, leading to the formation of ammonium carboxylate salts. We characterized the dynamics of the phase transition and found that it proceeds via a characteristic spatiotemporal pathway involving the nucleation, growth, and coalescence of isotropic domains, thus amplifying the atomic-scale acid-base reaction into an information-rich optical output. In contrast to TEA, we determined via both experiment and computation that neither hydrogen bonding donor or acceptor molecules, such as water, dimethyl methylphosphonate, ethylene oxide or formaldehyde, disrupt the heterodimers formed in the LC, hinting that the phase transition (including spatial-temporal characteristics of the pathway) induced in this class of hydrogen bonded LC may offer the basis of a facile and chemically selective way of reporting the presence of volatile amines. This proposal is supported by exploratory experiments in which we show that it is possible to trigger a phase transition in the LC by exposure to volatile amines emitted from rotting fish. Overall, these results provide new principles for the design of chemoresponsive soft matter based on hydrogen bonded LCs that may find use as the basis of low-cost visual indicators of chemical environments.

DMRG on Top of Plane-Wave Kohn-Sham Orbitals: A Case Study of Defected Boron Nitride.

In this paper, we analyze the numerical aspects of the inherent multireference density matrix renormalization group (DMRG) calculations on top of the periodic Kohn-Sham density functional theory using the complete active space approach. The potential of the framework is illustrated by studying hexagonal boron nitride nanoflakes embedding a charged single boron vacancy point defect by revealing a vertical energy spectrum with a prominent multireference character. We investigate the consistency of the DMRG energy spectrum from the perspective of sample size, basis size, and active space selection protocol. Results obtained from standard quantum chemical atom-centered basis calculations and plane-wave based counterparts show excellent agreement. Furthermore, we also discuss the spectrum of the periodic sheet which is in good agreement with extrapolated data of finite clusters. These results pave the way toward applying the DMRG method in extended correlated solid-state systems, such as point defect qubit in wide band gap semiconductors.

A bis(silylene)-stabilized diphosphorus compound and its reactivity as a monophosphorus anion transfer reagent.

In contrast to the well-established transition-metal-mediated activation of white phosphorus (P4), the metal-free direct functionalization of P4 has remained rare. The conversion of P4 into a reactive zero-valent diphosphorus compound (P2) has proven challenging to carry out without relying on metal reactivity. Herein, we describe the facile degradation of P4 mediated by two divalent silicon atoms in a bis(silylene) scaffold, resulting in a silylene-stabilized zero-valent P2 complex. The presence of two lone pairs of electrons on each P atom in the silylene-stabilized P2 complex enables a rich reactivity towards small molecules; reaction of the P2 species with CO2, water or a borane leads to the formation of P-C, P-H or P-B bonds, respectively. Notably, the P2 complex also serves as a single phosphorus anion (P-) transfer reagent towards metal carbonyls and a chlorogermylene compound, leading to the synthetically valuable phosphaketenide (PCO-) ligand and a phosphinidene germylene complex, respectively.

Bis(silylene)-Stabilized Monovalent Nitrogen Complexes.

The first series of bis(silylene)-stabilized nitrogen(I) compounds is described. Starting from the 1,2-bis(N-heterocyclic silylenyl) 1,2-dicarba-closo-dedocaborane(12) scaffold 1, [1,2-(LSi)2 C2 B10 H10 ; L=PhC(Nt Bu)2 ], reaction with adamantyl azide (AdN3 ) affords the terminal N-µ2 -bridged zwitterionic carborane-1,2-bis(silylium) AdN3 adduct 2 with an open-cage dianionic nido-C2 B10 cluster core. Remarkably, upon one-electron reduction of 2 with C8 K and liberation of N2 and adamantane, the two silylene subunits are regenerated to furnish the isolable bis(silylene)-stabilized NI complex as an anion of 3 with the nido-C2 B10 cluster cage. On the other hand, one-electron oxidation of 2 with silver(I) yields the monocationic bis(silylene) NI complex 4 with the closo-C2 B10 cluster core. Moreover, the corresponding neutral NI radical complex 5 results from single-electron transfer from 3 to 4.

Binding of Organophosphorus Nerve Agents and Their Simulants to Metal Salts.

Nerve agents (NAs) pose a great threat to society because they are easy to produce and are deadly in nature, which makes developing methods to detect, adsorb, and destroy them crucial. To enable the development of these methods, we report the use of first principles electronic structure calculations to understand the binding properties of NAs and NA simulants on metal salt surfaces. We report calculated Gibbs free binding energies (GBE) for four NAs (tabun (GA), sarin (GB), soman (GD), and venomous X (VX)) and five NA simulants (dimethyl methylphosphonate (DMMP), dimethyl chlorophosphate (DMCP), trimethyl phosphate (TMP), methyl dichlorophosphate (MDCP), and di-isopropyl methylphosphonate (DIMP)) on metal perchlorate and metal nitrate salts using density functional theory. Our results indicate a general trend in the binding strength of NAs and NA simulants to metal salt surfaces: MDCP < DMCP < GA < GD ≈ GB < TMP < VX ≈ DMMP < DIMP. Based on their binding properties on salt surfaces, we identify the most effective simulant for each of the studied NAs as follows: DMCP for GA, TMP for GB and GD, and DMMP for VX. To illustrate the utility of the binding energies calculated in our study, we address the design of NA sensors based on the competitive binding of NAs and liquid crystalline compounds on metal salts. We compare our results with previous experimental findings and provide a list of promising combinations of liquid crystal and metal salt systems to selectively and sensitively detect NAs. Our study highlights the great value of computational chemistry for designing selective and sensitive NA sensors while minimizing the number of very dangerous experiments involving NAs.

Facile Access to Dative, Single, and Double Silicon-Metal Bonds Through M-Cl Insertion Reactions of Base-Stabilized SiII Cations.

Silicon(II) cations can offer fascinating reactivity patterns due to their unique electronic structure: a lone pair of electrons, two empty p orbitals and a positive charge combined on a single silicon center. We now report the facile insertion of N-heterocyclic carbene (NHC)-stabilized silyliumylidene ions into M-Cl bonds (M=Ru, Rh), forming a series of novel chlorosilylene transition-metal complexes. Theoretical investigations revealed a reaction mechanism in which the insertion into the M-Cl bond with concomitant 1,2-migration of a silicon-bound NHC to the transition metal takes place after formation of an initial silyliumylidene transition-metal complex. The mechanism could be verified experimentally through characterization of the intermediate complexes. Furthermore, the obtained chlorosilylene complexes can be conveniently utilized as synthons to access Si-M and Si=M bonding motifs bonds through reductive dehalogenation.

Homocoupling of CO and isocyanide mediated by a C,C'-bis(silylenyl)-substituted ortho-carborane.

The unexpected reactivity of the o-carborane supported bis-silylene [(LSi:)C]2B10H101 {L= PhC(tBuN)2} towards carbon monoxide and 2,6-dimethylphenyl isocyanide is reported. While the reaction of 1 with CO leads selectively to the novel head-to-head coupling and C-O cleavage product 2 from two molecules 1 and four molecules CO, the reaction of 1 with 2,6-dimethylphenyl isocyanide affords solely the 1 : 2 molar head-to-tail coupling product 3 with a Si[double bond, length as m-dash]C bond.