Cluster Surface Research Articles

Micelle-like clusters in phase-separated Nanog condensates: A molecular simulation study.

The phase separation model for transcription suggests that transcription factors (TFs), coactivators, and RNA polymerases form biomolecular condensates around active gene loci and regulate transcription. However, the structural details of condensates remain elusive. In this study, for Nanog, a master TF in mammalian embryonic stem cells known to form protein condensates in vitro, we examined protein structures in the condensates using residue-level coarse-grained molecular simulations. Human Nanog formed micelle-like clusters in the condensate. In the micelle-like cluster, the C-terminal disordered domains, including the tryptophan repeat (WR) regions, interacted with each other near the cluster center primarily via hydrophobic interaction. In contrast, hydrophilic disordered N-terminal and DNA-binding domains were exposed on the surface of the clusters. Electrostatic attractions of these surface residues were responsible for bridging multiple micelle-like structures in the condensate. The micelle-like structure and condensate were dynamic and liquid-like. Mutation of tryptophan residues in the WR region which was implicated to be important for a Nanog function resulted in dissolution of the Nanog condensate. Finally, to examine the impact of Nanog cluster to DNA, we added DNA fragments to the Nanog condensate. Nanog DNA-binding domains exposed to the surface of the micelle-like cluster could recruit more than one DNA fragments, making DNA-DNA distance shorter.

Hydride Doping Effects on the Structure and Properties of Eight-Electron Rh/Ag Superatoms: The [RhHx@Ag21-x{S2P(OnPr)2}12] (x = 0-2) Series.

Three hitherto unknown eight-electron rhodium/silver alloy nanoclusters, [RhAg21{S2P(OnPr)2}12] (1), [RhHAg20{S2P(OnPr)2}12] (2), and [RhH2Ag19{S2P(OnPr)2}12] (3), have been isolated and fully characterized. Cluster 1 contains a regular Rh@Ag12 icosahedral core, whereas 2 and 3 exhibit distorted RhH@Ag12 and RhH2@Ag12 icosahedral cores. The single-crystal neutron structure of 2 located the encapsulated hydride at the center of an enlarged RhAg3 tetrahedron. A similar position was found by neutron diffraction for one of the hydrides in 3, whereas the other hydride is trigonally coordinated to Rh and an elongated Ag-Ag edge. The solid-state structures of 1-3 possess C1 symmetry due to the asymmetric arrangement of the surrounding capping Ag atoms. Our investigation shows that the insertion of one hydride dopant provokes the elimination of one capping silver atom on the cluster surface, resulting in the general formula [RhHx@Ag21-x{S2P(OnPr)2}12] (x = 0-2), which maintains the same number of cluster electrons as well as neutral charge. Clusters 1-3 exhibit an intense emission band in the NIR region. Contrarily to their PdAg21 and PdHAg20 relatives, the 4d orbitals of the encapsulated heterometal are somewhat involved in the optical processes.

Quantum cluster equilibrium prediction of liquid ethanol

Quantum cluster equilibrium theory (QCE) has been widely used to determine the properties of pure and binary mixture of liquids. The main limitation of the application of QCE is the exploration of different possible clusters formed by the solvent molecules. Therefore, in this study, we applied the QCE theory to predict liquid properties of ethanol after thorough exploration of the potential energy surfaces (PESs) of the ethanol clusters from dimer to hexamer. The exploration started by generating possible structures using classical molecular dynamics followed by optimizations at the MP2/aug-cc-pVDZ level of theory. 484 different configurations of the ethanol clusters have been finally used in the QCE theory. The results show that the population of liquid ethanol is constituted from the contribution of hexamer, pentamer, and tetramer. In addition, we noted that the ethanol monomer, dimer and trimer do not contribute to the population of liquid ethanol. Furthermore, based on the predicted population of the liquid ethanol, we calculated its infrared spectrum at different temperatures. The calculated infrared spectrum is found to be in qualitative agreement with experiment. Some thermodynamic properties, such as the heat capacity, are also predicted to be in good agreement with experiment.

Realization of high-pressure dry methane reforming by suppressing coke deposition with Co-Rh intermetallic clusters

It is economical to perform methane and carbon dioxide reforming under high-pressure and temperature conditions, but the harsh operation condition poses a grand challenge for coke-resistant catalyst design. We report herein that surface-segregation-free Co1Rh3 clusters are stable catalysts under 20 bars at 850 oC. Microkinetic analysis discloses that balanced and lowered surface coverages of C* and O* constitute the most abundant reaction intermediates on Co1Rh3 clusters at elevated pressures, with respect to that of the monometallic ones, thus avoiding surface carbon accumulation or surface oxidative deactivation. Moreover, density functional theory calculations of carbon atom nucleation discloses that adsorbed carbon transformation to refractory, graphene-like carbon is suppressed on Co1Rh3 cluster surface, owing to increased energetic barrier and ensemble size, hence, only CO2 gasifiable soft carbon could form. The revelation of the electronic/geometric features of Co1Rh3 is regarded to provide a guidance for future coke-resistant catalyst design.

Histidinehydroxamic acid as new biomolecule to produce molecular-like fluorescent gold nanoclusters: Possible mechanisms for metal ion sensing

We firstly demonstrate a new biomolecule, Histidinehydroxamic acid (HisHA), to produce few-atomic fluorescent gold nanoclusters (Au NCs) in aqueous medium. The preparation protocol has been optimized by studying the effect of metal ion/biomolecule molar ratio, metal ion concentration, pH, temperature, reaction time as well as the role of citrate as mild reducing agent on appearance of blue-emitting (λem = 440 nm, λex = 365 nm) molecular-like NCs. Structural studies confirmed that imidazole-N and amino-N of the HisHA stabilize the formed metallic cores. The quantum yield of ∼ 4% and fluorescence lifetime of 4.2 ns were determined. Moreover, these NCs show suitable stability under high inert salt (cNaCl = 2.5 M) concentration as well. Verifying its ability to detect metal ions, dual strategies were discovered. We confirmed that the copper ions cause fluorescence quenching (LOD = 2.49 µM) by pushing the higher amount of soft HisHA ligand from the metallic surface and forming complexes with dominantly hydroxamate-[O,O] coordination mode in the aqueous medium. For Zn2+-ions, a “turn-on” sensing mechanism was observed; the smallest detectable amount of Zn2+ is 7.5 µM. Linear increase of the quantum yield (from ∼ 4% to ∼ 11.5%) was identified above 75 μM of Zn2+ due to the binding of the Zn2+-ions on the cluster surface via hydroxamate-[O,O] donors.

Water binding and hygroscopicity in π-conjugated polyelectrolytes

The presence of water strongly influences structure, dynamics and properties of ion-containing soft matter. Yet, the hydration of such matter is not well understood. Here, we show through a large study of monovalent π-conjugated polyelectrolytes that their reversible hydration, up to several water molecules per ion pair, occurs chiefly at the interface between the ion clusters and the hydrophobic matrix without disrupting ion packing. This establishes the appropriate model to be surface hydration, not the often-assumed internal hydration of the ion clusters. Through detailed analysis of desorption energies and O–H vibrational frequencies, together with OPLS4 and DFT calculations, we have elucidated key binding motifs of the sorbed water. Type-I water, which desorbs below 50 °C, corresponds to hydrogen-bonded water clusters constituting secondary hydration. Type-II water, which typically desorbs over 50–150 °C, corresponds to water bound to the anion under the influence of a proximal cation, or to a cation‒anion pair, at the cluster surface. This constitutes primary hydration. Type-III water, which irreversibly desorbs beyond 150 °C, corresponds to water kinetically trapped between ions. Its amount varies strongly with processing and heat treatment. As a consequence, hygroscopicity—which is the water sorption capacity per ion pair—depends not only on the ions, but also their cluster morphology.

Alkali-promoted copper catalyst catalyze low-temperature water-gas shift reaction

The addition of alkali metal promoter is significant to improve the performance of copper catalyst in water–gas shift reaction (WGSR), but the previous theoretical work mainly focuses on the single crystal metal model like Cu(111), rather than the real supported copper clusters. In this paper, Cu4/TiO2(sub-nanometer), Cun/TiO2 (copper nanorod) were chosen to explore the effect of K on WGSR where K is located on TiO2 support, Cu/TiO2 interface, and copper clusters surface. Density functional theory calculation results show that only when K is on Cun/TiO2 and Cu(111) surfaces, can K promote water–gas shift reaction. Microkinetic modeling analysis shows that K mainly promotes the water–gas shift reaction by promoting the decomposition of H2O, and has little influence on the C-O bond formation like carboxyl and CO2. Combined with energetic, electronic and geometric analysis, on the surfaces of Cun/TiO2 and Cu(111), K mainly enhances the adsorption strength of H2O and reduces the barrier of H2O dissociation through direct bonding with oxygenate species, thereby improving the performance of WGSR. The copper clusters with sub-nanometer size cannot reveal the promotion effect of K, and the microkinetic modeling results of Cu(111) surface do not conform to the real situation, but the copper nanorod model can appropriately reflect the promotion effect of K. The structure unit of the active center is [TiO2−Cu−K]. The promotion of other alkali metals was explored, and the microkinetic modeling results show that the order of alkali metal promotion is as follows: Li < Na < K < Rb < Cs. The electronic structure analysis shows that the smaller the electronegativity of alkali metal, the lower the surface work function, the higher the surface dipole moment, and the stronger the promoting effect of alkali metal. This work not only reveals the origin of the K effect on WGSR, but also provides the accurate placement of K promoter.

D-Amino acid recognition of tripeptides studied by ultraviolet photodissociation spectroscopy of hydrogen-bonded clusters.

To understand the roles of D-amino acids, evaluating their chemical properties in living organisms is essential. Herein, D-amino acid recognition of peptides was investigated using a tandem mass spectrometer equipped with an electrospray ionization source and a cold ion trap. Ultraviolet (UV) photodissociation spectroscopy and water adsorption of hydrogen-bonded protonated clusters of tryptophan (Trp) enantiomers and tripeptides (SAA, ASA, and AAS, where S and A denote L-serine and L-alanine, respectively) were carried out at 8K in the gas phase. In the UV photodissociation spectrum of H+(D-Trp)ASA, the bandwidth of the S1-S0 transition, which corresponds to the ππ* state of the Trp indole ring, was narrower than those of the other five clusters, H+(D-Trp)SAA, H+(D-Trp)AAS, H+(L-Trp)SAA, H+(L-Trp)ASA, and H+(L-Trp)AAS. In the UV photoexcitation of H+(D-Trp)ASA(H2O)n, which were formed via water adsorption on gas-phase H+(D-Trp)ASA, the evaporation of water molecules was the main photodissociation pathway. An NH2CHCOOH-eliminated ion and H+ASA were observed in the product ion spectrum. By contrast, water molecules adsorbed on the other five clusters remained on the product ions for NH2CHCOOH elimination and Trp detachment after the UV photoexcitation. The results indicated that the indole ring of Trp was located on the surface of H+(D-Trp)ASA, and the amino and carboxyl groups of Trp formed hydrogen bonds in H+(D-Trp)ASA. For the other five clusters, the indole rings of Trp were hydrogen bonded in the clusters, and the amino and carboxyl groups of Trp were present on the cluster surfaces.

Adsorption Forms of NO Molecules Adsorbed on Surfaces of Gas-Phase Metal Clusters Investigated by Infrared Multiple Photon Dissociation Spectroscopy

Adsorption Forms of NO Molecules Adsorbed on Surfaces of Gas-Phase Metal Clusters Investigated by Infrared Multiple Photon Dissociation Spectroscopy

Unexpected Reactivity of Cationic-to-Cationic Thiolate Ligand-Exchange Reaction on Au25 Clusters.

Thiolate-protected molecular noble metal clusters have attracted significant attention due to their unique physicochemical properties, which make them applicable in diverse fields such as catalysis, sensing, and bioimaging. Ligand-exchange reactions are a crucial technique for synthesizing and functionalizing these clusters, as they allow for the introduction of new ligands onto the cluster surface, which can alter their properties. While numerous studies have investigated neutral-to-neutral, neutral-to-anionic, and neutral-to-cationic ligand-exchange reactions, the cationic-to-cationic ligand-exchange reaction has never been reported, making the study of such reactions intriguing. In this study, the cationic ligand-exchange reaction on Au25(4-PyET-CH3+)x(4-PyET)18-x (x ≈ 9) clusters, which contain both neutral and cationic ligands in nearly equivalent amounts, was investigated. Contrary to our expectation that the cationic-to-cationic ligand-exchange reaction would be suppressed due to Coulombic repulsion between the surface cationic ligands and incoming cationic ligands, the originally existing cationic ligand was selectively exchanged. The choice of counterions for cationic ligands played a crucial role in controlling the selectivity of ligand exchange. For instance, bulky and hydrophobic counterions such as PF6- can cause steric hindrance and reduce Coulombic repulsion, which promotes cationic-to-cationic ligand exchange. Conversely, counterions like Cl- can lead to neutral-to-cationic ligand exchange due to reduced steric hindrance and increased Coulombic repulsion between cationic ligands. These findings provide a novel method for tailoring the properties of molecular gold clusters through controlled ligand exchange without requiring the design of thiolate ligands with varying geometrical structures.

Proton-Coupled Electron Transfer at the Surface of Polyoxovanadate-Alkoxide Clusters.

ConspectusProton-coupled electron transfer (PCET) is a fundamental process involved in all areas of chemistry, with relevance to biological transformations, catalysis, and emergent energy storage and conversion technologies. Early observations of PCET were reported by Meyer and co-workers in 1981 while investigating the proton dependence of reduction of a molecular ruthenium oxo complex. Since that time, this conceptual framework has grown to encompass an enormous scope of charge transfer and compensation reactions. In this Account, we will discuss ongoing efforts in the Matson Laboratory to understand the fundamental thermodynamics and kinetics of PCET processes at the surface of a series of Lindqvist-type polyoxovanadate clusters. This project aims to provide atomistic resolution of net H atom uptake and transfer at the surfaces of transition-metal oxide materials.First, we discuss our efforts aimed at understanding PCET at metal oxide surfaces using the Lindqvist-type polyoxovanadate-alkoxide (POV-alkoxide) cluster [nBu4N]2[V6O13(TRIOLNO2)2]. These clusters reversibly bind H atom equivalents at bridging oxide sites, mirroring the proposed uptake and release of e-/H+ pairs at transition-metal oxide surfaces. Summarized results include the measurement of bond dissociation free energies of surface hydroxide moieties (BDFE(O-H)) as well as mechanistic analyses that verify concerted proton electron transfer as the operative pathway for PCET at the surface of POV-alkoxide clusters.Next, we discuss net proton and H atom uptake at the surface of reduced variants of the Lindqvist-type POV-alkoxide cluster, [V6O7(OR)12]n (R = Me, Et; n = -2, -1, 0, + 1). In the case of these low-valent POV-alkoxide clusters, nucleophilic bridging sites are kinetically inhibited by functionalization of the cluster surface with organic ligands. This molecular modification enables site-selectivity in proton and H atom uptake to terminal oxide sites. The impact of reaction site and cluster electronics on reaction driving force of PCET is explored, with core electron density playing a critical role in dictating thermodynamics of H atom uptake and transfer. Additional work described here contrasts the kinetics of PCET at terminal oxide sites to the reactivity observed at bridging oxides in POV-alkoxide clusters.Overall, this Account summarizes our foundational knowledge regarding the assessment of PCET reactivity at the surfaces of molecular metal oxides. Drawing analogies between POV-alkoxide clusters and nanoscopic metal oxide materials provide design principles for the advancement of materials applications with atomic precision. These complexes are additionally highlighted as tunable redox mediators in their own right; our studies demonstrate how cluster surface reactivities can be optimized by modifying electronic structure and surface functionalities.

Experimental and theoretical study of polysaccharides extracted from prickly pear nopales Pulp (PPUN) of Opuntia ficus-indica as corrosion inhibitors

Corrosion causes massive damage to our equipment and buildings, wasting billions of dollars every year. Identifying a workable answer to this problem is a priority. Scientists have begun using inhibitors derived from renewable resources and natural ingredients to have a minor possible detrimental impact on the environment. Though many synthetic structures exist to prevent corrosion during the acid pickling process, the persistence of the problem has prompted modern corrosion scientists to investigate this field further by studying natural products with increasingly intriguing moieties for practical applications and their applied capabilities against corrosion. Electrochemical studies have been used to fully explore the inhibitory effects of Pulp (PPUN) from prickly pear nopales of Opuntia ficus-indica (OFI) on mild steel corrosion in 1 M HCl solution. Scanning electron microscopy (SEM) and atomic force microscopy were used to investigate the surface of mild steel (AFM). The XPS method was used to learn about the mechanism of inhibition happening on mild steel. Through its absorption on the metal surface and its inhibition of the active corrosion site, the inhibitor is shown by a potentiodynamic polarization study to slow down both cathodic and anodic processes. The adsorption of PPUN on metal was demonstrated by a shift in impedance characteristics (charge transfer resistance Rt and double layer capacitance Cdl). The PPUN and product film have been analyzed using SEM and AFM. The computed values correspond well with those from the experiments. Based on MDS results, Galacturonic acid and Glucose constituents adsorb tightly on the carbon steel cluster surface.

Transfer-learned potential energy surfaces: Toward microsecond-scale molecular dynamics simulations in the gas phase at CCSD(T) quality.

The rise of machine learning has greatly influenced the field of computational chemistry and atomistic molecular dynamics simulations in particular. One of its most exciting prospects is the development of accurate, full-dimensional potential energy surfaces (PESs) for molecules and clusters, which, however, often require thousands to tens of thousands of abinitio data points restricting the community to medium sized molecules and/or lower levels of theory (e.g., density functional theory). Transfer learning, which improves a global PES from a lower to a higher level of theory, offers a data efficient alternative requiring only a fraction of the high-level data (on the order of 100 are found to be sufficient for malonaldehyde). This work demonstrates that even with Hartree-Fock theory and a double-zeta basis set as the lower level model, transfer learning yields coupled-cluster single double triple [CCSD(T)]-level quality for H-transfer barrier energies, harmonic frequencies, and H-transfer tunneling splittings. Most importantly, finite-temperature molecular dynamics simulations on the sub-μs time scale in the gas phase are possible and the infrared spectra determined from the transfer-learned PESs are in good agreement with the experiment. It is concluded that routine, long-time atomistic simulations on PESs fulfilling CCSD(T)-standards become possible.

Protonation Process of Porous Silica Cluster Surface using Molecular Dynamics Method

Using molecular dynamic simulation, we developed an algorithm to protonate the surface of an amorphous porous silica grain particle model and study its effect. In this work, the silica grain model can be used to study cosmic dust coagulation. The surface of the silica cluster was protonated by placing H atoms on oxygen atoms having only a single bond, namely, the non-bridging oxygens. The H atoms are placed opposite the Si–O bond with a distance of around 1 Å to form silanol (Si–O–H) group termination on the silica surface. The angular conformation of the silanol was optimized by relaxing the surface at low temperature. We evaluated the number of silanol groups, the angular distribution of the Si-O-H bond, and the average distance between Si-O particles using the radial distribution function (RDF). The result of the study shows that minimizing the energy of the silica surface changes the angular distribution of the silanol from 180° to about 110° and between 140°-160°. However, the average distance between Si-O particles remains at 1 Å, which demonstrates the correctness of the atomic interaction model. The addition of protons on the silica surface is an essential factor in the simulation of cosmic dust collision since the modification of the surface chemistry may alter the contact surface energy, thus changing the probability of particle coagulation.

Surface activation of platinum group metal clusters for efficient and durable oxygen reduction in proton exchange membrane fuel cells

To develop efficient and durable acidic oxygen–reduction–reaction (ORR) catalysts based on all platinum group metals (PGMs) is crucial for large-scale application of proton-exchange membrane fuel cells (PEMFCs) but challenging. Here, we report a nitrogen coordination-induced strong metal–support interaction that can tune the surface atoms of ORR-inactive PGM clusters into efficient and durable active sites. Taking Rh as an example, the carbonization of Rh-overdoped zeolitic imidazolate framework-8 results in a large number of Rh clusters (with a little atomic Rh) in porous nitrogen-doped carbon. The cluster surface atoms coordinate with the nitrogen of the carbon support, forming much stronger metal–support interactions than that of common N-doped carbon-supported metal nanoparticles. The activity of surface-activated Rh clusters is close to that of Pt/C. The regulation rules for the surface active sites inherit most of the characteristics of the corresponding single-atom catalysts, but without their severe instability problem. This surface activation strategy has also shown applicable to other PGMs, thereby it is a promising way to alleviate the reliance of PEMFCs on platinum.

First principles data-driven potentials for prediction of iron carbide clusters

Many have reported the use of quantum chemistry approaches for evaluating the catalytic properties of iron carbide clusters. Unfortunately, structural energy calculations are computationally expensive when using density functional theory. The computational cost is prohibitive for high-throughput simulations with large length and time scales. In this paper, we generate data from 177 k clusters and choose state-of-the-art machine learning models within physical chemistry to train the features of this data. The generated potential gives a very high prediction accuracy on the order of the structure stability and achieves better adaptability/tolerance to poor structures of clusters. In addition, we use the machine learning potential to assist in high-throughput data collection and the prediction of hydrogen adsorption sites on cluster surfaces. We achieve more stable adsorption locations of the hydrogen atom more rapidly compared with traditional quantum chemical calculations.

Revealing mechanism of non-Arrhenius viscosity change in metal melts based on Wulff cluster model

In some metal melts, like Pb and Al, viscosity as a function of temperature does not obey Arrhenius law in some temperature range, so called non-Arrhenius viscosity change. In this paper, the Wulff cluster model combined with phonon calculations is used to investigate the relation between the structural characteristic and the non-Arrhenius viscosity change in metallic melts. Although the surface energy of the Wulff clusters in melts does not change significantly with temperature, the Wulff shape changed evidently. For both Pb and Al, one surface of the Wulff shape disappears (Pb(321) and Al(100)) when the non-Arrhenius viscosity change occurs upon heating. The trend of the non-Arrhenius viscosity change is strongly related to the properties of the disappeared surface. The non-Arrhenius viscosity drop corresponds to the disappearance of the surface with the highest interaction energy between surface and free atoms (Pb(321) surface), while the non-Arrhenius viscosity rise corresponds to the disappearance of surface with the lowest interaction energy (Al(100) surface). All evidence indicates that the viscosity anomaly and the change of Wulff shape is closely related. The non-Arrhenius viscosity change in metallic melts is caused by the appearance/disappearance of certain surfaces of clusters, while the interaction strength of these surfaces determines the different features (non-Arrhenius drop/rise) of corresponding viscosity change.

Adsorption of CO, NO and SO on Fe2-10 clusters: A computational investigation on the metal catalysed activation of atmospheric pollutants

The adsorption and activation of CO, NO and SO gas molecules on the surface of Fe2-10 clusters were investigated. The calculated adsorption energies of –81 to –376 kJ mol−1 confirmed the chemisorption of the gases on the metal clusters. The chemical activations of the gas molecules were verified by analyzing the strength of the inter-nuclear bond by using various bond strength parameters. The metal-to-ligand charge transfers were investigated to understand the role of metals in the bond activation process. The current investigation is highly relevant to design efficient catalysts for the capture and recycling of gaseous pollutants.

Regioselective Derivatization of Silylated [20]Silafulleranes

Silafulleranes with endohedral Cl- ions are a unique, scarcely explored class of structurally well-defined silicon clusters and host-guest complexes. Herein, we report regioselective derivatization reactions on the siladodecahedrane [nBu4N][Cl@Si20(SiCl3)12Cl8] ([nBu4N][1]), which has its cluster surface decorated with 12 SiCl3 and 8 Cl substituents in perfect Th symmetry. The room-temperature reaction of [nBu4N][1] with excess iBu2AlH in ortho-difluorobenzene (oDFB) furnishes perhydrogenated [nBu4N][Cl@Si20(SiH3)12H8] ([nBu4N][2]) in 50% yield; the non-pyrophoric [2]- is the largest structurally authenticated (by X-ray diffraction) hydridosilane known to date. A simple switch from pure oDFB to an oDFB/Et2O solvent mixture suppresses core hydrogenation and results in the formation of [nBu4N][Cl@Si20(SiH3)12Cl8] ([nBu4N][3]). In addition to the exhaustive Cl/H exchange at all 44 Si-Cl bonds of [1]- and the regioselective 36-fold silyl group hydrogenation, we achieved the simultaneous introduction of Me substituents at all 8 SiCl vertices along with the conversion of all 12 SiCl3 to SiH3 groups by treating [nBu4N][1] with Me2AlH/Me3Al in oDFB ([nBu4N][Cl@Si20(SiH3)12Me8], [nBu4N][4]; 73%). Quantum-chemical free-energy calculations find an SN2-Si-type hydrogenation of the exohedral SiCl3 moieties in [1]- (trigonal-bipyramidal intermediate) slightly preferred over metathesis-like SNi-Si substitutions (four-membered transition state). Cage hydrogenation likely occurs via SNi-Si processes. The experimentally demonstrated influence of an Et2O co-solvent, which drastically increases the respective reaction barriers, is attributed to the increased stability of the resulting iBu2AlH-OEt2 adduct and its higher steric bulk compared to free iBu2AlH.

Surface Confinement of Finite-Size Water Droplets for SO3 Hydrolysis Reaction Revealed by Molecular Dynamics Simulations Based on a Machine Learning Force Field

As an important source for sulfuric acid in the atmosphere, hydrolysis of sulfur trioxide (SO3) takes place with water clusters of sizes from several molecules to several nanometers, resulting in various final products, including neutral (H2SO4)-(H2O) clusters and ionic (HSO4)--(H3O)+ clusters. The diverse products may be due to the ability of proton transfer and the formation of hydrated ions for water cluster of finite sizes, especially the sub-micrometer ones. However, the detailed molecular-level mechanism is still unclear due to the lack of available characterization and simulations tools. Here, we developed a quantum chemistry-level machine learning (ML) model to simulate the hydrolysis of SO3 with water clusters of sizes up to nanometers. The simulation results demonstrate diverse reaction paths taking place between SO3 and water clusters of different sizes. Generally, neutral (H2SO4)-(H2O) clusters are preferred by water clusters of ultra-small size, and a loop structure-mediated mechanism with SO3(H2O)n≤4 structures and a non-loop structure-mediated mechanism with structure relaxation are observed. As the water cluster size increases to (H2O)8, a (HSO4)--(H3O)+ ion-pair product emerges; and the Eigen-Zundel ion conversion-like proton transfer mechanism takes place and stabilizes the ion pairs. As the water cluster sizes further increase beyond several nanometers ((H2O)n≥32), the (SO4)2-[(H3O)+]2 ion-pair product appears. The reason could be that the surface of these water clusters is large enough to screen Coulomb repulsion between two tri-coordinated ion-pair complexes. These findings would provide new perspectives for understanding SO3 hydrolysis in the real atmosphere and sulfuric acid chemistry in atmospheric aerosols.