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Chemical processes taking place on ice-grain mantles are pivotal to the complex chemistry of interstellar environments. In this study, we conducted a comprehensive analysis of the catalytic effects of an amorphous solid water (ASW) surface on the reaction between ammonia (NH3) and formaldehyde (H2CO) to form aminomethanol (NH2CH2OH) using density functional theory. We identified potential catalytic sites based on the binding energy distribution of NH3 and H2CO reactants, on a set-of-clusters surface model composed of 22 water molecules and found a total of 14 reaction paths. Our results indicate that the catalytic sites can be categorized into four groups, depending on the interactions of the carbonyl oxygen and the amino group with the ice surface in the reactant complex. A detailed analysis of the reaction mechanism using Intrinsic Reaction Coordinate and reaction force analysis, revealed three distinct chemical events for this reaction: formation of the C–N bond, breaking of the N–H bond, and formation of the O–H hydroxyl bond. Depending on the type of catalytic site, these events can occur within a single, concerted, albeit asynchronous, step, or can be isolated in a step-wise mechanism, with the lowest overall transition state energy observed at 1.3 kcal mol−1. A key requirement for the low-energy mechanism is the presence of a pair of dangling OH bonds on the surface, found at 5% of the potential catalytic sites on an ASW porous surface. [ABSTRACT FROM AUTHOR]

The positions of grid points for representing a multidimensional potential energy surface (PES) have a non-negligible impact on its accuracy and the associated computational effort for its generation. Six different positioning schemes were studied for PESs represented by n-mode expansions as needed for the accurate calculation of anharmonic vibrational frequencies by means of vibrational configuration interaction theory. A static approach, which has successfully been used in many applications, and five adaptive schemes based on Gaussian process regression have been investigated with respect to the number of necessary grid points and the accuracy of the fundamental modes for a small set of test molecules. A comparison with a related, more sophisticated, and consistent approach by Christiansen et al. is provided. The impact of the positions of the ab initio grid points is discussed for multilevel PESs, for which the computational effort of the individual electronic structure calculations decreases for increasing orders of the n-mode expansion. As a result of that, the ultimate goal is not the maximal reduction of grid points but rather the computational cost, which is not directly related. [ABSTRACT FROM AUTHOR]

Efficiently creating a concise but comprehensive data set for training machine-learned interatomic potentials (MLIPs) is an under-explored problem. Active learning, which uses biased or unbiased molecular dynamics (MD) to generate candidate pools, aims to address this objective. Existing biased and unbiased MD-simulation methods, however, are prone to miss either rare events or extrapolative regions—areas of the configurational space where unreliable predictions are made. This work demonstrates that MD, when biased by the MLIP's energy uncertainty, simultaneously captures extrapolative regions and rare events, which is crucial for developing uniformly accurate MLIPs. Furthermore, exploiting automatic differentiation, we enhance bias-forces-driven MD with the concept of bias stress. We employ calibrated gradient-based uncertainties to yield MLIPs with similar or, sometimes, better accuracy than ensemble-based methods at a lower computational cost. Finally, we apply uncertainty-biased MD to alanine dipeptide and MIL-53(Al), generating MLIPs that represent both configurational spaces more accurately than models trained with conventional MD. [ABSTRACT FROM AUTHOR]

The accuracy of the training data limits the accuracy of bulk properties from machine-learned potentials. For example, hybrid functionals or wave-function-based quantum chemical methods are readily available for cluster data but effectively out of scope for periodic structures. We show that local, atom-centered descriptors for machine-learned potentials enable the prediction of bulk properties from cluster model training data, agreeing reasonably well with predictions from bulk training data. We demonstrate such transferability by studying structural and dynamical properties of bulk liquid water with density functional theory and have found an excellent agreement with experimental and theoretical counterparts. [ABSTRACT FROM AUTHOR]

The discovery of C60, C60+, and C70 in the interstellar medium has ignited a profound interest in the astrochemistry of fullerene and related systems. In particular, the presence of diffuse interstellar bands and their association with C60+ has led to the hypothesis that hydrogenated derivatives, known as fulleranes, may also exist in the interstellar medium and contribute to these bands. In this study, we systematically investigated the structural and spectroscopic properties of C60Hn+q (n = 0–4, q = 0,1) using an automated global minimum search and density functional theory calculations. Our results revealed novel global minimum structures for C60H2 and C60H4, distinct from previous reports. Notably, all hydrogenated fullerenes exhibited lower ionization potentials and higher proton affinities compared to C60. From an astrochemical perspective, our results exposed the challenges in establishing definitive spectroscopic criteria for detecting fulleranes using mid-infrared and UV-Vis spectroscopies. However, we successfully identified distinct electronic transitions in the near-infrared range that serve as distinctive signatures of cationic fulleranes. We strongly advocate for further high-resolution experimental studies to fully explore the potential of these transitions for the interstellar detection of fulleranes. [ABSTRACT FROM AUTHOR]

The first neutral and cationic Mo imido alkylidene cyclic alkyl amino carbene (CAAC) complexes of the general formulae [Mo(N−Ar)(CHCMe2Ph)(X)2(CAAC)] and [Mo(N−Ar)(CHCMe2Ph)(X)(CAAC)][B(ArF)4] (X=Br, Cl, OTf, OC6F5; CAAC=1‐(2,6‐iPr2‐C6H3)‐3,3,5,5‐tetramethyltetrahydropyrrol‐2‐ylidene) have been synthesized from molybdenum imido bishalide alkylidene DME precursors. Different combinations of the imido and "X" ligands have been employed to understand synthetic peculiarities. Selected complexes have been characterized by single‐crystal X‐ray analysis. Due to the pronounced σ‐donor/π‐acceptor characteristics of CAACs, the corresponding neutral and cationic molybdenum imido alkylidene CAAC complexes do not require the presence of stabilizing donor ligands such as nitriles. Calculations on the PBE0‐D3BJ/def2‐TZVP level for PBE0‐D3BJ/def2‐SVP optimized geometries revealed partial charges at molybdenum similar to the corresponding molybdenum imido alkylidene N‐heterocyclic carbene (NHC) complexes with a slightly higher polarization of the molybdenum alkylidene bond in the CAAC complexes. All cationic complexes have been tested in olefin metathesis reactions and showed improved activity compared to the analogous NHC complexes for hydrocarbon‐based substrates, allowing for turnover numbers (TONs) up to 9500 even at room temperature. Some Mo imido alkylidene CAAC complexes are tolerant towards functional groups like thioethers and sulfonamides. [ABSTRACT FROM AUTHOR]

Pyrazolones represent an important structural motif in active pharmaceutical ingredients. Their asymmetric synthesis is thus widely studied. Still, a generally highly enantio‐ and diastereoselective 1,4‐addition to nitroolefins providing products with adjacent stereocenters is elusive. In this article, a new polyfunctional CuII‐1,2,3‐triazolium‐aryloxide catalyst is presented which enables this reaction type with high stereocontrol. DFT studies revealed that the triazolium stabilizes the transition state by hydrogen bonding between C(5)−H and the nitroolefin and verify a cooperative mode of activation. Moreover, they show that the catalyst adopts a rigid chiral cage/pore structure by intramolecular hydrogen bonding, by which stereocontrol is achieved. Control catalyst systems confirm the crucial role of the triazolium, aryloxide and CuII, requiring a sophisticated structural orchestration for high efficiency. The addition products were used to form pyrazolidinones by chemoselective C=N reduction. These heterocycles are shown to be valuable precursors toward β,γ'‐diaminoamides by chemoselective nitro and N−N bond reductions. Morphological profiling using the Cell painting assay identified biological activities for the pyrazolidinones and suggest modulation of DNA synthesis as a potential mode of action. One product showed biological similarity to Camptothecin, a lead structure for cancer therapy. [ABSTRACT FROM AUTHOR]

Copyright of Angewandte Chemie is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In the reactions with NaN SB 3 sb , NaOAc, NaCl and H SB 2 sb O, the formation of the respective substitution products was detected by SP 1 sp H NMR spectroscopy and mass spectrometry of the reaction solution. SP 1 sp H NMR (400 MHz, CDCl SB 3 sb ): = 7.39-7.31 (m, 3 H, Ar- I H i ), 7.22-7.20 (m, 2 H, Ar- I H i ), 4.04-4.00 (m, 1 H, PhC I H i CO), 3.21 (dd, I J i = 18.6, 9.3 Hz, 1 H, C I H i HCO), 2.81 (dd, I J i = 18.6, 3.6 Hz, 1 H, CH I H i CO). SP 1 sp H NMR (400 MHz, DMSO- I d i SB 6 sb ): = 11.47 (bs, 1 H, N I H i ), 7.51-7.29 (m, 5 H, Ar- I H i ), 3.06 (d, I J i = 18.0 Hz, 1 H, C I H i HCO), 2.94 (d, I J i = 18.0 Hz, 1 H, CH I H i CO). SP 1 sp H NMR (400 MHz, CDCl SB 3 sb ): = 8.48 (bs, 1 H, N I H i ), 7.43-7.37 (m, 5 H, Ar- I H i ), 3.23 (d, I J i = 18.4 Hz, 1 H, C I H i HCO), 3.10 (d, I J i = 18.4 Hz, 1 H, CH I H i CO). [Extracted from the article]

Chemically complex multicomponent alloys possess exceptional properties derived from an inexhaustible compositional space. The complexity however makes interatomic potential development challenging. We explore two complementary machine-learned potentials—the moment tensor potential (MTP) and the Gaussian moment neural network (GM-NN)—in simultaneously describing configurational and vibrational degrees of freedom in the Ta-V-Cr-W alloy family. Both models are equally accurate with excellent performance evaluated against density-functional-theory. They achieve root-mean-square-errors (RMSEs) in energies of less than a few meV/atom across 0 K ordered and high-temperature disordered configurations included in the training. Even for compositions not in training, relative energy RMSEs at high temperatures are within a few meV/atom. High-temperature molecular dynamics forces have similarly small RMSEs of about 0.15 eV/Å for the disordered quaternary included in, and ternaries not part of training. MTPs achieve faster convergence with training size; GM-NNs are faster in execution. Active learning is partially beneficial and should be complemented with conventional human-based training set generation. [ABSTRACT FROM AUTHOR]

Hydroxylamine, NH2OH, is one of the already detected interstellar molecules with the highest prebiotic potential. Yet, the abundance of this molecule found by astronomical observations is rather low for a relatively simple molecule, ∼10−10 relative to H2. This seemingly low abundance can be rationalized by destruction routes operating on interstellar dust grains. In this work, we tested the viability of this hypothesis under several prisms, finding that the origin of a lower abundance of NH2OH can be explained by two chemical processes, one operating at low temperature (10 K) and the other at intermediate temperature (20 K). At low temperatures, enabling the hydrogen abstraction reaction HNO + H → NO + H2, even in small amounts, partially inhibits the formation of NH2OH through successive hydrogenation of NO, and reduces its abundance on the grains. We found that enabling a 15–30  per cent of binding sites for this reaction results in reductions of NH2OH abundance of approximately one to two orders of magnitude. At warmer temperatures (20 K, in our study), the reaction NH2OH + H → HNOH + H2, which was found to be fast (k ∼ 106 s−1) in this work, followed by further abstractions by adsorbates that are immobile at 10 K (O, N) are the main route of NH2OH destruction. Our results shed light on the abundance of hydroxylamine in space and pave the way to constraining the subsequent chemistry experienced by this molecule and its derivatives in the interstellar prebiotic chemistry canvas. [ABSTRACT FROM AUTHOR]

For decades there were many attempts to dispense with stoichiometric amounts of metal reagents for the synthesis of secondary alcohols. In 2021, the synthetic results of Newman and collaborators pioneered a synthesis still with metals, but not as reactants. Instead, they serverd as catalytic engines. Here we present a description by means of Density Functional Theory calculations of how this process can occur, and an attempt is made to shed light on the mechanism that facilitates the attainment of secondary alcohols, emphasizing the eternal cross‐coupling debate of whether the catalytically active species is Ni(0) or they are really taking shortcuts following the course of Ni(II). Effective Orbital analyses give a clear picture. Furthermore, this paper provides insight not only into the nature of the ligands of the metal catalyst but also the role of the base. [ABSTRACT FROM AUTHOR]

While ZrO2 is known to have a large effect on the activity and selectivity of the Cu/ZrO2 catalyst for methanol synthesis, its role in this process is poorly understood. Surface defects such as oxygen vacancies could play a role in the strong metal‐support interaction (SMSI) between Cu and ZrO2. However, due to the complexity of the surfaces, the exact molecular nature of this interaction is not at present known. Here, we make well‐defined models of both reduced and coordinatively unsaturated surface oxygen vacancies on ZrO2 using the molecular precursor [Cp2ZrCl]2(μ2‐O) (1). Complex 1 can be reduced to form a complex (2) containing one Zr(III) center and a bridging hydride ligand (according to EPR and IR spectroscopy) derived from C−H activation of either thf or the Cp ring. Complex 2 reacts with CO2 to largely produce CO, suggesting that surface defects with similar structures probably do not play a role in the industrial catalyst. Halide abstraction from complex 1 results in the Lewis acidic species 3, which has similar Lewis acid properties to acidic defects on the ZrO2 surface. Similarities of both of these model species to real surface oxygen vacancies and their role in the catalytic reaction are discussed. [ABSTRACT FROM AUTHOR]

A catalyst type is disclosed allowing for exceptional efficiency in direct 1,4‐additions. The catalyst is a zwitterionic entity, in which acetate binds to CuII, which is formally negatively charged and serving as counterion for benzimidazolium. All 3 functionalities are involved in the catalytic activation. For maleimides productivity was increased by a factor >300 compared to literature (TONs up to 6700). High stereoselectivity and productivity was attained for a broad range of other Michael acceptors as well. The polyfunctional catalyst is accessible in only 4 steps from N‐Ph‐benzimidazole with an overall yield of 96 % and robust during catalysis. This allowed to reuse the same catalyst multiple times with nearly constant efficiency. Mechanistic studies, in particular by DFT, give a detailed picture how the catalyst operates. The benzimidazolium unit stabilizes the coordinated enolate nucleophile and prevents that acetate/acetic acid dissociate from the catalyst. [ABSTRACT FROM AUTHOR]

Copyright of Angewandte Chemie is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Sulfurized poly(acrylonitrile) (SPAN) is a prominent example of a highly cycle stable and rate capable sulfur/polymer composite, which is solely based on covalently bound sulfur. However, so far no in‐depth study on the influence of nitrogen in the carbonaceous backbone, to which sulfur in the form of thioketones and poly(sulfides) is attached, exists. Herein, we investigated the role of nitrogen by comparing sulfur/polymer composites derived from nitrogen‐containing poly(acrylonitrile) (PAN) and nitrogen‐free poly(vinylacetylene) (PVac). Results strongly indicate the importance of a nitrogen‐rich, aromatic carbon backbone to ensure full addressability of the polymer‐bound sulfur and its reversible binding to the aromatic backbone, even at high current rates. This study also presents key structures, which are crucial for highly cycle and rate stable S‐composites. [ABSTRACT FROM AUTHOR]

Developing machine learning-based interatomic potentials from ab initio electronic structure methods remains a challenging task for computational chemistry and materials science. This work studies the capability of transfer learning, in particular discriminative fine-tuning, for efficiently generating chemically accurate interatomic neural network potentials on organic molecules from the MD17 and ANI data sets. We show that pre-training the network parameters on data obtained from density functional calculations considerably improves the sample efficiency of models trained on more accurate ab initio data. Additionally, we show that fine-tuning with energy labels alone can suffice to obtain accurate atomic forces and run large-scale atomistic simulations, provided a well-designed fine-tuning data set. We also investigate possible limitations of transfer learning, especially regarding the design and size of the pre-training and fine-tuning data sets. Finally, we provide GM-NN potentials pre-trained and fine-tuned on the ANI-1x and ANI-1ccx data sets, which can easily be fine-tuned on and applied to organic molecules. [ABSTRACT FROM AUTHOR]

The vibrational spectra of simple C H 3 + —Rg (Rg = He, Ne, Ar, Kr) complexes have been studied by vibrational configuration interaction theory relying on multidimensional potential energy surfaces (PESs) obtained from explicitly correlated coupled cluster calculations, CCSD(T)-F12a. In agreement with experimental results, the series of rare gas atoms leads to rather unsystematic results and indicates huge zero point vibrational energy effects for the helium complex. In order to study these sensitive complexes more consistently, we also introduce configuration averaged vibrational self-consistent field theory, which is a generalization of standard vibrational self-consistent field theory to several configurations. The vibrational spectra of the complexes are compared to that of the methyl cation, for which corrections due to scalar-relativistic effects, high-order coupled-cluster terms, e.g., quadruple excitations, and core-valence correlation have explicitly been accounted for. The occurrence of tunneling splittings for the vibrational ground-state of C H 3 + —He has been investigated on the basis of semiclassical instanton theory. These calculations and a direct comparison of the energy profiles along the intrinsic reaction coordinates with that of the hydronium cation, H3O+, suggest that tunneling effects for vibrationally excited states should be very small. [ABSTRACT FROM AUTHOR]

Aiming at merocyanine dyes with good linear optical and self-assembly properties, a series of rigid mono-, bi- and tricyclic merocyanines with O- and N-donor units as well as keto or malodinitrile acceptor units was prepared by a convergent approach. With particular focus on tailoring the donor unit, a selection of appropriate derivatives was investigated with respect to their dye properties in solution and in the bulk (UV/Vis, fluorescence, temperature-dependent fluorescence, lifetime). Determination of fluorescence quantum yields revealed the importance of the donor unit and the chromophore size. Larger chromophores and N-donors were beneficial for strong emission in solution, whereas small chromophores and O-donors favored emission in the solid state. To rationalize the different optical properties depending on their donor unit, density functional theory (DFT) calculations were performed. Liquid crystalline derivatives were additionally studied by optical polarization microscopy, differential scanning calorimetry, and X-ray diffraction experiments. For merocyanines with O-donor, fluorinated side chains were mandatory to get stable enantiotropic SmA phases regardless of chromophore size, side chain lengths or acceptor unit. Increased mesophase widths (up to 134 K) were observed upon increasing the chromophore lengths, chain lengths (up to C12) and F/C ratio in the side chain. On the other hand, merocyanines with N-donor and keto acceptor showed enantiotropic SmA phases in the presence of simple alkoxy side chains. The tricyclic merocyanine with N-donor shows an additional SmE phase at lower temperatures. The results revealed the importance of the donor unit to balance optical and mesomorphic properties in merocynanines. [ABSTRACT FROM AUTHOR]

MLL3, also known as KMT2C, is a lysine mono-methyltransferase in charge of the writing of an epigenetic mark on lysine 4 from histone 3. The catalytic site of MLL3 is composed of four tyrosines, namely, Y44, Y69, Y128, and Y130. Tyrosine residues are highly conserved among lysine methyltransferases' catalytic sites, although their complete function is still unclear. The exploration of how modifications on these residues from the enzymatic machinery impact the enzymatic activity of MLL3 could shed light transversally into the inner functioning of enzymes with similar characteristics. Through the use of QMMM calculations, we focus on the effect of the mutation of each tyrosine from the catalytic site on the enzymatic activity and the product specificity in the current study. While we found that the mutations of Y44 and Y128 by phenylalanine inactivated the enzyme, the mutation of Y128 by alanine reactivated the enzymatic activity of MLL3. Moreover, according to our models, the Y128A mutant was even found to be capable of di- and tri-methylate lysine 4 from histone 3, what would represent a gain of function mutation, and could be responsible for the development of diseases. Finally, we were able to establish the inactivation mechanism, which involved the use of Y130 as a water occlusion structure, whose conformation, once perturbed by its mutation or Y128 mutant, allows the access of water molecules that sequester the electron pair from lysine 4 avoiding its methylation process and, thus, increasing the barrier height. [ABSTRACT FROM AUTHOR]

High‐temperature sodium‐sulfur battery (HT Na–S) technology has attracted substantial interest in the stationary energy storage sector due to its low cost and high energy density. However, the currently used solid electrolyte (ß‐alumina) is expensive and can only be operated at high temperatures, which compromises safety. On the other hand, liquid electrolytes in room temperature sodium‐sulfur batteries (RT Na–S) are susceptible to dendrite formation and polysulfide shuttle. Consequently, an electrolyte with both solid (shuttle blocking) and liquid (ionic conductivity) properties to overcome the above‐mentioned issues is highly desired. Herein, a high‐performance quasi‐solid state crosslinked gel polymer electrolyte (GPE) prepared in situ using pentaerythritol triacrylate (PETA) exhibiting high ionic conductivity of 2.33 mS cm−1 at 25 °C is presented. The GPE‐based electrolyte shows high stability resulting in a high discharge capacity of >600 mAh gs−1 after 2500 cycles with an average Coulombic efficiency of 99.91%. Density functional theory calculations reveal a weak interaction between the Na+ ions and the oxygen molecules of the PETA moiety, which leads to a facile cation movement. The crosslinked polymer network is tightly connected to the cathode and can confine sulfides, thereby facilitating the conversion process. [ABSTRACT FROM AUTHOR]

Graph: Figure 2: The planar [C(CN)3]- anion in the crystal structure of Tl[tcm] exhibiting its ideal D3h point symmetry as viewed along [001] (left) and with its full Tl+-cation decoration (right). 4 Conclusions We report here the thallium(I) tricyanomethanide Tl[tcm] with the expected formula Tl[C(CN) SB 3 sb ] ( C SB 4 sb N SB 3 sb Tl) exhibiting a hitherto unknown crystal structure. This coordination sphere consists of six terminal nitrogen atoms ( I d i (Tl-N) = 299.2(4) pm) belonging to six different [tcm] SP - sp anions with three of those being part of the anionic sheet SP 1 sp / SB 3 sb I c i above and the remaining three belonging to the sheet SP 1 sp / SB 3 sb I c i below the cationic layer. Keywords: alkali metals; crystal structures; Raman spectra; thallium; tricyanomethanides EN alkali metals crystal structures Raman spectra thallium tricyanomethanides 237 243 7 05/13/22 20220501 NES 220501 Dedicated to Professor Christian Näther on the occasion of his 60th birthday. [Extracted from the article]

Microcanonical instanton theory offers the promise of providing rate constants for chemical reactions including quantum tunneling of atoms over the whole temperature range. We discuss different rate expressions, which require the calculation of stability parameters of the instantons. The traditional way of obtaining these stability parameters is shown to be numerically unstable in practical applications. We provide three alternative algorithms to obtain such stability parameters for non-separable systems, i.e., systems in which the vibrational modes perpendicular to the instanton path couple to movement along the path. We show the applicability of our algorithms on two molecular systems: H2 + OH → H2O + H using a fitted potential energy surface and HNCO + H → NH2CO using a potential obtained on-the-fly from density functional calculations. [ABSTRACT FROM AUTHOR]

The diffusion of atoms and radicals on interstellar dust grains is a fundamental ingredient for predicting accurate molecular abundances in astronomical environments. Quantitative values of diffusivity and diffusion barriers usually rely heavily on empirical rules. In this paper, we compute the diffusion coefficients of adsorbed nitrogen atoms by combining machine learned interatomic potentials, metadynamics, and kinetic Monte Carlo simulations. With this approach, we obtain a diffusion coefficient of nitrogen atoms on the surface of amorphous solid water of merely |$(3.5 \pm 1.1)\, \times 10^{-34}$|  cm2 s−1 at 10 K for a bare ice surface. Thus, we find that nitrogen, as a paradigmatic case for light and weakly bound adsorbates, is unable to diffuse on bare amorphous solid water at 10 K. Surface coverage has a strong effect on the diffusion coefficient by modulating its value over 9–12 orders of magnitude at 10 K and enables diffusion for specific conditions. In addition, we have found that atom tunnelling has a negligible effect. Average diffusion barriers of the potential energy surface (2.56 kJ mol−1) differ strongly from the effective diffusion barrier obtained from the diffusion coefficient for a bare surface (6.06 kJ mol−1) and are, thus, inappropriate for diffusion modelling. Our findings suggest that the thermal diffusion of N on water ice is a process that is highly dependent on the physical conditions of the ice. [ABSTRACT FROM AUTHOR]

Microemulsions provide a unique opportunity to tailor the polarity and liquid confinement in asymmetric catalysis via nanoscale polar and nonpolar domains separated by a surfactant film. For chiral diene Rh complexes, the influence of counterion and surfactant film on the catalytic activity and enantioselectivity remained elusive. To explore this issue chiral norbornadiene Rh(X) complexes (X=OTf, OTs, OAc, PO2F2) were synthesized and characterized by X‐ray crystallography and theoretical calculations. These complexes were used in Rh‐catalyzed 1,2‐additions of phenylboroxine to N‐tosylimine in microemulsions stabilized either exclusively by n‐octyl‐β‐D‐glucopyranoside (C8G1) or a C8G1‐film doped with anionic or cationic surfactants (AOT, SDS and DTAB). The Rh(OAc) complex showed the largest dependence on the composition of the microemulsion, yielding up to 59 % (90 %ee) for the surfactant film doped with 5 wt% of AOT as compared to 52 % (58 %ee) for neat C8G1 at constant surfactant concentration. Larger domains, determined by SAXS analysis, enabled further increase in yield and selectivity while the reaction rate almost remained constant according to kinetic studies. [ABSTRACT FROM AUTHOR]

We compare three methods for quantitatively distinguishing the location of noble metal (NM) particles in mesopores from those found on the external support surface. MCM‐41 and SBA‐15 with NM located in mesopores or on the external surface were prepared and characterized by TEM. 31P MAS NMR spectroscopy was used to quantify arylphosphines in complexes with NM. Phosphine/NM ratios drop from 2.0 to 0.2 when increasing the probe diameter from 1.08 to 1.54 nm. The reaction between NM and triphenylphosphine (TPP) within 3.0 nm MCM‐41 pores takes due to confinement effects multiple weeks. In contrast, external NM react with TPP instantly. A promising method is filling the pores by using the pore volume impregnation technique with tetraethylorthosilicate (TEOS). TPP loading revealed that 66 % of NMs are located on the external surface of MCM‐41. The pore filling method can be used in association with any probe molecule, also for the quantification of acid sites. [ABSTRACT FROM AUTHOR]

The recent wave of detections of interstellar aromatic molecules has sparked interest in the chemical behaviour of aromatic molecules under astrophysical conditions. In most cases, these detections have been made through chemically related molecules, called proxies, that implicitly indicate the presence of a parent molecule. In this study, we present the results of the theoretical evaluation of the hydrogenation reactions of different aromatic molecules (benzene, pyridine, pyrrole, furan, thiophene, silabenzene, and phosphorine). The viability of these reactions allows us to evaluate the resilience of these molecules to the most important reducing agent in the interstellar medium, the hydrogen atom (H). All significant reactions are exothermic and most of them present activation barriers, which are, in several cases, overcome by quantum tunnelling. Instanton reaction rate constants are provided between 50 and 500 K. For the most efficiently formed radicals, a second hydrogenation step has been studied. We propose that hydrogenated derivatives of furan and pyrrole, especially 2,3-dihydropyrrole, 2,5-dihydropyrrole, 2,3-dihydrofuran, and 2,5-dihydrofuran, are promising candidates for future interstellar detections. [ABSTRACT FROM AUTHOR]

The detection of phosphorous-bearing molecules in interstellar environments constitutes a fundamental task for understanding the formation of prebiotic molecules, but it is also a challenge. In cold interstellar environments, where rich chemistry is expected to happen, only PN and PO have been detected. Phosphine (PH3) must also play an essential role in these regions, since P is expected to deplete onto dust grains significantly, and hydrogenation reactions are dominant in such environments. Surface chemistry on dust grains shows a particular idiosyncrasy where an equilibrium between competitive reactions, photoconversion processes, and desorption are in continuous interplay, modifying both the dust composition and the gas composition. In this study, we theoretically study in detail the interconversion of P to PH3 via subsequent additions of H on cold dust grain analogs. For all reactions, we provide the binding energy of the adsorbates, reaction energies, and, when present, activation barriers and tunneling-corrected rate constants. We also present an estimate of the desorption temperature of these species based on transition state theory. Using recently available experimental results on PH3 desorption via chemical reactions, we conclude that all of the intermediate products of the hydrogenation sequence to phosphine may be released to the gas phase. [ABSTRACT FROM AUTHOR]

Amines are a ubiquitous class of compounds found in a variety of functional organic building blocks. Within the past years, hydrogen autotransfer catalysis has evolved as a new concept for the synthesis of amines. A through understanding of the mechanism of these reactions is necessary to design optimal catalysts. We investigate secondary amine formation catalyzed by a NNNN(P)Ru‐complex and provide understanding on the three reaction steps involved. We find that the ligand has to open one coordination site in order to allow the formation of a metal hydride intermediate. In a second step, a condensation reaction, which could also happen uncatalyzed in solution, is significantly enhanced by the presence of the ruthenium complex. The back‐transfer of the hydride to the substrate in a third step regenerates the catalyst. [ABSTRACT FROM AUTHOR]

Enantiopure secondary alcohols are fundamental high‐value synthetic building blocks. One of the most attractive ways to get access to this compound class is the catalytic hydroboration. We describe a new concept for this reaction type that allowed for exceptional catalytic turnover numbers (up to 15 400), which were increased by around 1.5–3 orders of magnitude compared to the most active catalysts previously reported. In our concept an aprotic ammonium halide moiety cooperates with an oxophilic Lewis acid within the same catalyst molecule. Control experiments reveal that both catalytic centers are essential for the observed activity. Kinetic, spectroscopic and computational studies show that the hydride transfer is rate limiting and proceeds via a concerted mechanism, in which hydride at Boron is continuously displaced by iodide, reminiscent to an SN2 reaction. The catalyst, which is accessible in high yields in few steps, was found to be stable during catalysis, readily recyclable and could be reused 10 times still efficiently working. [ABSTRACT FROM AUTHOR]

Enantiopure secondary alcohols are fundamental high‐value synthetic building blocks. One of the most attractive ways to get access to this compound class is the catalytic hydroboration. We describe a new concept for this reaction type that allowed for exceptional catalytic turnover numbers (up to 15 400), which were increased by around 1.5–3 orders of magnitude compared to the most active catalysts previously reported. In our concept an aprotic ammonium halide moiety cooperates with an oxophilic Lewis acid within the same catalyst molecule. Control experiments reveal that both catalytic centers are essential for the observed activity. Kinetic, spectroscopic and computational studies show that the hydride transfer is rate limiting and proceeds via a concerted mechanism, in which hydride at Boron is continuously displaced by iodide, reminiscent to an SN2 reaction. The catalyst, which is accessible in high yields in few steps, was found to be stable during catalysis, readily recyclable and could be reused 10 times still efficiently working. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbons (PAHs) constitute an essential family of compounds in interstellar (ISM) and circumstellar (CSM) media. Recently, formation routes for the corresponding polycyclic aromatic phosphorus heterocycles (PAPHs) in astrophysical environments have been proposed. In order to contribute to a better understanding of the phosphorus chemistry in the ISM, infrared (IR) spectra and selected properties of PAPHs were computed at the density functional theory level for neutral, cationic, and anionic species. Our results reveal that several protonated PAPHs do not have planar backbones, and all species have permanent dipole moments between 2D and 4D. Closed-shell PAPHs have similar ionization potentials compared to the parent PAHs, below the Lyman threshold limit. In addition, all PAPHs show positive electron affinities higher than those of PAHs. Protonation preferably occurs on the heteroatom but with lower proton affinities than those of the corresponding nitrogen analogues (polycyclic aromatic nitrogen heterocycles). In general, neutral species have similar IR spectra profile with the most intense bands around 800 cm−1 (12.5 μ m) related to C−H wagging. Charge and protonation affect the IR spectra mainly by decreasing the intensities of these modes and increasing the ones between 1000 (10.0 μ m) and 1800 cm−1 (5.6 μ m). The P−H stretching appears in a different spectral region, between 2300 (4.3 μ m) and 2700 cm−1 (3.7 μ m). Our results are discussed in the context of distinct sources where PAHs and phosphorus are detected. PAPHs, in particular the coronene derivatives, can contribute to the unidentified infrared emission band at 6.2 μ m. [ABSTRACT FROM AUTHOR]

Transition paths characterize chemical reaction mechanisms. In this paper, we present a new method to find mean reaction paths based on the free energy. A nudged elastic band (NEB) is optimized using gradients and Hessians of the free energy, which are obtained from umbrella integration. The transition state can be refined by a Newton-Raphson search starting from the highest point of the NEB path. All optimizations are done using Cartesian coordinates. Independent molecular dynamics (MD) runs are performed at each image used to discretize the path. This makes the method intrinsically parallel. In contrast to other free energy methods, the algorithm does not become more expensive when including more degrees of freedom in the active space. The method is applied to the alanine-dipeptide as a test case and compared to pathways that have been derived from metadynamics and forward flux sampling. [ABSTRACT FROM AUTHOR]

Dynamics of adsorption and desorption of (4S)-N on amorphous solid water are analysed using molecular dynamic simulations. The underlying potential energy surface was provided by machine-learned interatomic potentials. Binding energies confirm the latest available theoretical and experimental results. The nitrogen sticking coefficient is close to unity at dust temperatures of 10 K but decreases at higher temperatures. We estimate a desorption time-scale of 1 μ s at 28 K. The estimated time-scale allows chemical processes mediated by diffusion to happen before desorption, even at higher temperatures. We found that the energy dissipation process after a sticking event happens on the picosecond time-scale at dust temperatures of 10 K, even for high energies of the incoming adsorbate. Our approach allows the simulation of large systems for reasonable time-scales at an affordable computational cost and ab initio accuracy. Moreover, it is generally applicable for the study of adsorption dynamics of interstellar radicals on dust surfaces. [ABSTRACT FROM AUTHOR]

In this work, we present the results of our investigation into the chemistry of Z- and E-cyanomethanimine (HNCHCN), both of which are possible precursors to the nucleobase adenine. Ab initio quantum chemical calculations for a number of reactions with atomic hydrogen were carried out. We find that the reaction H + Z/E-HNCHCN leading both to H-addition as well as H2-abstraction proceed via similar short-range barriers with bimolecular rate coefficients on the order of ∼10−17 cm3 s−1. These results were then incorporated into astrochemical models and used in simulations of the giant molecular cloud G+0.693. The calculated abundances obtained from these models were compared with previous observational data and found to be in good agreement, with a predicted [Z/E] ratio of ∼3—somewhat smaller than the previously derived value of 6.1 ± 2.4. We find that the [Z/E] ratio in our simulations is due mostly to ion-molecule destruction rates driven by the different permanent dipoles of the two conformers. Based on these results, we propose a general rule-of-thumb for estimating the abundances of isomers in interstellar environments, which we call the "relative dipole principle." [ABSTRACT FROM AUTHOR]

Artificial neural network (ANN) potentials enable the efficient large-scale atomistic modeling of complex materials with near first-principles accuracy. For molecular dynamics simulations, accurate energies and interatomic forces are a prerequisite, but training ANN potentials simultaneously on energies and forces from electronic structure calculations is computationally demanding. Here, we introduce an efficient alternative method for the training of ANN potentials on energy and force information, based on an extrapolation of the total energy via a Taylor expansion. By translating the force information to approximate energies, the quadratic scaling with the number of atoms exhibited by conventional force-training methods can be avoided, which enables the training on reference datasets containing complex atomic structures. We demonstrate for different materials systems, clusters of water molecules, bulk liquid water, and a lithium transition-metal oxide that the proposed force-training approach provides substantial improvements over schemes that train on energies only. Including force information for training reduces the size of the reference datasets required for ANN potential construction, increases the transferability of the potential, and generally improves the force prediction accuracy. For a set of water clusters, the Taylor-expansion approach achieves around 50% of the force error improvement compared to the explicit training on all force components, at a much smaller computational cost. The alternative force-training approach thus simplifies the construction of general ANN potentials for the prediction of accurate energies and interatomic forces for diverse types of materials, as demonstrated here for water and a transition-metal oxide. [ABSTRACT FROM AUTHOR]

We investigated the behavior of H2, the main constituent of the gas phase in dense clouds, after collision with amorphous solid water (ASW) surfaces, one of the most abundant chemical species of interstellar ices. We developed a general framework to study the adsorption dynamics of light species on interstellar ices. We provide binding energies and their distribution, sticking probabilities for incident energies between 1 meV and 60 meV, and thermal sticking coefficients between 10 and 300 K for surface temperatures from 10 to 110 K. We found that the sticking probability depends strongly on the adsorbate kinetic energy and the surface temperature, but hardly on the angle of incidence. We observed finite sticking probabilities above the thermal desorption temperature. Adsorption and thermal desorption should be considered as separate events with separate time scales. Laboratory results for these species have shown a gap in the trends attributed to the differently utilized experimental techniques. Our results complement observations and extend them, increasing the range of gas temperatures under consideration. We plan to use our method to study a variety of adsorbates, including radicals and charged species. [ABSTRACT FROM AUTHOR]

Umbrella integration is a method to analyze umbrella sampling simulations. It calculates free-energy changes from distributions obtained from molecular dynamics. While it can be formulated on the full sampled distributions, they are generally approximated by normal distributions. This is equivalent to the truncation of a power series of the free energy with respect to the reaction coordinate after the quadratic term or by a truncation of a cumulant expansion. Here, expressions for additional terms in the power series are derived. They can be calculated from the central moments of the distributions. This extension allows to test the approximations in applications. [ABSTRACT FROM AUTHOR]

The instanton method allows to accurately calculate tunneling rates down to very low temperature. However, with lowering the temperature, the computational effort steeply increases as many more discretization points are required. This is caused in practical applications by the majority of the discretization points accumulating at a very small region in configuration space. Here, we describe a method to flexibly discretize the instanton path adapted to the temperature. Chosen appropriately, the discretization leads to a much more uniform distribution of the images (control points) along the path which reduces the number of required images by about a factor of two. Combined with a modified Newton-Raphson optimizer and successive updates of the Hessians, the proposed method provides converged reaction rates at computational costs reduced by more than an order of magnitude. We show the success of the method on analytic test potentials and on molecules with energies directly obtained from density functional theory calculations. [ABSTRACT FROM AUTHOR]

Umbrella integration is a method to analyze umbrella sampling simulations by calculating and integrating the mean force. Here, the method is extended to multidimensional reaction coordinates. Approximation of the probability distribution obtained from sampling by a multivariate normal distribution allows to calculate the mean force from the average and the covariance matrix of the reaction coordinate. Integration schemes of the free-energy gradient field are discussed. Integration on a real-space grid is compared to expansion of the gradient in a series of analytic functions (such as a Fourier analysis), which can be integrated, and the expansion of the gradient only at the window means in a series of analytic functions. The Fourier analysis was found particularly useful for periodic reaction coordinates, such as torsion angles. An expression is provided to calculate the Hessian of the free energy with respect to the reaction coordinates from sampling data. The utility of the method is demonstrated at the example of the free-energy surface of the alanine dipeptide in vacuum calculated with respect to the backbone torsion angles [uppercase_phi_synonym] and Ψ. Relevance of the Jacobian term for non-Cartesian reaction coordinates is discussed. [ABSTRACT FROM AUTHOR]

Algorithmic improvements of the dimer method [G. Henkelman and H. Jónsson, J. Chem. Phys. 111, 7010 (1999)] are described in this paper. Using the limited memory Broyden-Fletcher-Goldfarb-Shanno (L-BFGS) optimizer for the dimer translation greatly improves the convergence compared to the previously used conjugate gradient algorithm. It also saves one energy and gradient calculation per dimer iteration. Extrapolation of the gradient during repeated dimer rotations reduces the computational cost to one gradient calculation per dimer rotation. The L-BFGS algorithm also improves convergence of the rotation. Thus, three to four energy and gradient evaluations are needed per iteration at the beginning of a transition state search, while only two are required close to convergence. Moreover, we apply the dimer method in internal coordinates to reduce coupling between the degrees of freedom. Weighting the coordinates can be used to apply chemical knowledge about the system and restrict the transition state search to only part of the system while minimizing the remainder. These improvements led to an efficient method for the location of transition states without the need to calculate the Hessian. Thus, it is especially useful in large systems with expensive gradient evaluations. [ABSTRACT FROM AUTHOR]

Umbrella sampling simulations, or biased molecular dynamics, can be used to calculate the free-energy change of a chemical reaction. We investigate the sources of different sampling errors and derive approximate expressions for the statistical errors when using harmonic restraints and umbrella integration analysis. This leads to generally applicable rules for the choice of the bias potential and the sampling parameters. Numerical results for simulations on an analytical model potential are presented for validation. While the derivations are based on umbrella integration analysis, the final error estimate is evaluated from the raw simulation data, and it may therefore be generally applicable as indicated by tests using the weighted histogram analysis method. [ABSTRACT FROM AUTHOR]

We present a method to analyze biased molecular-dynamics and Monte Carlo simulations, also known as umbrella sampling. In the limiting case of a strong bias, this method is equivalent to thermodynamic integration. It employs only quantities with easily controllable equilibration and greatly reduces the statistical errors compared to the standard weighted histogram analysis method. We show the success of our approach for two examples, one analytic function, and one biological system. [ABSTRACT FROM AUTHOR]

The protonation of N2 bound to the active center of nitrogenase has been investigated using state-of-the-art density-functional theory calculations. Dinitrogen in the bridging mode is activated by forming two bonds to Fe sites, which results in a reduction of the energy for the first hydrogen transfer by 123 kJ/mol. The axial binding mode with open sulfur bridge is less reactive by 30 kJ/mol and the energetic ordering of the axial and bridged binding modes is reversed in favor of the bridging dinitrogen during the first protonation. Protonation of the central ligand is thermodynamically favorable but kinetically hindered. If the central ligand is protonated, the proton is transferred to dinitrogen following the second protonation. Protonation of dinitrogen at the Mo site does not lead to low-energy intermediates. [ABSTRACT FROM AUTHOR]

We have investigated the free energy surface of alanine tripeptide in water. To elucidate the secondary structure of the amide chain, information on the free energy surface with explicit water at room temperature, and the multidimensional reaction coordinates are required. We studied the minimum free energy paths (MFEPs) connecting reactants, transition structures (TS) and products. To solve this problem, we used the free energy reaction root mapping (FERRMap) method. This is an automated search method to find MFEPs by using umbrella integration and the scaled hypersphere search method. We calculated the four-dimensional free energy surface for alanine tripeptide in water using FERRMap and found 61 equilibrium structures (EQ) connected by 133 TS points. After elucidating the MFEP network, we analysed the structures of the EQ points and the MFEPs connecting beta-sheet structures and beta-turn structures or left-handed helix structures. [ABSTRACT FROM AUTHOR]