Revisiting falloff curves of thermal unimolecular reactions

Master equations for thermal unimolecular reactions and the reverse thermal recombination reactions are solved for a series of model reaction systems and evaluated with respect to broadening factors. It is shown that weak collision center broadening factors F(cent) (wc) can approximately be related to the collision efficiencies β(c) through a relation F(cent) (wc) ≈ max {β(c) (0.14), 0.64(±0.03)}. In addition, it is investigated to what extent weak collision falloff curves in general can be expressed by the limiting low and high pressure rate coefficients together with central broadening factors F(cent) only. It is shown that there cannot be one "best" analytical expression for broadening factors F(x) as a function of the reduced pressure scale x = k(0)∕k(∞). Instead, modelled falloff curves of various reaction systems, for given k(0), k(∞), and F(cent), fall into a band of about 10% width in F(x). A series of analytical expressions for F(x), from simple symmetric to more elaborate asymmetric broadening factors, are compared and shown to reproduce the band of modelled broadening factors with satisfactory accuracy.     

Some Advances in the Inorganic Chemistry of Bromine

Recent advances in the chemistry of bromine and its inorganic compounds are covered under the headings of technology, properties and reactions, liquid bromine as an inorganic solvent, analysis, and new or newly studied compounds. Much of what is new is bromine chemistry is characteristic also of what is new in inorganic chemistry as a whole. That is, more detailed information about compounds and reactions is becoming available from the application of new instrumental and theoretical techniques.     

Insights Gained by Modeling the Kinetics of Archetypal Hydrogen Atom Transfer Reactions: NH3 + ·H ⇆ H2N· + H2 and CH4 + ·H ⇆ H3C· + H2

Experimental measurements of the kinetics of the title reactions extend to temperature ranges of 1360 K for the ammonia‐hydrogen reaction and of 1602 K for the methane‐hydrogen reaction. Curved plots of ln(k) versus 1/T are obtained. Many theoretical calculations modeling these reactions routinely use tunneling corrections to match experiment. The steepness and curvatures of the plots are modeled successfully in this work and are shown to be caused solely by changes in the bond dissociation energies of the bonds involved in the reactions without invoking tunneling or any other adjustable parameters. The conclusion that tunneling does not contribute significantly to the rates in the temperature range of the measurements is in stark contrast with those theoretical calculations invoking large tunneling factors in the experimental temperature range. Support for the conclusion is provided by theoretical calculations of harmonic quantum transition state theory implementing instanton theory. There is direct experimental evidence that significant tunneling occurs in some H atom transfers, as with isotopomers of H₂ + ·H and other H transfers at very low temperatures. However, there is no direct experimental evidence of significant tunneling contributions to the rates of the title reactions in the temperature range of the measurements. Insights are gained into what specific forces must be overcome by the enthalpy of activation for reaction to occur.     

Decarbonylative C-C bond forming reactions mediated by transition metals

New methods for carbon-carbon (C-C) forming reactions are constantly emerging in the field of organic synthesis. In this review, a brief history followed by recent developments of decarbonylative C-C forming reactions mediated by transition metals is described. Many different substrates are presented and the review is organized by the different carbonyl precursors, such as acyl chlorides, aldehydes, anhydrides, esters and ketones, used in the respective transformations. Furthermore, the broad scope of these reactions is exhibited by the application to several reaction types (e.g. Heck-type reactions, Suzuki cross-coupling type reactions, C-H activation, etc.) as well as a natural product synthesis (e.g. muscroride A). While several examples are provided, this review marks the beginning of a new field that is still in its infancy and for what might be a new approach to achieve highly efficient reactions that come closer to meeting the standards of chemical economies (e.g. atom, redox, step, etc.) and green chemistry.     

En Route to Multisurface Chemistry

This article considers what happens when the energy required for a compound to react is supplied by an irradiation lamp instead of by a Bunsen burner. For this purpose real examples are selected from three typical groups of cases. The respective answers obtained should indicate significant moves in organic photochemistry which may be expected to affect the further development of chemistry as a whole in the near future. During this tour d'horizon particular attention is paid to photochemical processes in solids or solvent matrices, light-induced reactions are especially emphasized as key reactions in (natural product) syntheses, and a strong case is made for interpreting the reactions of electronically excited molecules in terms of Salem correlation diagrams.     

Synergic catalytic effects in confined spaces

Combining the merits of confined effects and synergic effects is a promising way to build efficient and versatile heterogeneous catalytic systems. Recently, heterogeneous bifunctional and even trifunctional catalysts have attracted more and more attention because the synergic catalysis between the multifunctional groups could be developed within confined spaces. Significantly, many incompatible functional groups have been successfully incorporated into one confined space and show superior catalytic performance. Understanding the synergic catalytic effects in confined spaces is of great significance for constructing sophisticated and efficient catalytic systems. This feature article summarizes the recent advances in synergic catalysis in confined spaces as well as the methods to build synergic catalysts. The confined spaces provided by the one- or three-dimensional rigid pores of mesoporous silicas or the two-dimensional flexible interlayer regions of layered double hydroxides (LDHs) are mainly involved. An important reason for choosing mesoporous silica and LDH solids is that they additionally participate in synergic effects through their intrinsic active sites, the acidic hydroxyl groups on mesoporous silicas and acid-base bifunctional sites on layered double hydroxides, for example. Visible enhancement of catalytic activity or enantioselectivity or both was observed in aldol, Michael, Friedel-Crafts and Henry reactions, cyanosilylation, hydrolytic kinetic resolution of epoxides, etc.     

Homogeneous Gold Catalysis Beyond Assumptions and Proposals—Characterized Intermediates

Gold catalysis is a very active area in the field of catalysis research. New reactions are published every week, amazing changes in the connectivity are often observed, the number of applications in total synthesis is increasing …?—but what are the mechanisms of these reactions? Sound information can be provided by knowledge about the intermediates of these reactions.     

Entropy,Binding Energy,and Enzymic Catalysis

Why are enzyme-catalyzed reactions so much faster than uncatalyzed reactions, and why are enzymes so specific? What is the effect of mere approximation of enzyme and substrate, and what is the influence of the strain energy? Attempts to answer these questions have led to comparisons between entropy changes in intermolecular and intramolecular reactions, and to determinations of the intrinsic energy of the bond arising by non-covalent interaction between enzyme and substrate.     

Factors Determining the Selection of Organic Reactions by Medicinal Chemists and the Use of These Reactions in Arrays (Small Focused Libraries)

Synthetic organic reactions are a fundamental enabler of small‐molecule drug discovery, and the vast majority of medicinal chemists are initially trained—either at universities or within industry—as synthetic organic chemists. The sheer breadth of synthetic methodology available to the medicinal chemist represents an almost endless source of innovation. But what reactions do medicinal chemists use in drug discovery? And what criteria do they use in selecting synthetic methodology? Why are arrays (small focused libraries) so powerful in the lead‐optimization process? In this Minireview, we suggest some answers to these questions and also describe how we have tried to expand the number of robust reactions available to the medicinal chemist.     

β‐Peptides with Different Secondary‐Structure Preferences: How Different Are Their Conformational Spaces?

The conformational spaces accessible to two β‐hexapeptides in MeOH at 298 K and 340 K are investigated by molecular‐dynamics simulation with an atomistic model of both solute and solvent. The structural properties of these peptides have been previously studied by NMR in MeOH at room temperature. The experimental data could be fitted to a model (P)‐12/10‐helix for one of the peptides and a model hairpin with a ten‐membered H‐bonded turn for the other. The goal of the present work is to determine whether the conformational spaces accessible to these two peptides of seemingly different conformational properties contain any common regions. In other words, to what extent are the evident differences found at the macroscopic level also present at the microscopic structural level? It is found that, for the two peptides studied, the conformational spaces sampled in the respective simulations show significant overlap.     

Sesquiterpene synthases: passive catalysts or active players?

Sesquiterpene synthases catalyse the metal dependent turnover of farnesyl diphosphate to generate a class of natural products characterised by an enormous diversity in structure, stereochemistry, biological function and application. It has been proposed that these enzymes take a passive role in the reactions they catalyse and that they serve mostly as stereochemical templates, within which the reactions take place. Here, recent research into the structure and function of sesquiterpene synthases and the mechanisms of the reactions that they catalyse will be reviewed to suggest that these fascinating enzymes play multifaceted active roles in what are arguably the most complex biosynthetic reactions.     

In Operando Analysis of Diffusion in Porous Metal-Organic Framework Catalysts

The potential to exert atomistic control over the structure of site-isolated catalyst sites, as well as the topology and chemical environment of interstitial pore spaces, has inspired efforts to apply porous metal-organic frameworks (MOFs) as catalysts for fine chemical synthesis. In analogy to enzyme-catalyzed reactions, MOF catalysts have been proposed as platforms in which substrate confinement could be used to achieve chemo- and stereoselectivities that are orthogonal to solution-phase catalysts. In order to leverage the tunable pore topology of MOFs to impact catalyst selectivity, catalysis must proceed at interstitial catalyst sites, rather than at solvent-exposed interfacial sites. This Minireview addresses challenges inherent to interstitial MOF catalysis by 1) describing the diffusional processes available to sorbates in porous materials, 2) discussing critical factors that impact the diffusion rate of substrates in porous materials, and 3) presenting in operando experimental strategies to assess the relative rates of substrate diffusion and catalyst turnover in MOF catalysis. It is anticipated that the continued development of in operando tools to evaluate substrate diffusion in porous catalysts will advance the application of these materials in fine chemical synthesis.     

Multiply-charged ions and interstellar chemistry

Gaseous molecules and ions, and even dust grains, can accumulate charge in the interstellar medium (ISM) by harvesting the energy of UV photons, cosmic rays, helium ions and metastable atoms. This Perspective views the various modes of gas-phase formation of multiply-charged cations and the possible impact of their reactions on the chemical and ionization structure of the ISM, in the light of what is still very limited knowledge. Emphasis is given to gas-phase reactions of multiply-charged cations with atoms, molecules and electrons that lead to charge reduction, charge separation and chemical bond formation and these are examined for multiply-charged atoms, small molecules, hydrocarbons, polycyclic aromatic hydrocarbons and fullerenes, primarily as dications but also as a function of charge state. The increased electrostatic interaction due to multiple charge is seen to promote bonding to individual charge sites on large molecules (e.g. fullerenes) and allow ensuing "surface" chemistry under the influence of Coulomb repulsion. The unique ability of multiply charged cations to undergo charge separation reactions, either unimolecular or bimolecular, can feature in the production in the ISM of internally cold, but translationally hot, cations of lower charge state or hot atoms that may provide the driving force for subsequent chemical reactions in what is otherwise an ultracold environment. Available chemical kinetic models that account for the role of multiply-charged ions in the ISM are few and of limited scope and the observation of these ions in the ISM has remained elusive.     

计量合成学的产生与发展

对计量合成学的产生与发展及其研究与应用进行了综论，强调了计量化学在合成中的应用，集中在开发有机合成反应的决策及战术方面，通过试验设计对反应体系深入探索及通过自适应建模对试验条件的系统优化和通过综合评价对合成产物的全面考察，将有机反应开发为合成方法，参考文献３５篇。     

Parsing of the free energy of aromatic–aromatic stacking interactions in solution

We report an analysis of the energetics of aromatic–aromatic stacking interactions for 39 non-covalent reactions of self- and hetero-association of 12 aromatic molecules with different structures and charge states. A protocol for computation of the contributions to the total energy from various energetic terms has been developed and the results are consistent with experiment in 92% of all the systems studied. It is found that the contributions from hydrogen bonds and entropic factors are always unfavorable, whereas contributions from van-der-Waals, electrostatic and/or hydrophobic effects may lead to stabilizing or destabilizing factors depending on the system studied. The analysis carried out in this work provides an answer to the questions “What forces stabilize/destabilize the stacking of aromatic molecules in aqueous-salt solution and what are their relative importance?”     

Molecular approach to catalysis of electrochemical reaction in porous films

Current attempts to bridge the fields of what is conventionally called ‘electrocatalysis’ and of molecular catalysis of electrochemical reactions are surveyed and discussed. It amounts in many cases to ‘heterogenize’ molecular catalytic systems. Information on the meso- to nanostructures of the resulting catalytic films forms the basis of the understanding of new modes of transport of the reactants (catalysts, substrates and cosubstrates, products) that may govern the mechanistic competitions and consequently selectivity. Efforts to adapt benchmarking procedures developed in homogeneous molecular catalysis (catalytic Tafel plots) should be encouraged, taking into account, as additional factors, the transport of electrons and reacting species (including gases) through the catalytic system.     

Ultrasound and Sonochemistry for Radical Polymerization: Sound Synthesis

The use of ultrasound as an external stimulus for promoting polymerization reactions has received increasing attention in recent years. In this Review article, the fundamental processes that can lead to either the homolytic cleavage of polymer chains, or the sonolysis of solvent (or other) small molecules, under the application of ultrasound are described. These reactions promote the production of reactive radicals, which can be utilized in chain-growth radical polymerizations under the right conditions. A full historical overview of the development of ultrasound-assisted radical polymerization is provided, with special attention given to the recently described systems that are “controlled” by methods of reversible (radical) deactivation. Perspectives are shared on what challenges still remain in polymer sonochemistry, as well as new areas that are yet to be explored.     

Shared-memory parallelization of the TURBOMOLE programs AOFORCE, ESCF, and EGRAD: how to quickly parallelize legacy code

The programs ESCF, EGRAD, and AOFORCE are parts of the TURBOMOLE program package and compute excited-state properties and ground-state geometric hessians, respectively, for Hartree-Fock and density functional methods. The range of applicability of these programs has been extended by allowing them to use all CPU cores on a given node in parallel. The parallelization strategy is not new and duplicates what is standard today in the calculation of ground-state energies and gradients. The focus is on how this can be achieved without needing extensive modifications of the existing serial code. The key ingredient is to fork off worker processes with separated address spaces as they are needed. Test calculations on a molecule with about 80 atoms and 1000 basis functions show good parallel speedup up to 32 CPU cores.     

Use of Quantum Chemical Methods to Study Cyclodextrin Chemistry

Studies of cyclodextrin chemistry by quantum chemical methods are briefly surveyed. Emphases are put on what types of quantum chemical methods can be used for cyclodextrin chemistry, how to use quantum chemical methods to find the global minimum, to study the structures, binding energies, driving forces for cyclodextrin complexes, as well as chemical reactions occurring inside cyclodextrin cavities. Problems associated with the application of quantum chemical methods in cyclodextrin chemistry are also discussed.