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This Perspective outlines a panoramic description of the nature of the chemical bond according to valence bond theory. It describes single bonds and demonstrates the existence of a "forgotten family" of charge-shift bonds (CSBs) in which the entire/most of the bond energy arises from the resonance between the covalent and ionic structures of the bond. Many of the CSBs are homonuclear bonds. Hypervalent molecules (e.g., XeF2) are CSBs. This Perspective proceeds to describe multiple bonded molecules with an emphasis on C2 and 3O2. C2 has four electron pairs in its valence shell and, hence, 14 covalent structures and 1750 ionic structures. This Perspective outlines an effective methodology of peeling the electronic structure to the minimal and important number of structures: a dominant structure that displays a quadruple bond and two minor structures with π + σ bonds, which stabilize the quadruple bond by resonance. 3O2 is chosen because it is a diradical, which is persistent and life-sustaining. It is shown that the persistence of this diradical is due to the charge-shift bonding of the π -3-electron bonds. This section ends with a discussion of the roles of π vs σ in the geometric preferences of benzene, acetylene, ethene, and their Si-based analogs. Subsequently, this Perspective discusses bonding in clusters of univalent metal atoms, which possess only parallel spins (n+1Mn), and are nevertheless bonded due to the resonance interactions that stabilize the repulsive elementary structure (all spins are up). The bond energy reaches ∼40 kcal/mol for a pair of atoms (in n+1Cun; n ∼ 10–12). The final subsection discusses singlet excited states in ethene, ozone, and SO2. It demonstrates the capability of the breathing-orbital VB method to yield an accurate description of a variety of excited states using merely 10 or few VB structures. Furthermore, the method underscores covalent structures that play a key role in the correct description and bonding of these excited states. [ABSTRACT FROM AUTHOR]

Oriented external electric fields (EEFs) act as catalysts that can induce selectivity in chemical reactions. The responses of the Diels–Alder (DA) reaction between butadiene and ethylene (BDE‐DA) as well as cyclopentadiene and ethylene (CPDE‐DA) towards EEF stimuli are investigated here using density functional theory (B3LYP) calculations. EEF is a vector that catalyzes the reaction in one direction while inhibiting it in the opposite direction. Here we report that the inhibitive direction becomes rate‐enhancing after some increase in the EEF. The EEF value that brings about the maximum possible inhibition for the reaction is defined as the electrostatic resistance point (ERP). The possibility of both normal and inverse electron‐demand DA reactions causes catalytic activity in both directions of the EEF starting at a unique ERP value. The C5 substituents of cyclopentadiene control the ERP values depending upon the resistance power that the functional group provides against the EEF. The endo and exo diastereomeric transition states of the DA reaction have distinct ERP values and the difference (ΔERP) provides the through‐space electrostatic contribution to the stereoselectivity on a relative scale. Thus, the ERP values can be used as a gauge for the electrostatic interactions between substituent groups and external stimuli. [ABSTRACT FROM AUTHOR]

Oriented external electric fields (EEFs) act as catalysts that can induce selectivity in chemical reactions. The responses of the Diels–Alder (DA) reaction between butadiene and ethylene (BDE‐DA) as well as cyclopentadiene and ethylene (CPDE‐DA) towards EEF stimuli are investigated here using density functional theory (B3LYP) calculations. EEF is a vector that catalyzes the reaction in one direction while inhibiting it in the opposite direction. Here we report that the inhibitive direction becomes rate‐enhancing after some increase in the EEF. The EEF value that brings about the maximum possible inhibition for the reaction is defined as the electrostatic resistance point (ERP). The possibility of both normal and inverse electron‐demand DA reactions causes catalytic activity in both directions of the EEF starting at a unique ERP value. The C5 substituents of cyclopentadiene control the ERP values depending upon the resistance power that the functional group provides against the EEF. The endo and exo diastereomeric transition states of the DA reaction have distinct ERP values and the difference (ΔERP) provides the through‐space electrostatic contribution to the stereoselectivity on a relative scale. Thus, the ERP values can be used as a gauge for the electrostatic interactions between substituent groups and external stimuli. [ABSTRACT FROM AUTHOR]

The present work reports a DFT‐based mechanistic investigation of aryl C−O bond activation in anisole catalysed by a Rh−Al pincer‐type complex at room temperature. The study is extended to analogues Rh−E complexes based on Group 13 elements (E=B/Ga). Our results show a preference for the heterolytic cleavage pathway over oxidative addition in the C−O bond activation. The calculated barriers range from 16 to 36 kcal/mol, following the order: E=Al

Helmut Schwarz is a great scientist and a statesman who has made important fundamental contributions to chemistry as a science, and to the globalization of chemistry when he served as the Vice President of the German Research Foundation and subsequently as the President of the Alexander von Humboldt (AvH) Foundation. [ABSTRACT FROM AUTHOR]

The [4Fe‐4S]‐dependent radical S‐adenosylmethionine (SAM) proteins is one of large families of redox enzymes that are able to carry a panoply of challenging transformations. Despite the extensive studies of structure–function relationships of radical SAM (RS) enzymes, the electronic state‐dependent reactivity of the [4Fe‐4S] cluster in these enzymes remains elusive. Using combined MD simulations and QM/MM calculations, we deciphered the electronic state‐dependent reactivity of the [4Fe‐4S] cluster in Dph2, a key enzyme involved in the biosynthesis of diphthamide. Our calculations show that the reductive cleavage of the S−C(γ) bond is highly dependent on the electronic structure of [4Fe‐4S]. Interestingly, the six electronic states can be classified into a low‐energy and a high‐energy groups, which are correlated with the net spin of Fe4 atom ligated to SAM. Due to the driving force of Fe4−C(γ) bonding, the net spin on the Fe4 moiety dictate the shift of the opposite spin electron from the Fe1‐Fe2‐Fe3 block to SAM. Such spin‐regulated electron transfer results in the exchange‐enhanced reactivity in the lower‐energy group compared with those in the higher‐energy group. This reactivity principle provides fundamental mechanistic insights into reactivities of [4Fe‐4S] cluster in RS enzymes. [ABSTRACT FROM AUTHOR]

The [4Fe‐4S]‐dependent radical S‐adenosylmethionine (SAM) proteins is one of large families of redox enzymes that are able to carry a panoply of challenging transformations. Despite the extensive studies of structure–function relationships of radical SAM (RS) enzymes, the electronic state‐dependent reactivity of the [4Fe‐4S] cluster in these enzymes remains elusive. Using combined MD simulations and QM/MM calculations, we deciphered the electronic state‐dependent reactivity of the [4Fe‐4S] cluster in Dph2, a key enzyme involved in the biosynthesis of diphthamide. Our calculations show that the reductive cleavage of the S−C(γ) bond is highly dependent on the electronic structure of [4Fe‐4S]. Interestingly, the six electronic states can be classified into a low‐energy and a high‐energy groups, which are correlated with the net spin of Fe4 atom ligated to SAM. Due to the driving force of Fe4−C(γ) bonding, the net spin on the Fe4 moiety dictate the shift of the opposite spin electron from the Fe1‐Fe2‐Fe3 block to SAM. Such spin‐regulated electron transfer results in the exchange‐enhanced reactivity in the lower‐energy group compared with those in the higher‐energy group. This reactivity principle provides fundamental mechanistic insights into reactivities of [4Fe‐4S] cluster in RS enzymes. [ABSTRACT FROM AUTHOR]

Bonding in the recently synthesized NaBH3− cluster is investigated using the high level Valence Bond BOVB method. Contrary to earlier conclusions, the Na−B bond is found to be neither a genuine dative bond, nor a standard polar‐covalent bond at equilibrium. It is rather revealed as a split and polarized weakly coupled electron‐pair, which allows this cluster to be more effectively stabilized by a combination of (major) dipole‐dipole electrostatic interaction and (secondary) resonant one‐electron bonding mechanism. Our analysis of this unprecedented bonding situation extends to similar clusters, and the VB model unifies and articulates the previously published variegated views on this exotic "bond". [ABSTRACT FROM AUTHOR]

Bonding in the recently synthesized NaBH3− cluster is investigated using the high level Valence Bond BOVB method. Contrary to earlier conclusions, the Na−B bond is found to be neither a genuine dative bond, nor a standard polar‐covalent bond at equilibrium. It is rather revealed as a split and polarized weakly coupled electron‐pair, which allows this cluster to be more effectively stabilized by a combination of (major) dipole‐dipole electrostatic interaction and (secondary) resonant one‐electron bonding mechanism. Our analysis of this unprecedented bonding situation extends to similar clusters, and the VB model unifies and articulates the previously published variegated views on this exotic "bond". [ABSTRACT FROM AUTHOR]

This essay tells the story of the conception of the Two‐State Reactivity (TSR) notion. Since scientific career is part of life's flow, the story blends sub‐stories of scientific colleagues and events. This is also a story of a beguiling paradigm, which has started from a puzzling reactivity of the diatomic oxidant FeO+, has continued to larger oxidants, like the active species of Cytochrome P450 and of nonheme enzymes, and its extension to reductive processes. Finally, the essay discusses prospects of experimental probing of the reactive spin‐state, and of the transition state constitution for these reactions by means of tunneling‐augmented kinetic isotope effects. [ABSTRACT FROM AUTHOR]

This quantity is interpreted, on the basis of Koopmans' theorem, as the force exerted on the nuclei along this interatomic distance by removal of one electron from the MO, in the frozen MO approximation. In MO terms, such bonds display a doubly occupied bonding MO and a corresponding singly occupied antibonding one. Another problem with the DOF analysis of [1.1.1]propellane is that it cannot cleanly separate the wing bonds from the central bridgehead bond in the canonical MOs representation, due to the fundamentally delocalized nature of these MOs. Thus, the largely negative DOF of -0.109 a.u. for this MO is better interpreted as being due to the out-of-phase combination of bonding wing orbitals rather than to the minor contribution of the central bond. [Extracted from the article]

Charge‐shift bonds (CSBs) constitute a new class of bonds different than covalent/polar‐covalent and ionic bonds. Bonding in CSBs does not arise from either the covalent or the ionic structures of the bond, but rather from the resonance interaction between the structures. This Essay describes the reasons why the CSB family was overlooked by valence‐bond pioneers and then demonstrates that the unique status of CSBs is not theory‐dependent. Thus, valence bond (VB), molecular orbital (MO), and energy decomposition analysis (EDA), as well as a variety of electron density theories all show the distinction of CSBs vis‐à‐vis covalent and ionic bonds. Furthermore, the covalent–ionic resonance energy can be quantified from experiment, and hence has the same essential status as resonance energies of organic molecules, e.g. benzene. The Essay ends by arguing that CSBs are a distinct family of bonding, with a potential to bring about a Renaissance in the mental map of the chemical bond, and to contribute to productive chemical diversity. [ABSTRACT FROM AUTHOR]

Charge‐shift bonds (CSBs) constitute a new class of bonds different than covalent/polar‐covalent and ionic bonds. Bonding in CSBs does not arise from either the covalent or the ionic structures of the bond, but rather from the resonance interaction between the structures. This Essay describes the reasons why the CSB family was overlooked by valence‐bond pioneers and then demonstrates that the unique status of CSBs is not theory‐dependent. Thus, valence bond (VB), molecular orbital (MO), and energy decomposition analysis (EDA), as well as a variety of electron density theories all show the distinction of CSBs vis‐à‐vis covalent and ionic bonds. Furthermore, the covalent–ionic resonance energy can be quantified from experiment, and hence has the same essential status as resonance energies of organic molecules, e.g. benzene. The Essay ends by arguing that CSBs are a distinct family of bonding, with a potential to bring about a Renaissance in the mental map of the chemical bond, and to contribute to productive chemical diversity. [ABSTRACT FROM AUTHOR]

This Essay projects the spark of genius of Mendeleev, whose efforts led to the effective formulation of the periodic table, which has placed the entire world of chemical matter on a palm. The periodic table gave rise to a central paradigm, which did for chemistry what Newton had done for physics and Darwin for biology. Subsequently the Essay recounts the popularization of the Periodic Table through literature by Primo Levi, Oliver Sacks and others, and through music and art by composers and artists, such as Jerry Feldman, the King Crimson band, Tom Lehrer, and George Brecht, Blair Bradshaw, Eugènia Balcells, etc. [ABSTRACT FROM AUTHOR]

Dieser Essay beleuchtet den sprühenden Geist Mendeleevs. Seine Leistungen führten zur Aufstellung des Periodischen Systems, durch das die gesamte Welt chemischer Materie auf einem Handteller Platz hat. Das Periodensystem basiert auf einem zentralen Denkmuster und bedeutet für die Chemie so viel wie Newtons Errungenschaften für die Physik oder Darwins Thesen für die Biologie. Darüber hinaus beschäftigt sich der Essay mit der Verbreitung des Periodensystems durch die Literatur von Primo Levi, Oliver Sacks und anderen, sowie durch Musik und die darstellende Kunst, von Musikern und Künstlern wie Jerry Feldman, der King Crimson Band, Tom Lehrer und George Brecht, Blair Bradshaw, Eugènia Balcells, etc. [ABSTRACT FROM AUTHOR]

Flavoenzymes can mediate a large variety of oxidation reactions through the activation of oxygen. However, the O2 activation chemistry of flavin enzymes is not yet fully exploited. Normally, the O2 activation occurs at the C4a site of the flavin cofactor, yielding the flavin C4a‐(hydro)hydroperoxyl species in monooxygenases or oxidases. Using extensive MD simulations, QM/MM calculations and QM calculations, our studies reveal the formation of the common nucleophilic species, Flavin‐N5OOH, in two distinct flavoenzymes (RutA and EncM). Our studies show that Flavin‐N5OOH acts as a powerful nucleophile that promotes C−N cleavage of uracil in RutA, and a powerful base in the deprotonation of substrates in EncM. We reason that Flavin‐N5OOH can be a common reactive species in the superfamily of flavoenzymes, which accomplish generally selective general base catalysis and C−X (X=N, S, Cl, O) cleavage reactions that are otherwise challenging with solvated hydroxide ion base. These results expand our understanding of the chemistry and catalysis of flavoenzymes. [ABSTRACT FROM AUTHOR]

Flavoenzymes can mediate a large variety of oxidation reactions through the activation of oxygen. However, the O2 activation chemistry of flavin enzymes is not yet fully exploited. Normally, the O2 activation occurs at the C4a site of the flavin cofactor, yielding the flavin C4a‐(hydro)hydroperoxyl species in monooxygenases or oxidases. Using extensive MD simulations, QM/MM calculations and QM calculations, our studies reveal the formation of the common nucleophilic species, Flavin‐N5OOH, in two distinct flavoenzymes (RutA and EncM). Our studies show that Flavin‐N5OOH acts as a powerful nucleophile that promotes C−N cleavage of uracil in RutA, and a powerful base in the deprotonation of substrates in EncM. We reason that Flavin‐N5OOH can be a common reactive species in the superfamily of flavoenzymes, which accomplish generally selective general base catalysis and C−X (X=N, S, Cl, O) cleavage reactions that are otherwise challenging with solvated hydroxide ion base. These results expand our understanding of the chemistry and catalysis of flavoenzymes. [ABSTRACT FROM AUTHOR]

Investigates various atomic and molecular multiplet states using a proposed spin-restricted open-shell Kohn-Sham (ROKS) method. Range of multiplets considered; ROKS equations; States of atomic species; Angular momentum states of linear molecules; Diradicaloid states of perpendicular ethylene and of square cyclobutadiene.

Graphical abstract Highlights • Helmut Schwarz' 75th Birthday. • collaboration with Schwarz on two-state reactivity. • collaboration with Schwarz on mechanistic variants of methane activation. • Helmut Schwarz and the story of the Lise Meitner-Minerva Center. • Helmut Schwarz as a statesman. • Schwarz and the Alexander von Humboldt Foundation. Abstract This essay is dedicated to Helmut Schwarz on his 75th Birthday. It is written based on my diary notes, and covers the period from the early 1980s to the present time. It describes two periods of our scientific collaboration, on: (a) Two-state reactivity (TSR), which has become a widely used concept in bioinorganic and metallo-enzymatic chemistry; and (b) the mechanistic variants of methane activation, which is still ongoing. In between, the essay tells the story of the Lise Meitner-Minerva Center for Computational Quantum Chemistry, in which Helmut played a major role during the establishment of the Center and as the Chairman of its Advisory Board for 20 years. A few words are devoted to the role of Helmut as a statesman and for his making the Alexander von Humboldt Foundation a vehicle for worldwide scientific influence. This essay is basically a story of science and friendship. [ABSTRACT FROM AUTHOR]

This is a tutorial on use of external-electric-fields (EEFs) as effectors of chemical change. The tutorial instructs readers how to conceptualize and design electric-field effects on bonds, structures, and reactions. Most effects can be comprehended as the field-induced stabilization of ionic structures. Thus, orienting the field along the “bond axis” will facilitate bond breaking. Similarly, orienting the field along the “reaction axis”, the direction in which “electron pairs transform” from reactants- to products-like, will catalyse the reaction. Flipping the field's orientation along the reaction-axis will cause inhibition. Orienting the field off-reaction-axis will control stereo-selectivity and remove forbidden-orbital mixing. Two-directional fields may control both reactivity and selectivity. Increasing the field strength for concerted reactions (e.g., Diels–Alder's) will cause mechanistic-switchover to stepwise mechanisms with ionic intermediates. Examples of bond breaking and control of reactivity/selectivity and mechanisms are presented and analysed from the “ionic perspective”. The tutorial projects the unity of EEF effects, “giving insight and numbers”. [ABSTRACT FROM AUTHOR]

The driving force for the electron transfer (ET) step in the catalytic cycle of cytochrome P450BM3 is investigated using three sets of 1μs molecular dynamic simulations for the resting state of P450 in complex with the flavin (FMN) in the reductase domain. These sets involve the following: (i) substrate-free (SF), (ii) substrate (N-palmitoyl glycine, i.e., NPG)-bound (SB), and (iii) SB with the semiquinone radical anion (SQ-) of FMN. Starting from the X-ray structure of the SF heme domain, we observe that the α1-helix (of the reductase) and the C-helix (of the heme) undergo reorientation to a parallel orientation, which is the thermochemically stable form. The reorientation of the helices pushes away the FMN to a distance of 18.4 Å from the heme's center. When the substrate binds it causes the I-helix of the heme domain to kink and push the C-helix toward the α1-helix, thereby locking the latter two into a stabilized perpendicular conformation, wherein the FMN -- heme distance is 12 Å. The distance drops further in the SQ- form, and upon QM/MM geometry optimization the two moieties approach 8.8 Å, which enhances the ET rate (by 104-106 fold) to the heme's Fe[sup 3+] ion. These motions are driven by hydrogen bond strengthening between the C- and the α1-helices. Finally, substrate binding leads to formation of an organized water chain connecting the FMN and heme moieties. The water channel assists the ET and couples it to the proton transfer steps that should activate O2 and create the oxo -- iron active species. [ABSTRACT FROM AUTHOR]

This Perspective discusses a story of one molecule (methane), a few metal-oxide cationic clusters (MOCCs), dopants, metal-carbide cations, orientedelectric fields (OEFs), and a dizzying mechanistic landscape of methane activation! One mechanism is hydrogen atom transfer (HAT), which occurs whenever the MOCC possesses a localized oxyl radical (M-O*). Whenever the radical is delocalized, e.g., in [MgO]n *+ the HAT barrier increases due to the penalty of radical localization. Adding a dopant (Ga2O3) to [MgO]2 *+ localizes the radical and HAT transpires. Whenever the radical is located on the metal centers as in [Al2O2]*+ the mechanism crosses over to proton-coupled electron transfer (PCET), wherein the positive Al center acts as a Lewis acid that coordinates the methane molecule, while one of the bridging oxygen atoms abstracts a proton, and the negatively charged CH3 moiety relocates to the metal fragment. We provide a diagnostic plot of barriers vs reactants' distortion energies, which allows the chemist to distinguish HAT from PCET. Thus, doping of [MgO]2 *+ by Al2O3 enables HAT and PCET to compete. Similarly, [ZnO]*+ activates methane by PCET generating many products. Adding a CH3CN ligand to form [(CH3CN)- ZnO]*+ leads to a single HAT product. The CH3CN dipole acts as an OEF that switches off PCET. [MC]+ cations (M = Au, Cu) act by different mechanisms, dictated by the M+-C bond covalence. For example, Cu+, which bonds the carbon atom mostly electrostatically, performs coupling of C to methane to yield ethylene, in a single almost barrier-free step, with an unprecedented atomic choreography catalyzed by the OEF of Cu+. [ABSTRACT FROM AUTHOR]

What do experimentally measured kinetic isotope effects (KIEs) tell us about H-abstraction reactions with multispin-state reactivity options? Using DFT calculations with tunneling corrections for experimentally studied H-abstraction reactions of porphyrin-Compound II species (Chem.-Eur. J. 2014, 20, 14437; Angew. Chem., Int. Ed. 2008, 47, 7321) with cyclohexane, dihydroanthracene (DHA), and xanthene (Xan), we show here that KIE is a selective probe that identifies the experimentally reactive spin state. At the same time, comparison of calculated and experimental KIE values permits us to determine the structural orientation of the transition states, as well as the presence/absence of an axial ligand, and the effect of porphyrin substituents. The studied compound II (Cpd II) species involve porphine, and porphyrin ligands with different meso-substituents, TPFPP (tetrakis(pentafluorophenyl)-porphyrin dianion) and TMP (tetramesitylporphyrin dianion), with and without imidazole axial ligands. The DFT calculations reveal three potential pathways: quintet and triplet σ-pathways (5Hσ and 3Hσ) that possess linear transition state (TS) structures, and a triplet π -pathway (3Hπ) having a bent TS structure. Without an axial ligand, the 5Hσ pathways for these Cpd II complexes cross below the triplet states. The axial ligand raises the barriers for the quintet and triplet σ-pathways and quenches any chances for two-state reactivity, thus proceeding via the 3Hπ pathway. All of these pathways exhibit characteristic KIE values: very large for 3Hπ (48−200), small for 5Hσ (3−9), and intermediate for 3Hσ (23−51). The calculated KIEs for (TPFPP)FeIV=O without an axial ligand reveal that 3Hσ is the only pathway having a KIE that matches the experimental values, for the reactions with DHA and Xan (Angew. Chem., Int. Ed. 2008, 47, 7321). Indeed, theory shows that tunneling significantly lowers the 3Hσ barrier rendering it the sole reactive state for the reaction. A prediction is made for the reactivity and KIE of (TMP)FeIV=O complex, and a comparison is made with the analogous nonheme complexes. [ABSTRACT FROM AUTHOR]

QM/MM calculations reveal that the nickel pincer complex in lactate racemase functions as a reversible 'single-center electrode' that accepts and donates back an electron. In this way, it catalyzes the isomerization process d-lactate⇌ l-lactate through successive proton-coupled electron-transfer steps. [ABSTRACT FROM AUTHOR]

QM/MM calculations reveal that the nickel pincer complex in lactate racemase functions as a reversible 'single-center electrode' that accepts and donates back an electron. In this way, it catalyzes the isomerization process d-lactate⇌ l-lactate through successive proton-coupled electron-transfer steps. [ABSTRACT FROM AUTHOR]

In response to the comment by Frenking and Hermann on our work in this journal (Chem. Eur J. 2016, 22, 4116) it is is shown once again that C2 has a quadruple bond with three strong bonds and one weaker exo ‐ bond. All other bonding forms are less stable. [ABSTRACT FROM AUTHOR]

As we approach the Lewis model centennial, it may be timely to discuss novel bonding motifs. Accordingly, this review discusses no-pair ferromagnetic (NPFM) bonds that hold together monovalent metallic atoms using exclusively parallel spins. Thus, without any traditional electron-pair bonds, the bonding energy per atom in these clusters can reach 20 kcal mol−1. This review describes the origins of NPFM bonding using a valence bond (VB) analysis, which shows that this bonding motif arises from bound triplet electron pairs that are delocalized over all the close neighbors of a given atom in the cluster. The VB model accounts for the tendency of NPFM clusters to assume polyhedral shapes with rather high symmetry and for the very steep rise of the bonding energy per atom. The advent of NPFM clusters offers new horizons in chemistry of highly magnetic species sensitive to magnetic and electric fields. [ABSTRACT FROM AUTHOR]

Ever since Lewis depicted the triple bond for acetylene, triple bonding has been considered as the highest limit of multiple bonding for main elements. Here we show that C2 is bonded by a quadruple bond that can be distinctly characterized by valence-bond (VB) calculations. We demonstrate that the quadruply-bonded structure determines the key observables of the molecule, and accounts by itself for about 90 % of the molecule's bond dissociation energy, and for its bond lengths and its force constant. The quadruply-bonded structure is made of two strong π bonds, one strong σ bond and a weaker fourth σ-type bond, the bond strength of which is estimated as 17-21 kcal mol−1. Alternative VB structures with double bonds; either two π bonds or one π bond and one σ bond lie at 129.5 and 106.1 kcal mol−1, respectively, above the quadruply-bonded structure, and they collapse to the latter structure given freedom to improve their double bonding by dative σ bonding. The usefulness of the quadruply-bonded model is underscored by 'predicting' the properties of the 3Σu+ state. C2’s very high reactivity is rooted in its fourth weak bond. Thus, carbon and first-row main elements are open to quadruple bonding! [ABSTRACT FROM AUTHOR]

The study of C-H bond activation reactions by nonheme FeIVO species with nine hydrocarbons shows that the kinetic isotope effect (KIE) involves strong tunneling and is a signature of the reactive spin states. Theory reproduces the observed spike-like appearance of plots of KIEexp against the C-H bond dissociation energy, and its origins are discussed. The experimentally observed Bell-Evans-Polanyi correlations, in the presence of strong tunneling, are reproduced, and the pattern is rationalized. [ABSTRACT FROM AUTHOR]

We tell the story of the Lise Meitner-Minerva Center, its establishment and activities, its members and their scientific activity, and its instrumental role in weaving intense relationships with the theoretical community in Germany, and in amalgamating the Israeli community of computational quantum chemistry into a national center that enjoys a high international reputation. [ABSTRACT FROM AUTHOR]

This is the story of the establishment of the Hebrew University (HU), and within it, the sub-story of the first school of chemistry in the Land of Israel, from 1923. It spans more than 100 years, from the publication of the pamphlet, Eine Jüdische Hochschule, in 1902, all the way to 2015. The story starts with a dream, almost in the realm of science fiction, the audacious vision of building a university in a stateless and impoverished region, and all the way to an Institute of Chemistry of HU that has served as the mother of chemistry in the State of Israel. [ABSTRACT FROM AUTHOR]

Bonding is a glue of chemical matter and is also a useful concept for designing new molecules. Despite the fact that electron pairing remains the bonding mechanism in the great majority of molecules, in the past few decades scientists have had a growing interest in discovering novel bonding motifs. As this Account shows, monovalent metallic atoms having exclusively parallel spins, such as 11Li10, 11Au10, and 11Cu10, can nevertheless form strongly bound clusters, without having even one traditional bond due to electron pairing. These clusters, which also can be made chiral, have high magnetic moments. We refer to this type as no-pair ferromagnetic (NPFM) bonding, which characterizes the n+1Mnclusters, which were all predicted by theoretical computations. The small NPFM alkali clusters that have been “synthesized” to date, using cold-atom techniques, support the computational predictions. [ABSTRACT FROM AUTHOR]

Abstract: Two-state reactivity (TSR) has become a widely used concept in bioinorganic and metallo-enzymatic chemistry. This concept was conceived in response to puzzling data on the reactivity of Fe O+ with H2[1], and in the reactivity patterns of the enzyme Cytochrome P450. The TSR concept has been developed through a very intense interaction of the author with Helmut Schwarz and the late Detlef Schröder between the years 1993 and 1998. The paper is written as a tribute to Detlef Schröder who passed away untimely, and is still remembered by his many friends as a wonderful scientist with scintillating energy, goodness and enthusiasm. [Copyright &y& Elsevier]

What could be simpler than C2, a diatomic molecule that has the second strongest homonuclear bond? This molecule turns out to be a microcosm of the bonding issues that bother chemists, as is shown in this trialogue. Join the three authors in their lively debate, light a candle, as Faraday did, and see the excited states of C2! [ABSTRACT FROM AUTHOR]

Was könnte einfacher sein als C2, ein zweiatomiges Molekül mit der zweitstärksten homonuklearen Bindung? Es stellt sich heraus, dass dieses Molekül – wie der vorliegende Trialog zeigt – viele Fragen zur chemischen Bindung vereint, die Chemiker beschäftigen. Folgen Sie den drei Autoren in ihrer Debatte, zünden Sie eine Kerze an – so wie es Faraday tat – , und sehen Sie die Anregungen von C2! [ABSTRACT FROM AUTHOR]

Triple bonding is conventionally considered to be the limit for multiply bonded main group elements, despite higher metal-metal bond orders being frequently observed for transition metals and lanthanides/actinides. Here, using high-level theoretical methods, we show that C2 and its isoelectronic molecules CN+, BN and CB? (each having eight valence electrons) are bound by a quadruple bond. The bonding comprises not only one ?- and two ?-bonds, but also one weak 'inverted' bond, which can be characterized by the interaction of electrons in two outwardly pointing sp hybrid orbitals. A simple way of assessing the energy of the fourth bond is proposed and is found to be ~12-17 kcal mol?1 for the isoelectronic species studied, and thus stronger than a hydrogen bond. In contrast, the analogues of C2 that contain higher-row elements, such as Si2 and Ge2, exhibit only double bonding. [ABSTRACT FROM AUTHOR]

This commentary focuses on the conceptual interpretation of the bonding of O and NO to heme in oxyheme and nitrosylheme complexes, using high-level ab initio complete active space self-consistent field (CASSCF)/molecular mechanical (MM), gas-phase CASSCF, and CASSCF followed by second-order perturbation theory (CASPT2) calculations as well as density functional theory (DFT) calculations. The commentary shows that expanding the complex multiconfigurational (MC) wave functions into valence bond (VB)-type configurations based on localized orbitals provides significant insight into bonding and precise definitions of oxidation states. Furthermore, the VB 'reading' of the wave function unifies the descriptions of DFT and MC theories, reconciling controversies and surprises. In so doing, we demonstrate the impact of the protein bulk polarity and its hydrogen-bonding capability on the bonding. The insight provided by 'reading' the VB content of the MC wave functions highlights the potential of this approach as a general paradigm in future computational bioinorganic chemistry. A great deal of insight lies in this road. [ABSTRACT FROM AUTHOR]

In view of recent reports of high reactivity of ferric-superoxide species in heme and nonheme systems (Morokuma et al. J. Am. Chem. Soc. 2010, 132, 1199312005; Que et a!. lnorg. Chem. 2010, 49, 3618-3628; Nam et a!. J. Am. Chem. Soc. 2010, 132, 59585959; J. Am. Chem. Soc. 2010, 132, 10668-10670), we use herein combined quantum mechanics/molecular mechanics (QM/MM) methodsto explore the potential reactivity of P450C ferric-superoxide toward hydroxylation, epoxidation, and sulfoxidation. The calculations demonstrate that P450 ferric-superoxide is a sluggish oxidant compared with the high-valent oxoiron porphyrin cation-radical species. As such, unlike heme enzymes with a histidine axial ligand, the P450 superoxo species does not function as an oxidant in P450cam. The origin of this different behavior of the superoxo species of P450 vis-à-vis other heme enzymes like tryptophan 2, 3-dioxygenase (TDO) is traced to the ability of the latter superoxo species to make a stronger FeOO-X (X = H,C) bond and to stabilize the corresponding bond-activation transition states by resonance with charge-transfer configurations. By contrast, the negatively charged thiolate ligand in the P450 superoxo species minimizes the mixing of charge transfer configurations in the transition state and raises the reaction barrier. However, as we demonstrate, an external electric field oriented along the Fe-O axis with a direction pointing from Fe toward 0 will quench Cpd I formation by slowing the reduction of ferric-superoxide and will simultaneously lower the barriers for oxidation by the latter species, thereby enabling observation of superoxo chemistry in P450. Other options for nascent superoxo reactivity in P450 are discussed. [ABSTRACT FROM AUTHOR]

A computational study of a catalytic cycle generates state energies (the E-representation), whereas experiments lead to rate constants (the k-representation). Based on transition state theory (TST), these are equivalent representations. Nevertheless, until recently, there has been no simple way to calculate the efficiency of a catalytic cycle, that is, its turnover frequency (TOF), from a theoretically obtained energy profile. In this Account, we introduce the energetic span model that enables one to evaluate TOFs in a straightforward manner and in affinity with the Curtin−Hammett principle. As shown herein, the model implies a change in our kinetic concepts. Analogous to Ohm's law, the catalytic chemical current (the TOF) can be defined by a chemical potential (independent of the mechanism) divided by a chemical resistance (dependent on the mechanism and the nature of the catalyst). This formulation is based on Eyring's TST and corresponds to a steady-state regime. In many catalytic cycles, only one transition state and one intermediate determine the TOF. We call them the TOF-determining transition state (TDTS) and the TOF-determining intermediate (TDI). These key states can be located, from among the many states available to a catalytic cycle, by assessing the degree of TOF control (XTOF); this last term resembles the structure−reactivity coefficient in classical physical organic chemistry. The TDTS−TDI energy difference and the reaction driving force define the energetic span (δE) of the cycle. Whenever the TDTS appears after the TDI, δE is the energy difference between these two states; when the opposite is true, we must also add the driving force to this difference. Having δE, the TOF is expressed simply in the Arrhenius−Eyring fashion, wherein δE serves as the apparent activation energy of the cycle. An important lesson from this model is that neither one transition state nor one reaction step possess all the kinetic information that determines the efficiency of a catalyst. Additionally, the TDI and TDTS are not necessarily the highest and lowest states, nor do they have to be adjoined as a single step. As such, we can conclude that a change in the conceptualization of catalytic cycles is in order: in catalysis, there are no rate-determining steps, but rather rate-determining states. We also include a study on the effect of reactant and product concentrations. In the energetic span approximation, only the reactants or products that are located between the TDI and TDTS accelerate or inhibit the reaction. In this manner, the energetic span model creates a direct link between experimental quantities and theoretical results. The versatility of the energetic span model is demonstrated with several catalytic cycles of organometallic reactions. [ABSTRACT FROM AUTHOR]

Reactivity principles based on orbital overlap and bonding/antibonding interactions are well established to describe the reactivity of organic species, and atomic structures are typically predicted by Hund's rules to have maximum single-electron occupancy of degenerate orbitals in the ground state. Here, we extend the role of exchange to transition states and discuss how, for reactions and kinetics of bioinorganic species, the analogue of Hund's rules is exchange-controlled reactivity. Pathways that increase the number of unpaired and spin-identical electrons on a metal centre will be favoured by exchange stabilization. Such exchange-enhanced reactivity endows transition states with a stereochemistry different from that observed in cases that are not exchange-enhanced, and is in good agreement with the reactivity observed for iron-based enzymes and synthetic analogues. We discuss the interplay between orbital- and exchange-controlled principles, and how this depends on the identity of the transition metal, its oxidation number and its coordination sphere. [ABSTRACT FROM AUTHOR]

Valence bond calculations utilizing the Xiamen package have been carried out on the bimolecular (E2) elimination reaction X- + HCH2CH2Y ? XH + CH2-CH2 + Y- where X,Y = F,F; F,Cl; Cl,F; Cl,Cl for anti and syn reactant complexes, transition states, and product complexes. The calculations were supplemented by MO-based calculations at MP2/6-311++G**//MP2/6-311++G**. The valence bond calculations give reasonable energies with eight contributors to the resonance hybrid. Charge-localized contributors dominate the transition states. NPA charges from the MO calculations confirm that the transition states possess a significant degree of localized charge and can be described by the key resonance structure X(-)-H(+)-CH2(-)-CH2(+)-Y(-). At the same time, the MO calculations show that electronically and geometrically the reactions are clearly concerted though not synchronous. Valence bond state correlation diagrams (VBSCD) show that a simple proton transfer such as that in the E1cB irreversible reaction is predicted to have a lower barrier than a synchronous concerted (E2) reaction. The E2 transition state evidently avoids this energetic disadvantage by becoming localized and nonsynchronous, though with important electronic and geometric changes at all of the reacting centers. [ABSTRACT FROM AUTHOR]

Abstract: This paper interrogates the efficiency of a model cross-coupling reaction catalyzed by palladium complexes, using a quantum mechanical/molecular mechanical (QM/MM) methodology that permits ‘independent’ treatments of the inductive and steric effects of the phosphine ligands of the palladium catalysts. To test the efficiency of the catalyst we calculate turnover frequencies (TOFs), based on ‘the energetic-span model’ which considers the TOF determining transition state (TDTS) and TOF determining intermediate (TDI) as the reaction-rate controlling factors. Four different TDTS and TDI species are considered: the square planar diphosphine palladium complex, the anionic monophosphine, the T-shaped monophosphine, and the anionic phosphine-free species. Two different requirements are found to typify the most efficient catalysts: either bulky ligands that hinder the stabilization of diphosphines, or medium size ligands with high electron withdrawing power. Both conditions reduce the energetic span of the cycle by destabilizing the TDI more than the TDTS, thereby leading to enhanced TOFs. The approach used here can serve for future theoretical or experimental designs of new palladium catalysis. [Copyright &y& Elsevier]

The article presents a research which investigates the selectivity, reactivity, and structure of the enzyme cytochrome P450 (CYP) using quantum mechanical/molecular mechanical (QM/MM) calculations. It offers information on several low-lying spin states that contribute to the enzymes' product formation and reactivity. It concludes that further investigation is needed to provide additional insights into the ability of the enzyme to find solutions for performing complex transformations.

Electron-pair bonding is a central chemical paradigm. Here, we show that alongside the two classical covalent and ionic bond families, there exists a class of charge-shift (CS) bonds wherein the electron-pair fluctuation has the dominant role. Charge-shift bonding shows large covalent–ionic resonance interaction energy, and depleted charge densities, and features typical to repulsive interactions, albeit the bond itself may well be strong. This bonding type is rooted in a mechanism whereby the bond achieves equilibrium defined by the virial ratio. The CS bonding territory involves, for example, homopolar bonds of compact electronegative and/or lone-pair-rich elements, heteropolar bonds of these elements among themselves and with other atoms (for example, the metalloids, such as silicon and germanium), hypercoordinated molecules, and bonds whose covalent components are weakened by exchange-repulsion strain (as in [1.1.1]propellane). Here, we discuss experimental manifestations of CS bonding in chemistry, and outline new directions demonstrating the portability of the new concept. [ABSTRACT FROM AUTHOR]

The paper outlines the fundamental factors that govern the mechanisms of alkane hydroxylation by cytochrome P450 and the corresponding barrier heights during the hydrogen abstraction and radical rebound steps of the process. This is done by a combination of density functional theory calculations for 11 alkanes and valence bond (VB) modeling of the results. The energy profiles and transition states for the various steps are reconstructed using VB diagrams (Shaik, S. S. J. Am. Chem. Soc. 1981, 103,3692-3701. Shaik, S.; Shurki, A. Angew. Chem. Int. Ed. 1999, 38, 586-625.) and the DFT barriers are reproduced by the VB model from raw data based on C-H bond energies. The model explains a variety of other features of P450 hydroxylations: (a) the nature of the polar effect during hydrogen abstraction, (b) the difference between the activation mechanisms leading to the Fe" vs the Fe" electromers, (c) the difference between the gas phase and the enzymatic reaction, and (d) the dependence of the rebound barrier on the spin state. The VB mechanism shows that the active species of the enzyme performs a complex reaction that involves multiple bond making and breakage mechanisms by utilizing an intermediate VB structure that cuts through the high barrier of the principal transformation between reactants and products, thereby mediating the process at a low energy cost. The correlations derived in this paper create order and organize the data for a process of a complex and important enzyme. This treatment can be generalized to the reactivity patterns of nonheme systems and synthetic iron-oxo porphyrin reagents. [ABSTRACT FROM AUTHOR]

We have investigated C-H hydroxylation of camphor by Compound I (Cpd I) of cytochrome P450cam in different electronic states and by its one-electron reduced and oxidized forms, using QM/MM calculations in the native protein/solvent environment. Cpd I species with five unpaired electrons (pentaradicaloids) are Ca. 12 kcal/mol higher in energy than the ground state Cpd I species with three unpaired electrons (triradicaloids). The H-abstraction transition states of pentaradicaloids lie Ca. 21 (9) kcal/mol above the triradicaloid (pentaradicaloid) reactants. Hydroxylation via pentaradicaloids is thus facile provided that they can react before relaxing to the ground-state triradicaloids. Excited states of Cpd I with an Fe(V)-oxo moiety lie more than 20 kcal/mol above the triradicaloid ground state in single-point gas-phase calculations, but these electronic configurations are not stable upon including the point-charge protein environment which Causes SCF convergence to the triradicaloid ground state. One-electron reduced species (Cpd II) show sluggish reactivity compared with Cpd I in agreement with experimental model studies. One-electron oxidized species are more reactive than Cpd I but seem too high in energy to be accessible. The barriers to hydrogen abstraction for the various forms of Cpd I are generally not affected much by the chosen protonation states of the Asp297 and His355 residues near the propionate side chains of the heme or by the appearance of radical character at Asp297, His355, or the propionates. [ABSTRACT FROM AUTHOR]

Starting from the 1980s and onwards, Valence Bond theory has been enjoying renaissance that is characterized by the development of a growing number of ab initio methods, and by many applications to chemical reactivity and to the central paradigms of chemistry. Owing the increase of computational power of modern computers and to significant advances in the methodology, valence bond theory begins to offer a sound and attractive alternative to Molecular Orbital theory. This review aims at summarizing the most important developments of ab initio valence bond methods during the last two or three decades, and is primarily devoted to a description of what the various methods can actually achieve within their specific scopes and limitations. Key available softwares are surveyed. © 2006 Wiley Periodicals, Inc. J Comput Chem, 2007 [ABSTRACT FROM AUTHOR]

Is chemistry a science without a territory? I argue that “chemical bonding” has been a traditional chemical territory ever since the chemical community amalgamated in the seventeenth century, and even before. The modern charter of this territory is Gilbert Newton Lewis, who started the “electronic structure revolution in chemistry.” As a tribute to Lewis, I describe here three of his key papers from the years 1913, 1916, and 1923, and analyze them. Lewis has defined the quantum unit, the “electron pair bond,” for construction of a chemical universe, and in so doing he charted a vast chemical territory and affected most profoundly the mental map of chemistry for generations ahead. Nevertheless, not all is known about the chemical bond” the chemical territory is still teaming with new and exciting problems of in new materials, nanoparticles, quantum dots, metalloenzymes, bonding at surface-vapor interfaces, and so on and so forth. © 2006 Wiley Periodicals, Inc.J Comput Chem, 2007 [ABSTRACT FROM AUTHOR]

Quantum mechanics/molecular mechanics calculations were utilized to study the process of oxidation of a native substrate (ferulic acid) by the active species of horseradish peroxidase (Dunford, H. B. Heme Peroxidases; Wiley-VCH: New York, 1999), Compound I and Compound II, and the manner by which the enzyme returns to its resting state. The results match experimental findings and reveal additional novel features. The calculations demonstrate that both oxidation processes are initiated by a proton-coupled electron-transfer (PCET) step, in which the active species of the enzyme participate only as electron-transfer partners, while the entire proton-transfer event is being relayed from the substrate to and from the His42 residue by a water molecule (W402). The reason for the observed (Henriksen, A; Smith, A. T.; Gajhede, M. J. Biol. Chem. 1999, 274, 35005–35011) similar reactivities of Compound I and Compound II toward ferulic acid is that the reactive isomer of Compound II is the, hitherto unobserved, Por+FeIIIOH isomer that resembles Compound I. The PCET mechanism reveals that His42 and W402 are crucial moieties and they determine the function of the HRP enzyme and account for its ability to perform substrate oxidation (Poulos, T. L. Peroxidases and Cytochrome P450. In The Porphyrin Handboolc,, Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2000; Vol. 4, pp 189). In view of the results, the possibility of manipulating substrate oxidation by magnetic fields is an intriguing possibility. [ABSTRACT FROM AUTHOR]

The hydrogen abstraction reaction of camphor in cytochrome P450cam has been investigated in the native enzyme environment by combined quantum mechanical/molecular mechanical (QM/MM) calculations and in the gas phase by density functional calculations. This work has been motivated by contradictory published QM/MM results. In an attempt to pinpoint the origin of these discrepancies, we have systematically studied the factors that may affect the computed barriers, including the QM/MM setup, the optimization procedures, and the choice of QM region, basis set, and protonation states. It is found that the ChemShell and QSite programs used in the published QM/MM calculations yield similar results at given geometries, and that the discrepancies mainly arise from two technical issues (optimization protocols and initial system preparation) that need to be well controlled in QM/MM work. In the course of these systematic investigations, new mechanistic insights have been gained. The crystallographic water 903 placed near the oxo atom of Compound I lowers the hydrogen abstraction barrier by ca. 4 kcal/mol, and thus acts as a catalyst for this reaction. Spin density may appear at the A-propionate side chain of the heme if the carboxylate group is not properly screened, which might be expected to happen during protein dynamics, but not in static equilibrium situations. There is no clear correlation between the computed A-propionate spin density and the hydrogen abstraction barrier, and hence, no support for a previously proposed side-chain mediated transition state stabilization mechanism. Standard QM/MM optimizations yield an A-propionate environment close to the X-ray structure only for protonated Asp297, and not for deprotonated Asp297, but the computed barriers are similar in both cases. An X-ray like A-propionate environment can also be obtained when deprotonated Asp297 is included in the QM region and His355 is singly protonated, but this Compound II-type species with a closed-shell porphyrin ring has a higher hydrogen abstraction barrier and should thus not be mechanistically relevant. © 2006 Wiley Periodicals, Inc. J Comput Chem 27: 1324–1337, 2006 [ABSTRACT FROM AUTHOR]