Orbital mechanism of upright CO activation on Fe(100)

The knowledge of bond activation forms a cornerstone for modern chemistry, wherein symmetry rules of electronic activation lie in the heart of bond activation. However, the question as to how a chemical bond is activated remains elusive. By taking CO activated on Fe(100), herein, we have resolved the long-standing fundamental question; we have found that excitations in the adsorbate feature the bond activation. We essentially have discovered contrasting electronic processes in respective σ and π electron systems of the adsorbed CO molecule. The σ electron system is involved in reversible hidden excitations/deexcitations between two occupied σ orbitals, whereas the π electron system is subject to irreversible π to π* excitations dispersed along the d-band region, which is coupled to the rotational 2π electron couplings depending on the strength of molecule-metal interactions. The σ excitations pertain to the Pauli repulsion mediated quantum nature with energy and entropy marked by the two energy levels, whereas the π to π* excitations fall into a new category of electronic excitations contributing to energy and entropy exchanges in a wide and continuous d-band region. The findings that the internal states of the adsorbate are excited and that fundamental connections between the frontier orbitals and low-lying orbitals are established as the molecule comes to the surface may open up new channels to realize more efficient bond activation and renew our thinking on probing the quantum mechanical nature of bond activation at surfaces with further possible impact on manipulation of orbital activation in femtochemistry and attochemistry.     

π-Orbital electron–energy contributions to chemical reaction heats,I. Reactions involving only linear molecules containing up to three first–row atoms

Ab initio calculations of various expectation energies have been made for the reactant and product species in six reactions that involve only small linear molecules. The reactions include fission by hydrogen, addition of hydrogen, exchange of triply bonded atoms, fluorination, and oxygen atom transfer. The change in total electronic energy is not invariably the result of changes in inner shell energy and outer shell σ- and π-electron energies simply augmenting each other, but in several cases there is a complex interplay of opposing effects. This approach gives a different insight into the energetic aspects of changes in bonding from that derived from the concept of shared electron pairs in σ and π bonds together with lone pairs in valence shells. Changes in π-electron energy are shown to be important in a reaction in which neither reactant nor product molecules contain π bonds in the usual chemical sense. While in a reaction in which there is a complete change in the nature of the triple bonds, and hence the π bonding, the change in π-electron energy makes a smaller contribution than either the change in inner shell or the outer shell σ-electron energies.     

Performance of the VBSCF method for pericyclic and π bond shift reactions

The performance of the valence bond self-consistent field (VBSCF) method was investigated in this paper by predicting the activation barriers and reaction energies in pericyclic and π bond shift reactions for hydrocarbons. The benchmarking set includes 3 electrocyclic reactions, 3 sigmatropic shifts, 3 cycloadditions, 2 cycloreversions, and 7 π bond shift reactions, where the first 11 reactions are taken from Houk's standard set (J. Phys. Chem. A 2003, 107, 11445). Computational results reveal that the VB(CI) method, which performs VBSCF calculations first with full covalent structures and then includes all mono- and di-ionic structures to compute the total energy without further orbital optimization, matches the accuracy of the complete active space SCF (CASSCF) method very well. The mean absolute error values (the deviations from the CASSCF data) are 0.01 kcal/mol for the reaction energy, and 0.26 and 0.32 kcal/mol for the activation energy with the 6-31G and 6-31G(d) basis sets, respectively. © 2018 Wiley Periodicals, Inc.     

Effects of the ionization in the tautomerism of uracil: A reaction electronic flux perspective

The one‐step tautomerization processes of uracil and its radical cation and radical anion have been investigated in the light of the reaction force and reaction electronic flux (REF) formalisms. The relative energies of the different tautomers as well as the corresponding tautomerization barriers have been obtained through the use of the G4 high‐level ab initio method and by means of B3LYP/6‐311+G(3df,2p)//B3LYP/6‐311+G(d, p) calculations. Systematically, the enol radical cations are more stable in relative terms than the neutral, due to the higher ionization energy of the diketo forms with respect to the enolic ones. Conversely, the enol radical anions, with the only exception of the 2‐keto‐N1 anion, are found to be less stable than the neutral. The effects of the ionization are also sizable on the tautomerization barriers although this effect also depends on the particular tautomerization process. The reaction force analysis shows that all reactions are mainly activated through structural rearrangements that initiate the electronic activity. This electronic activity is monitored along the reaction coordinate through the REF that obeys a delicate balance between the acid and basic character of the atoms involved in the hydrogen transfer. © 2015 Wiley Periodicals, Inc.     

Geminal bond participation in the uncatalyzed Mukaiyama aldol reaction

We predicted that uncatalyzed Mukaiyama aldol reactions are under the control of the geminal bond participation. At the transition state of the model reaction, addition of CH₂O and CH₂CH(OSiH₃), the interbond energy IBE is negative between the σ-bonding orbital at the Z-position of the enolate terminal and , and positive between that at the E-position and . These results led us to predict that the electron-donating σ-bond at the Z-position enhances the reactivity. We calculated the enthalpies of activation of the reactions of a variety of the R-substituted silyl enolates and confirmed the prediction by showing that the reactivity of the Z-isomers relative to that of the E-isomers increases with the energy of the bonding orbital of the σ bond at the Z-position (the axial position at the transition state of the chair form). We demonstrate that the geminal bond participation is general not only to the pericyclic reactions but also to the aldol reaction, which is one of the most fundamental C-C bond forming reactions.     

The Quadruple Bonding in C2 Reproduces the Properties of the Molecule

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 ³ 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!     

Epoxidation of Alkenes by Peracids: From Textbook Mechanisms to a Quantum Mechanically Derived Curly-Arrow Depiction

Using the intrinsic bond orbital (IBO) analysis based on accurate quantum mechanical calculations of the reaction path for the epoxidation of propene using peroxyacetic acid, we find that the four commonly used curly arrows for representing this reaction mechanism are insufficient and that seven curly arrows are required as a result of changes to σ and π bonding interactions, which are usually neglected in all textbook curly arrow representations. The IBO method provides a convenient quantitative method for deriving curly arrows in a rational manner rather than the normal ad hoc representations used ubiquitously in teaching organic chemistry.     

Transmetalation reactions from Fischer carbene complexes to late transition metals: a DFT study

Transmetalation reactions from chromium(0) Fischer carbene complexes to late-transition-metal complexes (palladium(0), copper(I), and rhodium(I)) have been studied computationally by density functional theory. The computational data were compared with the available experimental data. In this study, the different reaction pathways involving the different metal atoms have been compared with each other in terms of their activation barriers and reaction energies. Although the reaction profiles for the transmetalation reactions to palladium and copper are quite similar, the computed energy values indicate that the process involving palladium as catalyst is more favorable than that involving copper. In contrast to these transformations, which occur via triangular heterobimetallic species, the transmetalation reaction to rhodium leads to a new heterobimetallic species in which a carbonyl ligand is also transferred from the Fischer carbene to the rhodium catalyst. Moreover, the structure and bonding situation of the so far elusive heterobimetallic complexes are briefly discussed.     

Organometallic alkane CH activation

The title topic is reviewed with emphasis on catalysis and on recent advances. Alkane σ complexes, Shilov chemistry and oxidative addition routes are covered. Attention is also given to σ bond metathesis, surface-bound organometallics and CH activation involving carbene complexes. Closely related reactions of non-alkane substrates such as the Murai reaction are also discussed.     

Analysis of carbon-carbon bonding in small hydrocarbons and dicarbon using dynamic orbital forces: Bond energies and sigma/pi partition. Comparison with sila compounds

The CC bonding is analyzed using dynamic orbital forces (DOF) in the series cyclopropane-ethane- benzene-ethylene-acetylene. The sum Σ(DOF)_t of the DOF over occupied molecular orbitals (MOs) is found linearly correlated to bond energies and thus can be used as a tool for determination of CC bond strength. A partition of bonding into σ and π components indicates a weakening of the σ bonding along the series, mainly due to the decrease of the bonding character of the highest σ MO. For C₂ molecule, Σ(DOF) _t was computed taking into account the four dominant configurations. On the basis of the preceding correlation, the C₂ bond was found about 15 kcal/mol weaker than that of acetylene, with a 25% σ participation; the bond order of C₂ can be evaluated at about 2.8 if we assume bond orders of 3 for acetylene and 2 for ethylene. Some sila homologs of the preceding carbon compounds have been studied. They exhibit characteristics generally close to the carbon compounds. A quite good correlation between Σ(DOF)_t and bond energies is also observed.     

第一过渡系金属硅烯配合物的理论研究

以HF/6-311+G^*基组研究了硅烯SiH₂同第一过渡系金属的配合物MSiH₂的分子轨道特征及键解离能.MSiH₂为共平面构型.其中基态的3TiSiH₂和4CoSiH₂带有明显的双键特征.M-Si键具有共价性质.M-Si的键解离能,从Sc到Cu呈现周期性变化,这种变化趋势同M的金属离子激发能之间存在近似的线性关系.     

Beryllium Atom Mediated Dinitrogen Activation via Coupling with Carbon Monoxide

The reactions of laser‐ablated beryllium atoms with dinitrogen and carbon monoxide mixtures form the end‐on bonded NNBeCO and side‐on bonded (η²‐N₂)BeCO isomers in solid argon, which are predicted by quantum chemical calculations to be almost isoenergetic. The end‐on bonded complex has a triplet ground state while the side‐on bonded isomer has a singlet electronic ground state. The complexes rearrange to the energetically lowest lying NBeNCO isomer upon visible light excitation, which is characterized to be an isocyanate complex of a nitrene derivative with a triplet electronic ground state. A bonding analysis using a charge‐ and energy decomposition procedure reveals that the electronic reference state of Be in the NNBeCO isomers has an 2s⁰2p² excited configuration and that the metal‐ligand bonds can be described in terms of N₂→Be←CO σ donation and concomitant N₂←Be→CO π backdonation. The results demonstrate that the activation of N₂ with the N?N bond being completely cleaved can be achieved via coupling with carbon monoxide mediated by a main group atom.     

A Valence Bond Study of the Low‐Lying States of the NF Molecule

The electronic structures of the three lowest‐lying states of NF are investigated by means of modern valence bond (VB) methods such as the VB self‐consistent field (VBSCF), breathing orbital VB (BOVB), and VB configuration interaction (VBCI) methods. The wave functions for the three states are expressed in terms of 9–12 VB structures, which can be further condensed into three or four classical Lewis structures, whose weights are quantitatively estimated. Despite the compactness of the wave functions, the BOVB and VBCI methods reproduce the spectroscopic properties and dipole moments of the three states well, in good agreement with previous computational studies and experimental values. By analogy to the isoelectronic O₂ molecule, the ground state ³Σ^? possesses both a σ bond and 3‐electron π bonds. However, here the polar σ bond contributes the most to the overall bonding. It is augmented by a fractional (19 %) contribution of three‐electron π bonding that arises from π charge transfer from fluorine to nitrogen. In the singlet ¹Δ and ¹Σ⁺ excited states the π‐bonding component is classically covalent, and it contributes 28 % and 37 % to the overall bonding picture for the two states, respectively. The resonance energies are calculated and reveal that π bonding contributes at least 24, 35 and 42 kcal mol^?1 to the total bonding energies of the ³Σ^?, ¹Δ and ¹Σ⁺ states, respectively. Some unusual properties of the NF molecule, like the equilibrium distance shortening and bonding energy increasing upon excitation, the counterintuitive values of the dipole moments and the reversal of the dipole moments as the bond is stretched, are interpreted in the light of the simple valence bond picture. The overall polarity of the molecule is very small in the ground state, and is opposite to the relative electronegativity of N vs F in the singlet excited states. The values of the dipole moments in the three states are quantitatively accounted for by the calculated weights of the VB structures.     

Model of the radical addition reaction as the superposition of three potential curves

A new semiempirical model of the reaction of radical addition to molecules with multiple bonds has been developed. In the framework of this model, the transition state (TS) of the reaction X^. + Y=Z → XYZ^. is considered as the result of the intersection of the potential curve of the stretching vibration of the forming bond X-Y with the curve that is the difference between the amplitudes of stretching vibrations of the Y-Z and Y=Z bonds and the stretching vibrations are considered harmonic. The kinetic parameters describing the activation energy as a function of the enthalpy of the reaction were calculated for 34 classes of addition reactions using the new model. The factors determining the activation energy of the addition reactions are analyzed: triplet repulsion in the TS, the π electrons in the α position to the reaction center, the electronegativity of atoms of the reaction center of the TS, the steric factor, the interaction of polar groups in the TS, and the force constants of the reacting bonds. The increments characterizing the contribution of these factors to the activation energy are calculated. The model is also used to describe the energy of 12 classes of cyclization reactions and 16 classes of radical decomposition reactions. The parameters that make it possible to estimate the activation energy of the reaction from its enthalpy are calculated for these classes of reactions.     

Theoretical study on two types of weak interactions between methylenecyclopropane and XY (X,Y = H,F, Cl,and Br)

We present theoretical evidence that the two types of interactions exist in the complexes formed between methylenecyclopropane (MECP) and XY (X, Y = H, F, Cl, and Br). Two seats of XY interacted with MECP are located: (a) is via the pseudo‐π bonding electron pair associated with a C? C bond of the cyclopropane ring and (b) is via the typical‐π bonding of electron pair of the C?C bond of MECP. These two types of weak interactions are compared based on the calculated geometries, interaction energies, frequency changes, and topological properties of electron density. The integration of electron density over the interatomic surface is found to be a good measure for the strength of weak interaction. Furthermore, the total electron density and separated σ and π electron densities are also computed and discussed in this article. The separated electron density shows σ electron density determined the strength and π electron density influenced the direction of the hydrogen/halogen bond. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

The role of three-center/four-electron bonds in superelectrophilic dirhodium carbene and nitrene catalytic intermediates

Three-center/four-electron (3c/4e) bonds are important bonding motifs that dictate the electronic structure, and thereby the reactivity, of metal-metal bonded carbene and nitrene intermediate complexes that are crucial to the dirhodium-catalyzed functionalization of hydrocarbons. In this Perspective article, general features of the 3c/4e bond are presented and discussed in comparison to two-center/two-electron (2c/2e) bonds. Specifically, 3c/4e bonding interactions lead to longer distances between the atoms involved and measurably weaker bonds. Additionally, excited states derived from the 3c/4e bonding manifold are lower in energy than those derived from a 2c/2e manifold, signifying a greater degree of reactivity in the former case. Three coterminous 3c/4e Ru-Ru-N bonds are present in metal-metal/metal-ligand multiply bonded diruthenium terminal nitrido compounds. This bonding situation results in an unusual superelectrophilic character of the nitride nitrogen atom, exemplified by its insertion into aryl C-H bonds via an electrophilic aromatic substitution mechanism. The key catalytic intermediates in dirhodium-catalyzed C-H functionalization reactions, dirhodium carbene and dirhodium nitrene complexes, may also be described as superelectrophilic by virtue of 3c/4e Rh-Rh-C(or N) σ and π bonds. These 3c/4e bonding interactions set apart dirhodium carbene and nitrene intermediates from other, less electrophilic, carbene or nitrene species.     

Catalyst‐Free Formal Thioboration to Synthesize Borylated Benzothiophenes and Dihydrothiophenes

The first ring‐forming thioboration reaction of C?C π bonds is reported. This catalyst‐free method proceeds in the presence of a commercially available external electrophilic boron source (B‐chlorocatecholborane) in good to high yields. The method is scalable and tolerates a variety of functional groups that are intolerant of other major borylation methods. The resulting borylated benzothiophenes participate in a variety of in situ derivatization reactions, showcasing that these borylated intermediates do not need to be isolated prior to downstream functionalization. This methodology has been extended to the synthesis of borylated dihydrothiophenes. Mechanistic experiments suggest that the operative mechanistic pathway is through boron‐induced activation of the alkyne followed by electrophilic cyclization, as opposed to S?B σ bond formation, providing a mechanistically distinct pathway to the thioboration of C?C π bonds.     

Intramolecular proton transfer in the isomerization of hydroxyacetone: Characterization based on reaction force analysis and the bond fragility spectrum

The mechanism of isomerization of hydroxyacetone to 2-hydroxypropanal is studied within the framework of reaction force analysis at the M06-2X/6-311++G(d,p) level of theory. Three unique pathways are considered: (a) a step-wise mechanism that proceeds through the formation of the Z-isomer of their shared enediol intermediate, (b) a step-wise mechanism that forms the E-isomer of the enediol, and (c) a concerted pathway that bypasses the enediol intermediate. Energy calculations show that the concerted pathway has the lowest activation energy barrier at 45.7 kcal mol⁻¹. The reaction force, chemical potential, and reaction electronic flux are calculated for each reaction to characterize electronic changes throughout the mechanism. The reaction force constant is calculated in order to investigate the synchronous/asynchronous nature of the concerted intramolecular proton transfers involved. Additional characterization of synchronicity is provided by calculating the bond fragility spectrum for each mechanism.