What is a Good Approximation for the Transition State of an SN2 Reaction?

It has been previously proposed, based on a qualitative application of the Curve Crossing model, that the transition state for a reaction is found in the vicinity of the crossing point of reactant and product electronic configurations. Computational data on Sfj2 reactions are found to support this contention and the application of this result for the Leffler-Hammond postulate are discussed

The Periodic Table - A universal icon: Its birth 150 years ago, and its popularization through literature, art and music

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

Flavin-N5OOH: A most powerful nucleophile and base in nature

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The physical origin of large covalent-ionic resonance energies in some two-electron bonds

This study uses valence bond (VB) theory to analyze in detail the previously established finding that alongside the two classical bond families of covalent and ionic bonds, which describe the electron-pair bond, there exists a distinct class of charge-shift bonds (CS-bonds) in which the fluctuation of the electron pair density plays a dominant role. Such bonds are characterized by weak binding, or even a repulsive, covalent component, and by a large covalent-ionic resonance energy RECS that is responsible for the major part, or even for the totality, of the bonding energy. In the present work, the nature of CS-bonding and its fundamental mechanisms are analyzed in detail by means of a VB study of some typical homonuclear bonds (H-H, H3C-CH3, H2N-NH2, HO-OH, F-F, and Cl-Cl), ranging from classical-covalent to fully charge-shift bonds. It is shown that CS-bonding is characterized by a covalent dissociation curve with a shallow minimum situated at long interatomic distances, or even a fully repulsive covalent curve. As the atoms that are involved in the bond are taken from left to right or from bottom to top of the periodic table, the weakening effect of the adjacent bonds or lone pairs increases, while at the same time the reduced resonance integral, that couples the covalent and ionic forms, increases. As a consequence, the weakening of the covalent interaction is gradually compensated by a strengthening of CS-bonding. The large RECS quantity of CS-bonds is shown to be an outcome of the mechanism necessary to establish equilibrium and optimum bonding during bond formation. It is shown that the shrinkage of the orbitals in the covalent structure lowers the potential energy, V, but excessively raises the kinetic energy, T, thereby tipping the virial ratio off-balance. Subsequent addition of the ionic structures lowers T while having a lesser effect on V, thus restoring the requisite virial ratio (T/-V=1/ 2). Generalizing to typically classical covalent bonds, like H-H or C-C bonds, the mechanism by which the virial ratio is obeyed during bond formation is primarily orbital shrinkage, and therefore the charge-shift resonance energy has only a small corrective effect. On the other hand, for bonds bearing adjacent lone pairs and/or involving electronegative atoms, like F-F or Cl-Cl, the formation of the bond corresponds to a large increase of kinetic energy, which must be compensated for by a large participation or covalent - ionic mixing

Stereochemistry and regiochemistry in model electron transfer and substitution reactions of a radical anion with an alkyl halide

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Mechanistic crossover induced by steric hindrance: a theoretical study of electron transfer and substitution mechanisms of cyanoformaldehyde anion radical and alkyl halides

This paper describes a mechanistic crossover driven by steric hindrance, from C-alkylation (SUB(C)) to dissociative electron transfer (ET), in the reactions between cyanoformaldehyde anion radical and alkyl chlorides of variable steric size (alkyl = Me, Et, i-Pr, t-Bu). The computations provide structural details on the transition state (TS) structures which undergo this mechanistic transformation, and thereby enable links to experimental investigations on the relationship between classical substitution mechanisms and their ET counterparts to be drawn. The TS's of the interchanging mechanisms possess the C- - -C- - -Cl structure, where the first C is the carbon atom of the formyl group. It is found that the TS's for the less hindered substrates (Me, Et), with R(CC) = 2.35 and 2.45 Å, collapse to C-alkylation product, hence a SUB(C) mechanism. As steric hindrance increases (i-Pr, t-Bu) and the C- - -C distance increases to 2.57 Å and then to 2.96 Å, the TS falls apart to dissociated ET products, hence an ET mechanism. This is therefore an isostructural mechanistic transformation within a narrow range of change in the C- - -C distance. A third mechanism of O-alkylation (SUB(O)) is also observed, but while its TS undergoes O- - -C loosening by the steric hindrance, no mechanistic transformation occurs. This dichotomy of the steric hindrance is analyzed with use of the valence bond configuration mixing (VBCM) method and shown to originate in the parity (odd vs even) of the number of electrons which participate in the bond reorganization. The VBCM method projects that ET and SUB(C) mechanisms are nascent from the VB mixing of the same set of configurations, and as such the two mechanisms are “entangled” and their corresponding TS's involve hybrid characters. Near the changeover zone (e.g., where the TS for the i-PrCl substrate is located in Figure 6), the degree of entanglement is strong, and may lead to surface bifurcation. The origins of the experimentally observed residual stereoselectivity of ET reactions are discussed in this respect and as a result of radical collapse. The ET-TS which emerges from the computations possesses significant and variable bonding which conforms to simple orbital selection rules (refs 1, 10, and 11). The importance of probing the bonding is discussed along with potential strategies thereof