Electronic Structure of Transition States

Chemists use curved arrows to show the electronic changes that occur during a chemical reaction. Fot example, the arrows describing the Sn2 reaction below show formation of a CC bond and loss of a Cl bond. 

Examine the highest-occupied molecular orbital (HOMO of cyanide anion. Is the larger lobe on carbon or nitrogen Would you expect cyanide to act as a carbon or nitrogei nucleophile Does this lead to the lower energy transitioi state (compare the energy of cyanide-l-methyl iodide ( attack and cyanide+methyl iodide N attack)  

Examine atomic charges and the electrostatic potentit map for the lower-energy transition state. Which atom appear to be most electron rich in each Is the negativ charge concentrated on a single atom in the transition stat or delocalized Add this charge information (either or 5- ) to the molecular structure for the transition stat which you drew previously. 

Does your transition state drawing look more like a sing Lewis structure or a resonance hybrid If the latter, whi resonance contributors must you combine to generate a of the features of this hybrid  

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The ability to provide quality data relating to the intimate details of reaction mechanisms is of critical importance to their understanding. While experimental studies have the potential of providing significant mechanistic insight into a multitude of chemical reactions, the fine detail able to be provided by quality quantum computational data is unrivalled in no other way can the geometry and electronic structure of transition states and unstable hypervalent intermediates along reaction pathways be understood. 

Electron correlation plays an important role in determining the electronic structures of many solids. Hubbard (1963) treated the correlation problem in terms of the parameter, U. Figure 6.2 shows how U varies with the band-width W, resulting in the overlap of the upper and lower Hubbard states (or in the disappearance of the band gap). In NiO, there is a splitting between the upper and lower Hubbard bands since IV relative values of U and W determine the electronic structure of transition-metal compounds. Unfortunately, it is difficult to obtain reliable values of U. The Hubbard model takes into account only the d orbitals of the transition metal (single band model). One has to include the mixing of the oxygen p and metal d orbitals in a more realistic treatment. It would also be necessary to take into account the presence of mixed-valence of a metal (e.g. Cu ", Cu ). 

Analysis of the valence-band spectrum of NiO helped to understand the electronic structure of transition-metal compounds. It is to be noted that th.e crystal-field theory cannot explain the features over the entire valence-band region of NiO. It therefore becomes necessary to explicitly take into account the ligand(02p)-metal (Ni3d) hybridization and the intra-atomic Coulomb interaction, 11, in order to satisfactorily explain the spectral features. This has been done by approximating bulk NiO by a cluster (NiOg) ". The ground-state wave function Tg of this cluster is given by,... 

During the last two decades it has been found that there is a special group of chemical reactions, essentially redox reactions, for which the catalytic influence of solids can be interpreted in terms of the catalyst s electronic structure and the controlled variations of that structure. The study of single-phase catalysts and the relationship between function and electronic structure of solid state catalysts show that redox reactions may be divided into two classes. Donor reactions are reactions in which the rate-determining step involves an electron transition from the reactant molecule to the catalyst acceptor reactions are those where the reactant must accept electrons from the catalyst in order to form the activated state. Broadly speaking, donor reactions mobilize reducing agents like... 

In this chapter, we have developed the information content of different excited state spectroscopic methods in terms of ligand field theory and the covalency of L—M bonds. Combined with the ground-state methods presented in the following chapters, spectroscopy and magnetism experimentally define the electronic structure of transition metal sites. Calculations supported by these data can provide fundamental insight into the physical properties of inorganic materials and their reactivities in catalysis and electron transfer. The contribution of electronic structure to function has been developed in Ref. 61. 

CONTENTS Introduction, Thom H. Dunning, Jr. Electronic Structure Theory and Atomistic Computer Simulations of Materials, Richard P. Messmer, General Electric Corporate Research and Development and the University of Pennsylvania. Calculation of the Electronic Structure of Transition Metals in Ionic Crystals, Nicholas W. Winter, Livermore National Laboratory, David K. Temple, University of California, Victor Luana, Universidad de Oviedo and Russell M. Pitzer, The Ohio State University. Ab Initio Studies of Molecular Models of Zeolitic Catalysts, Joachim Sauer, Central Institute of Physical Chemistry, Germany. Ab Inito Methods in Geochemistry and Mineralogy, Anthony C. Hess, Battelle, Pacific Northwest Laboratories and Paul F. McMillan, Arizona State University. 

The electronic structure of transition metals in their ground state and oxidation states often causes considerable confusion. As electrons are negatively charged particles, removal of electrons makes the metal more positive (hence -h sign), and conversely when electrons are added to a metal it becomes more negative (hence-sign). When considering metals and electrons there are three important terms to remember ... 

The usefulness of moments in electronic state calculations is evident in the pioneering paper of Cyrot-Lackmann. In early attempts at describing the electronic structure of transition metals, a conjecture about its features was made. For instance, a common procedure consisted of approximating the unknown density of states by the product of a Gaussian and a polynomial, with coefficients fitted to the first moments. 

Ballhausen s latest book [30], Molecular Electronic Structures of Transition Metal Complexes appeared in 1979, 25 years after his first article. It can be seen as his answer to the question What is a molecule - in particular a transition metal complex He starts with his conclusion from a series of articles on the chemical bond [31], Chemistry is one huge manifestation of quantum mechanics . He then introduces the Bom-Oppenheimer approximation as the basis for applying electronic and nuclear coordinates, and lets the picture of a molecule unfold itself with the concepts of electronic states, potential surfaces, transitions, vibronic couplings, etc. The presentation is traditional, but contains many refinings in the discussion of a molecule s ground state as well as its excited states. The world of transition metal complexes is favoured through the choice of examples. 

A more powerful experimental technique to probe the electronic structure of transition-metal clusters is size-selected anion photoelectron spectroscopy (PES) [70. 71. 72. 73. 74. 75 and 76]. In PES experiments, a size-selected anion cluster is photodetached by a fixed wavelength photon and the kinetic energies of the photoemitted electrons are measured. PES experiments provide direct measure of the electron affinity and electronic energy levels of neutral clusters. This technique has been used to study many types of clusters over a large cluster size range and can probe how the electronic structures of transition-metal clusters evolve from molecular to bulk [77. 78. 79, 80 and M] Research has focused on the 3d transition-metal clusters, for which there have also been many theoretical studies [82, M, M, 86, M and 89]. It is found that the electronic structure of the small transition-metal clusters is molecular in nature, with discrete electronic states. However, the electronic structure of the transition-metal clusters approaches that of the bulk rapidly. Figure Cl. 14 shows that the electronic structure of vanadium clusters with 65 atoms is already very similar to that of bulk vanadium [90]. Other 3d transition-metal clusters also show bulk-like electronic structures in similar size range [78]. 

Stereoelectronic control by deactivation of unproductive stabilization The alternative way to analyze chemical reactivity is based on the comparison of ground state structures with the stractural distortions needed to satisfy the electronic demands of transition states and/or evolution of delocahzing effects on route from the starting material to the product (Figure 10.6). This analysis can readily reveal the stereoelectronic approaches to efficient control of reactivity in a direct and specific way. 

The nature of this group of phases has been associated with the characteristic feature of the electronic structure of transition metal atoms [6,7] d electrons which do not participate in the formation of Me-Si bonds. It was concluded that the d states are discrete only in CrSi2 and do not have any appreciable effect on the transport properties of the compound. 

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