Molecular modelling electronic structure

Most theoretical studies of outer-sphere (nonbond-breaking) electron transfer reactions at the metal-solution interface involve major simplifying assumptions regarding the molecular and electronic structure of the solvents and the metal. Although the importance of molecular structure and the dynamics of the solvent has been recognized, most of the theoretical work in this area has been based on a highly simplified continuum model. ... 

In tackling problems of molecular and electronic structure such as those posed by these sulfur-bridged tetrametallic clusters, two approaches are possible. One is the construction, often heavily reliant upon symmetry arguments, of a general, essentially qualitative model intended to provide a broad description of a series of molecules the other is a detailed quantum mechanical study, at an appropriate level of theory, of individual molecules, followed by a search for generalization or patterns. A combination of both approaches is usually the most valuable for chemical understanding, and much effort along these lines has been expanded on cubane-type clusters and their derivatives. 

Jameson, C. J. In Theoretical Models of Chemical Bonding Maksic, Z. B., Ed. Molecular Spectroscopy, Electronic Structure, and Intramolecular Interactions, Part 3 Springer-Verlag Berlin, 1991, pp. 457-519. 

Fig. 38a-f. One-electron level °u pictures and self-energy diagrams for fluctuation and relaxa- °g tion of hole levels within a molecular model-level structure oj, (for explanation of (a) to (f) see text)... 

The molecular and electronic structures are discussed on the basis of B3LYP and CASPT2 quantum chemical calculations. Paracyclophanes are attractive model compounds for studying specific intramolecular interactions [146, 147]. Gerson et al. [148] showed the radical anion of [2,2] paracyclophane 71 is an unassociated specie, with the unpaired electron being equally distributed over both 7r-systems. 

Finally, we would note that the state of the art in computational chemistry has improved dramatically during the last decade. This is especially important for transition metals, for which quantitative experimental data (for example, thermodynamic quantities) are frequently unavailable. There is now an opportunity to use high-quality calculations to systematically study the molecular and electronic structures, the bond energies, and the nature of chemical bonding of transition metals in a very broad range of chemical environments. Such studies will provide quantitative tests and analyses of the very successfiil qualitative models that we have come to rely on. 

Mo(Tp )(E)(bdt)j, [Mo(Tp )(E)(tdt)], [Mo(Tp )(E)(bdtCl2)]82 83 (E = O, NO), and [MoO(qdt)(Tp )]84 have been investigated as models for various pyranopterin Mo enzyme active sites, including sulfite oxidase. Solution redox potentials and heterogeneous electron transfer rate constants for these species have been also reported.85 The interactions between the sulfur tt-orbitals of arene dithiolates and high-valent Mo in [MoO(Tp )(bdt)] have been investigated by gas-phase photoelectron spectroscopy and DFT methods in order to understand the properties of the active site of pyranopterin Mo-W enzymes.86 Temperature-dependent measurements of potential and electron-transfer rate constants are also reported for electrochemical reduction of a series of [MoO(Tp )(X,Y)] complexes.87 The molecular and electronic structures of the SO active site [MoO(Tp )(bdt)] have been also reported.88... 

The rest of this chapter reviews what is known about the nature of the surface chemical bond. It will become clear that a combination of techniques, which yield diverse information on the atomic, molecular, and electronic structure of the adsorbate-substrate compound, are needed to obtain a complete physical-chemical picture of bonding at surfaces and interfaces. We will summarize the information available and present the current models of the surface chemical bond, along with the unique properties of these bonds that have been uncovered by surface-science studies. 

Jagadeesh MN, Makur A, Chandrasekhar J (2000) The interplay of angle strain and aromaticity molecular and electronic structures of [0n]paracyclophanes. J Mol Model 6(2) 226-233... 

Thermal organic reactions are often classified in terms of the molecular and electronic structure of their transition state or reactive intermediate (which is often taken as a model of the transition state). Thus, for instance, one has the Sn2 transition state for concerted bimolecular nucleophilic substitution reactions one has the E2 and Ei transition states in elimination reactions, etc. Given the transient nature of the transition states, the use of quantum chemical methodologies is essential for the determination of their detailed geometrical and electronic structure. Furthermore, the computation of the associated transition vectors provides information on the reactive mode... 

The central mechanistic feature in most photochemical mechanisms is the conical intersection. Thus we hope to present some thoughts about how to predict and rationalize the molecular and electronic structure of such mechanistic features using VB ideas. It turns out that one can derive analytical results for n orbitals with n electrons so we shall develop the main ideas with reference to the photochemistry of some simple model systems such as the cycloaddition of two ethylene molecules and the radiationless decay of benzene. Once one allows zwitterionic systems, lone pairs and heteroatoms, the same principles apply but analytical results are not available so easily and one must be content with a qualitative analysis at the moment. 

From the standpoint of molecular and electronic structure, carbocations present a much greater variety and more significant conceptual challenges than carbanions, radicals, or carbenes. Many aspects of carbocation chemistry have been at times quite controversial. Here, we describe the bonding models for basic carbocation structures. First, however, we must explain some nomenclature. 

C. Tanaka and J. Tanaka, Molecular and electronic structure of model compounds of doped polyacetylene, Bull. Chem. Soc. Jpn. 66 357 (1993). 

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No examples of fourteen electron cyclo-additions have come to light, but one sixteen electron case is known, (Equation 6.76). Of the two predicted pathways, [ 14j + 2s] or [ I4s + 2j], the one involving an antarafacial interaction on the 2 component appears most unlikely from the consideration of molecular models. The structure of the adduct, which was confirmed by single crystal X-ray crystallographic analysis, indicates the assigned [jrl + 2 ] pathway to be correct. The antarafacial interaction on the heptafuivalene molecule is possible because of its twisted shape. 

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