Chemical property bonding calculations

For many years, our group has been using quantum-chemical methods to calculate properties of TM compounds. During the last decade, we focused on theoretical analysis of chemical bonds in TM compounds [3-7] and on elucidation of mechanisms of TM-mediated reactions [8],... 

This paper reports the results of full-potential-linear-augmented-slater-type-orbital (FLASTO) band calculations on the early transition metal monocarbides. Using the results of these calculations we will develop a bonding model which accurately accounts for the chemical properties of these compounds. This model does not require one to determine either the direction or magnitude of charge transfer between metal and non-metal atoms. 

IR and -NMR investigations (as well as non-empirical and empirical calculations of interaction energies136) of it facial interactions between hydroxy groups and C=C double bonds of olefins137,138 emphasize the importance of these interactions in explaining physical and chemical properties of molecules bearing double bonds and acid protons, mainly of O—H and N—H groups. 

Measurement of electron density by X-ray diffraction cannot distinguish each electron in different orbitals however, it provides overall information on the asphericity of the d electron distribution in a molecule as well as on the redistribution of electrons upon chemical bond formation. At the present stage, experimental charge distributions can be compared only qualitatively with theoretical calculations. The method, however, will be of considerable value in clarifying poorly understood bonding situations. If the electron-density distribution and the geometrical arrangement of the atomic nuclei in a complex are known, it is possible, at least in principle, to predict all its physical and chemical properties on the basis of quantum mechanics. 

Although X-ray crystallography has mainly been used for structure determination in the past, the diffraction data, especially if measured carefully and to high orders, contain information about the total electron density distribution in the crystal. This may be analyzed to provide essential information about the chemical properties of molecules, in particular the characterization of covalent and hydrogen bonds, both from the point of view of the valence electron density, the Laplacian of the density and derived energy density distribution. In addition, calculation of the molecular electrostatic potential indicates direction of chemical attack as well as how molecules can interact with their environment. 

Quantitative Calculations. These attempts to connect physical and chemical properties with models are helpful to chemists and, in the hands of organic chemists, molecular models have led to marvelous advances, but physicists are inclined to object to them because they are inadequate in more quantitative fields. We were rather pleased with the assignment of definite bands in the spectrum of ethyl bromide to corresponding bonds in the molecule and... 

Quantum-chemical ab initio calculations have become an alternative to experiments for determining accurately structures, vibrational frequencies and electronic properties as well as intermolecular forces and molecular reactivity.28-31 Two specific approximations were developed to solve the problems of surface chemistry periodic approximation, where quantum-chemical method employs a periodic structure of the calculated system and cluster approximation, where a model of solid phase of finite size is created as a cutoff from the system of solid phase (it produces unsaturated dangling bonds at the border of the cluster). Cluster approximation has been widely used for studying interactions of molecules with all types of solids and their surfaces.32 This approach is powerful in calculating the systems with deviations from the ideal periodic structure like doping and defects. 

There are currently three different approaches to understanding chemical bonding. Quantum mechanical calculations see Ab Initio Calculations, Molecular Orbital Theory), even though they give the most complete picture, offer few insights into the nature of chemical bonds themselves because the concept of a bond does not arise naturally from a formahsm based on the interactions between nuclei and electrons rather than the interaction between atoms. Even though quantum mechanics gives accurate values for measurable properties, its calculations are compnter intensive and it becomes more difficult to use the more complex the chemical system. 

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