Carbide diatomics

A series of carbide diatomics, CaC, ZnC, BeC, and MgC, were another challenge for the MR BWCCSD to treat systems of multireference nature caused by near-degeneracy effects. The task was to examine theoretically [79,80] the competing and states. In Table 18.3 we present results for CaC. 

Silicon is important to plant and animal life. Diatoms in both fresh and salt water extract Silica from the water to build their cell walls. Silica is present in the ashes of plants and in the human skeleton. Silicon is an important ingredient in steel silicon carbide is one of the most important abrasives and has been used in lasers to produce coherent light of 4560 A. 

As far as phenomenological modeling is concerned, an excellent review of earlier thermodynamic approaches to chemisorption and surface reactivity was given by Benziger (156), who also developed some general thermodynamic criteria for dissociative versus nondissociative adsorption of diatomic and polyatomic molecules on transition metal surfaces (137, 156). In particular, for quantitative estimates of QA, A = C, N, or O, Benziger (156) used the heats of formation of bulk metal carbides, nitrides, and oxides. The BOC-MP approach is different, however, not only analytically but also in making direct use of experimental values of QA. 

The four types of interaction that occur in the solid state have been shown to be dictated by a common principle. Pure covalent interaction only occurs in a few crystals, such as diamond, Si, Ge, silicon carbide and gray, or, a-Sn. However, it is the dominant interaction in most small molecules, including diatomic molecules, in which the three-dimensional distribution is easily over-... 

Fig. 39. A compilation of data for diatomic bond strengths of meta] oxides (solid data points) and carbides (open points) against valence electron number in the transition scries. Circles for first row elements, squares for second row and triangles for third row.

The bond order of the carbide ion is 3 and that of C2 is only 2. With what homonuclear diatomic molecule is the carbide ion isoelectronic ... 

The carbide systems are next to oxides in importance both in condensed state as well as in gaseous state. However, a few significant differences between the carbides and the oxides systems should be noted. Carbon, unlike the diatomic gaseous oxygen, not only is solid at ambient temperature, but even at high temperatures the carbon vapors in equilibrium with graphite contain many more species besides the diatomic carbon species. This difference between the carbide systems and the oxide systems is also found in the gaseous rare-earth carbides. 

The simple molecular orbital (MO) concept for diatomic molecules can be applied to describe the covalent bonds between adjacent atoms in the crystal. By a linear combination of atomic orbitals (LCAO) on neighboring atoms, bonding or antibonding MOs can be constructed. Applying this description to the case of the transition metal carbides and nitrides, several types of covalent bonds can be formed. The transition metal atoms participate in covalent bonds mainly by their d electrons, which are split by the octahedral crystal field, generated by the nearest nonmetal neighbours, into the tig and the Cg manifold. 

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