Diatomic molecules bonding analysis

The limitation of the above analysis to the case of homonuclear diatomic molecules was made by imposing the relation Haa = Hbb> as in this case the two nuclei are identical. More generally, Haa and for heteronuclear diatomic molecules Eq. (134) cannot be simplified (see problem 25). However, the polarity of the bond can be estimated in this case. The reader is referred to specialized texts on molecular orbital theory for a development of this application. 

Hiickel s application of this approach to the aromatic compounds gave new confidence to those physicists and chemists following up on the Hund-Mulliken analysis. It was regarded by many people as the simplest of the quantum mechanical valence-bond methods based on the Schrodinger equation. 66 Hiickel s was part of a series of applications of the method of linear combination of atom wave functions (atomic orbitals), a method that Felix Bloch had extended from H2+ to metals in 1928 and that Fowler s student, Lennard-Jones, had further developed for diatomic molecules in 1929. Now Hiickel extended the method to polyatomic molecules.67... 

R, the sum of atomic radii plays important role in this formalism. As demonstrated in Sect 1.5, equivalence between the absolute atomic hardness and atomic radius is best for van der Waals or ionic radii. Thus, the R distance may be much larger than the normal bond in the AB molecule. For the purpose of this analysis, however, R/2 = Rab as a working approximation might be used, which is strictly valid for van der Waals radii and homonuclear diatomic molecules. Another choice is to follow the ehemieal approximation and use some available hardness parameters to estimate R as R = (rA + Tb) = (r A )-... 

In this chapter some systems containing a homo-bimetallic multiple bond, which have been recently synthesized and studied with quantum chemical methods, are analyzed. The chapter is organized as follows. In Section 9.2 a brief description of the computational tools employed in this analysis is presented. In Section 9.3 the classical case of Re2Clg is reviewed. In Section 9.4 group VI homo-nuclear diatomic molecules, namely Cr2, M02, and Wj are discussed. In Section 9.5 homo-bimetallic compounds containing the Cr2, M02 and W2 moieties are described. In Section 9.6 some recently synthesized Fe2 compounds are reported. Finally in Section 9.7 some perspective and some conclusions are offered. 

The second part of this chapter is devoted to a discussion of surface elementary reactions. We limit ourselves mainly to an analysis of the mles that define the nature of the transition states of diatomic molecules such as CO or NO with a molecular k bond as well as of molecules such as CH and NHj with a chemical bonds. Our interest is to understand differences in elementary rate constants as a function of surface topology and again as a function of coordinative unsaturation of the surface atoms. 

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