Valence bond theory diatomic molecules

This chapter consists of the application of the symmetry concepts of Chapter 2 to the construction of molecular orbitals for a range of diatomic molecules. The principles of molecular orbital theory are developed in the discussion of the bonding of the simplest molecular species, the one-electron dihydrogen molecule-ion, H2+, and the simplest molecule, the two-electron dihydrogen molecule. Valence bond theory is introduced and compared with molecular orbital theory. The photo-electron spectrum of the dihydrogen molecule is described and interpreted. 

Chapter 1 Homonuclear diatomic molecules valence bond (VB) theory 27... 

Valence bond theory does agree fairly well with molecular orbital (MO) theory for homonuclear diatomic molecules that can obey the octet rule H2 (single bond, bond order = 1), Li2 (single bond, bond order = 1), N2 (triple bond, bond order = 3), 02 (double bond, bond order = 2), F2 (single bond, bond order = 1). However, for those molecules that don t, it is more difficult to know if they exist or not and what bond orders they have. MO theory allows us to predict that He2, Be2 and Ne2 do not exist since they have bond orders = 0, and that B2 has bond order = 1 and C2 has bond order = 2. 

When carbon vaporizes at extremely high temperatures, among the species present in the vapor is the diatomic molecule C2. Write a Lewis formula for C2. Does your Lewis formula of C2 obey the octet rule (C2 does not contain a quadruple bond.) Does C2 contain a single, a double, or a triple bond Is it paramagnetic or diamagnetic Show how molecular orbital theory can be used to predict the answers to questions left unanswered by valence bond theory. 

It is well known that certain univalent metals (for example, lithium) form diatomic molecules in the vapour in which the interatomic binding is presumably covalent in character. On the valence-bond theory it is assumed that such bonds also operate in the solid state, but since the number of electrons available is inadequate to give rise to covalent bonds between each atom and all its neighbours (eight in lithium) resonance is assumed to take place throughout the solid in a way which may be symbolized, in two dimensions, thus ... 

In this chapter we will focus on MO theory because this is the most widely used method of calculating molecular properties, but valence bond theory will be discussed where appropriate. Before entering into a detailed discussion of the molecular orbitals of simple diatomic molecules, it will be useful to delve a little deeper into quantum mechanics, and take a look at ways of evaluating approximate solutions of the Schrodinger equation. 

The validity of molecular orbital theory is supported by its ability, unlike valence bond theory, to correctly predict certain properties of homonuclear diatomic molecules of elements in the first and second periods. What prediction would valence bond theory make about the paramagnetism of these molecules For which molecules does molecular orbital theory make a different prediction ... 

Metal ligand bonds are called bond depending on their symmetry with respect to rotation by 180° along the M-C, M-N, or M-0 axes. In terms of simple valence bond theory, the atoms in CO, and the other isoelectronic diatomic molecides, are sp-hybiidized. In all these molecules there are three bonds between the two atoms one valence shell there are six electrons between the two atoms, and each atom has a lone pair in an sp hybrid. Similarly, in dioxygen or NO the atoms are sp -hybridized with a double bond between them. 

In valence bond theory, rr bonds are always accompanied by a cr bond. Can the same also be said of the molecular orbital theory for diatomic molecules ... 

Provocative experimental evidence, at variance with conventional theory, is provided by the estimates of molecular diameters for diatomic molecules. Bonding theory requires the concentration of valence densities between the nuclei to increase as a function of bond order, in agreement with observed bond lengths (1.097, 1.208, 0.741 A) and force constants (22.95, 11.77, 5.75 Ncm-1) of the species N=N, 0=0 and H-H respectively. Molecular diameters can be measured by a variety of techniques based on gas viscosity, heat conductivity, diffusion and van der Waals equation of state. The results are in excellent agreement at values of 3.75, 3.61 and 2.72 A, for N2, O2 and H2, respectively. Conventional bonding theory cannot account for these results. 

Relationship between Valence Bond and Spin Valence Theories.—We consider first for simplicity a diatomic molecule AB. The basic physical idea behind all the variants of VB theory is that the wavefunction for the molecule, Pab, should in some way be written as a product of the wavefunctions Pa, Pb for particular states of the participating atoms. Thus... 

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