Valence Bond Mixing Diagrams

In fact, what we saw in the VB output could have been reasoned qualitatively at the same level and logic of the computational method. This lucidity is part of the beauty inherent in VB theory. This is demonstrated by appeal to Fig. 2.1. Let us start from the VB structures at infinite separation between the atoms. At this asymptotic limit, the ionic structures lie above the covalent one, by an energy quantity given as the difference between the ionization potential of H, /H, and the corresponding electron affinity, An  

Using experimental values for these quantities, /H = 313.6 kcal/mol, while Ah = 17.4 kcal/mol the difference is 296 kcal/mol. As we let the two H fragments approach each other, the HL-structure will be stabilized by the spin-pairing energy, while the ionic structures will go down by virtue of electrostatic interactions. We may assume that the two effects roughly cancel out, so that at the equilibrium distance the gap is still significant, 200 kcal/ mol or more. 

With such a large energy gap between the VB structures, we can justifiably use perturbation theory to construct the states, and predict the stabilization energy of the ground state by the covalent—ionic resonance energy. This 

FIGURE 2.1 A VB mixing diagram showing the formation of the states of the H—H bond from the covalent and ionic structures. 

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S. Shaik, P. C. Hiberty, Adv. Quant. Chem. 26, 100 (1995). Valence Bond Mixing and Curve Crossing Diagrams in Chemical Reactivity and Bonding. 

Shaik S, Hiberty PC (1995) Valence bond mixing and curve crossing diagrams in chemical reactivity and bonding. Adv Quantum Chem 26 99-163... 

VALENCE BOND CONFIGURATION MIXING DIAGRAMS GENERAL FEATURES... 

VALENCE BOND CONFIGURATION MIXING DIAGRAM WITH IONIC INTERMEDIATE CURVES... 

Valence Bond Configuration Mixing Diagram for Proton-Transfer Processes... 

Insights from Valence Bond Configuration Mixing Diagrams One Electron Less-One Electron More... 

To construct the total MO diagram for the B2 molecule, we make the assumption that the 2s and 2p orbitals combine separately (in other words, there is no 2s-2p mixing). The resulting diagram is shown in Fig. 14.38. Note that B2 has six valence electrons. (Remember that the Is orbitals and electrons are assumed not to participate in the bonding.) This diagram predicts a bond order of 1 ... 

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