Are the Failures of VB Theory Real

As mentioned in the introduction, VB theory has been accused of a few failures that are occasionally used to dismiss the theory, and have caused it to have an unwarranted reputation. The next few subsections use the simple VB guidelines drawn above to demonstrate that VB theory is free of these failures . 

Goddard et al. and subsequently the present two authors also provided a simple VB explanation for the choice of the ground state. Let us reiterate this explanation based on om quafitative VB theory, outlined above. 

Similarly, 26 and 26 yield a resonant Ag combination and an antireso-nant one. Thus, it is seen that simple qualitative VB considerations not only predict the ground state of O2 to be a triplet, but also yield a correct energy ordering for the remaining low-lying excited states. 

As discussed in the introduction, the development of PES showed that the spectra could be simply interpreted if one assumed that electrons occupy delocalized molecular orbitals. By contrast, VB theory, which uses localized bond orbitals (LBOs), seems completely useless for interpretation of PES. Moreover, since VB theory describes equivalent electron pairs that occupy LBOs, the PES results seem to be in disagreement with this theory. An iconic example of this failure of VB theory is the PES of methane that displays two different ionization peaks. These peaks correspond to the aj and tz MOs, but not to the four equivalent C—H LBOs in Pauling s hybridization theory. 

There remains to diagonalize the Hamiltonian matrix in the space of the four configurations, to get the four states of CH4. This can be done by diag- 

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This chapter was dedicated to demonstrations that all the so-called failures of VB theory are in fact not real. It was shown that in each such failure , one could use a simple VB theory, based on the principles outlined in Chapter 3, and arrive at the correct predictions—results. In so doing, this chapter also provided the reader with an opportunity to apply qualitative VB theory to some classical problems in bonding. Having done so, the reader is now more prepared for the material in Chapter 6, where VB theory is applied to chemical reactivity. 

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