Are The “Failures” of Valence Bond Theory Real?

Chapter 5 examines the mythical failures of valence bond theory (VBT) that have long been used to disrepute this theory. The first case is the ground state of dioxygen, which was allegedly described by VBT as a singlet state with all electrons being spin-paired. In fact, VBT produces the correct energy ordering for the four lowest states of dioxygen ! Thus, the chapter shows that even at the simple qualitative VBT level, the lowest state is a triplet diradical that displays two three-electron π-bonds, in agreement with the molecular orbital theory (MOT) result and with the experimental characterization. The same qualitative level of VBT is shown to account for the aromaticity/antiaromaticity in the neutral and ionic rings C n H n •/+/ . VBT also accounts for the valence ionization spectra of methane and water and demonstrates that both molecules display two ionization peaks, in agreement with the experiment.