Weak single bonds lone pairs

In practice, the NBO program labels an electron pair as a lone pair (LP) on center B whenever cb 2 > 0.95, i.e., when more than 95% of the electron density is concentrated on B, with only a weak (<5%) delocalization tail on A. Although this numerical threshold produces an apparent discontinuity in program output for the best single NBO Lewis structure, the multi-resonance NRT description depicts smooth variations of bond order from uF(lon) = 1 (pure ionic one-center) to bu 10n) = 0 (covalent two-center). This properly reflects the fact that the ionic-covalent transition is physically a smooth, continuous variation of electron-density distribution, rather than abrupt hopping from one distinct bond type to another. 

Single-bond dissociation is the simplest when it takes place between atoms that carry no multiple bonds, lone pairs, or empty orbitals. In that case it is bitopic (two active orbitals). If the single bond is covalent and uncharged, the a,o ) state (which often represents S,) is weakly bound and often fluorescent, and the a,o ) state (which often represents T ) is purely dissociative, as shown schematically in Figure 4.5 for the dissociation of H2. Another example is the photodissociation of the Si—Si bond, mentioned in Section 7.2.3. Bitopic photodissociation normally takes place in the triplet state, which correlates with a radical pair, and not in the singlet state, which correlates with an ion pair. 

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