Delocalisation energy

The r-delocalisation in the parent phospholide anion I (Fig. 3, R =R =H) can be expressed in the valence bond picture by resonance between the canonical structures lA-IC (and their mirror images). Phosphonio-sub-stituents (R =R =PH3 ) increase the weight of the 1,2-dipolaric canonical structure IB and induce thus, in essence, a partial r-bond localisation and a shift of r-electron density from the phosphorus to the adjacent carbon atoms [16]. Consequences of this effect are the decrease in delocalisation energy for reaction (1) depicted in Fig. 4, and lower C2-C3/C4-C5 and higher C3-C4 bond orders which are reproduced in concomitant variations of computed bond distances [16]. 

Fig. 4 Delocalisation energies AEi,2 (in kcal mol ) in (benzo)phospholides as computed from the isodesmic reactions (1) and (2). (R=H, from [16])...

Delocalisation energies of the deprotonated anions and their parent acids. 

The delocalisation energy (AEdeioc) of the parent acids and their anions is defined as the energy difference between the localised wavefunction, < )ioc, and the delocalised ground state, ( >dei in their optimal geometry, i.e. the adiabatic delocalisation energy [104]... 

As expected, because the parent acids of these two anions can be reasonably well described by a single Lewis structure, they have much smaller delocalisation energies. In ethanol and its deprotonated anion, for which no resonance between low-lying Lewis structures may be expected, the delocalisation energy is relatively small. 

It would be of interest to relate this to what we conventionally describe as the resonance energy, or the delocalisation energy, of the molecule. 

One final word of caution may be sounded on the delocalisation energies discussed in 3.2a and tabulated in Table 3-1. They have been calculated without reference whatever to any changes in the ex-framework which may be required to go from a hypothetical Kekule-structure to an actual, delocalised , conjugated system—changes which, energetically, may sometimes be quite large. For example, to convert a benzene Kekule-... 

Delocalisation energy, 35-38, 43-45 Delocalised bonding, 4-5 Differential relationships connected with reactivity, 126-132 Dipole moments, 82, 111, 138... 

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