Some Computed Properties of Polyprismanes

4 valence electrons from C and 4 from the Hs are available for bonding, 8 electrons in all. 

The C-central H bond is moderately electron-deficient (Lowdin bond order 0.75) and the three other bonds are of conventional order (1.00), which accords with the calculated bond lengths of 1.325 A (central) and for the other three, 1.125 A, although these three are a bit long (the MP2/6-3IG C-H length for tetrahedral CH4 is 1.090 A). The MO scheme deduced from the energy levels and by visualization of the orbitals is shown in Fig. 12.13. 

We ll review the salient points in the inaugural paper [1] and augment this (anticipatory to experiment) knowledge of polyprismanes with a few more calculations. 

The total strain in [3,3]-, [3,4]-, [3,5]-, and [3,6]-prismanes was calculated by homodesmotic reactions, hypothetical reactions with the same number of each kind of bond in reactants and products homodesmotic reactions are a refinement of isodesmic reactions [33], and have been extensively used to estimate strain energies [34,35]. The exothermicity of such a reaction is a measure of the SE of the molecule whose rings are being brokea Consider [3,4]prismane, for which a homodesmotic reaction is shown in Fig. 12.14. The energetics of this reaction at the B3LYP/6-3IG level are (my calculations, a.u. or hartrees, omitting ZPE)  

The reaction is calculated to be exothermic (this is a 0 K enthalpy change) by 1585kJ mol, which we take as the SE. At the considerably higher B3LYP/ 6-311(2df, p) level [1] the value was 1561 kJ mol (373kcal mol ), essentially 

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