Prismanes strain energy

The above trend is also applicable to the larger members of the [njprismane family (M2 H2 , n > 4). As Figure 1 shows5,10,15, the strain energies of persila[n]prismanes... 

FIGURE 1. The strain energies of the tetrahedrane and [n]prismane system (M2 H2 ) calculated at the HF/6-31G level for M = C and Si and the HF/DZ(d) level for M = Ge and Sn. Reprinted with permission from Reference 10. Copyright 1995 American Chemical Society... 

All of the polycyclic polystannanes reported to date are thermally robust in solution to temperatures of at least 200°C. For the bicyclo[2.2.0]hexa-slannane derivative 87, this is not too surprising, since the calculated strain-energy value for the parent compound is still ca. 12 kcal mol" less than the parent cyclotristannane system (102). However, the parent perstanna-[ ]-prismanes for n = 4 and 5 are expected to be more ring-strained than cyclotristannane (cf. 73.2 and 61.0 kcal mol" , respectively) (99,I04) therefore, the thermal stability of the two compounds, 90 and 91, suggest that these compounds are kinetically stable by virtue of signifi-... 

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) ... 

Hg. 12.16 Variation of strain energy per C-C bond with number of staeked rings, for the cyclopropane ([2,3]- to [7,3]prismane) and cyclohexane ([2,6]- to [7,6]prismane) prismane series. Calculated by homodesmotic reactions (cf Fig. 12.14) at the B3LYP/6-31 lG(2df,p) (for stack heights 2-4, [1]) and B3LYP/6-31G (for stack heights 5-7, this work) levels the two basis sets give essentially the same SE/C-C, see text. The SE/C-C levels off for the cyclopropane series at about 100 kJ mol , and for the cyclohexane series at about 85 kJ moG ... 

This synthesis is over 20 years old. No new prismane has appeared since the 1980s. It is not clear whether the will and resources exist for the pursuit of these types of molecules, but they do represent exciting opportunities to examine concepts of bonding and strain energy, and they present a unique set of challenges that motivate the development of synthetic methodology. 

Shown below are the values of AH (in kcal / mol) for hexamethy Ibenzene, hexamethyl Dewar benzene, and hexamethyl-prismane. Determine the strain energy for each compound. You will need to make estimates for some group increments. Justify your choices for estimates. 

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