Hexakis prismane

There are also no experimental thermochemical studies113 of prismane (57), per se, but measurements of the enthalpy of isomerization of its hexamethyl derivative to hexam-ethylbenzene have been reported114. These experimentally measured isomerization enthalpies differ by some 40 kJ moT1. There are also measurements of the isomerization enthalpy for hexakis(trifluoromethyl)prismane to the corresponding benzene115. 

Ultraviolet irradiation causes changes in carbon skeleton of many compounds. A classical example is conversion of hexafluorobenzene to hexafluoro Dewar benzene [759], and isomerization of benzene to three isomers. The same is true of hexakis(trifluoromethyl)benzene, which rearranges to the perfluorinated isomers of hexamethyl Dewar benzene (F), prismane (G), and benzvalene (H) [140]. 

As noted above, there are no measurements on the enthalpy of formation of prismane, per se, but experimental studies of the rearrangement enthalpy of hexamethylprismane to hexamethylbenzene have been reported". But the two relevant numerical values, 382 and (345 3) kJ mol" respectively, are markedly different from each other, and from the ca 570 kJmol" suggested by modern calculational theory" for the parent species. While the measured rearrangement enthalpy" of hexakis(trifluoromethyl)prismane to hexakis(trifluo-romethyl)benzene is ca 135 kJ mol" smaller than the above hexamethyl results, so be it CF3 substitution is significantly different from that of Me". But it eludes us why there is... 

Hexakis-[tri fluormethyl-J- II exakis-[trifluormethyl]-bicyclo 2.2.0] hexudien prisman... 

By contrast, hexakis(pentafluoroethyl)benzene gives only a para-bonded isomer and then a prismane. These perfluoroalkyl-substituted isomers show remarkable thermal stability, half-lives at 170°C ranging from 135 h for isomer (117) to 9 h for isomer (116). ... 

The photochemistry of six-membered Af-heterocycles has been much less extensively investigated. That of pentakis(pentafluoroethyl)pyridine resembles that of hexakis(pentafluoroethyl)benzene. U.v. irradiation at longer wavelengths produces the para-bonded isomer (146), which isomerizes to the aza-prismane (147) upon excitation at shorter wavelengths ... 

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