The kinetics and thermochemistry of valence isomerization by differential scanning calorimetry: The case of hexamethylprismane and hexamethyldewarbenzene

Abstract

On heating hexamethylbicyclo-[2,2,0]-hexa-2,5,-diene (hexamethyldewarbenzene, wish to present the first application of this unique and novel mechanistic tool to the thermal isomerization of hexamethyldewarbenzene (11) and hexamethylprismane (111). In Figure I the thermograms for the isomerizations I11 ---f I + I1 (solid curve) and I1 -+ I (broken curve) are illustrated. The rate constant for the unimolecular reaction is conveniently given by the quotient of the ordinate (dH/dt) and the fractional area under the peak ( A -a) at a particular temperature in the thermogram, that is k( T ) = (dH/dt)/(A -a ) [S]. From

Theoretical studies of [n]paracyclophanes and their valence isomers

“…the energy minima of prismane P, Dewar benzene D, the hilltop H and the intermediate I (and I') and the energies of the transition states C1 and C2 (and CI' and C2'), are given in ' Table 4. They are in very good agreement with experimental data, known for substituted compounds, such as for hexamethylprismane [1] and perfluorohexamethylprismane [2]. The calculated heats of reaction AH~,] are about -32kcal/mol for the unbridged compounds (from the RHF-and the UHF-approaches) and -34 kcal/ mol for [6]prismane.…”

“…The same form of the potential energy surface was postulated for the reaction prismane~Dewar benzene in other work [1]. The same form of the potential energy surface was postulated for the reaction prismane~Dewar benzene in other work [1].…”

“…This reaction has been experimentally studied in detail for many substituted benzenes [1][2][3][4][5]. For example, benzene gives Dewar benzene.…”

Ab initio study of the stability of [n]paracyclophanes and their Dewar benzene-type isomers

“…As far as we know, this quantity has not been determined for the unsubstituted compound. Detailed measurements are available for hexamethylbenzene [36] and find AH°=-56.2+0.6kcal/mol (2.44 eV) or 59.4 kcal/mol (2.58 eV) [39]. Our SCF calculations (Table 6) find an energy difference between the two hexamethylbenzene isomers of 3.05 eV, i.e., considerably lower than for benzene itself.…”

“…One generally reads that the thermal rearrangement of Dewar benzenes to benzenes is about 60 kcal/ mol [36][37][38] or 2.60eV. As far as we know, this quantity has not been determined for the unsubstituted compound.…”