Heat of hydrogenation of 1,5-dehydroquadricyclane. A computational and experimental study of a highly pyramidalized alkene.
The radical anion of the highly pyramidalized alkene 1,5-dehydroquadricyclane (1) was generated in the gas phase from the Squires reaction of 1,5-bis(trimethylsilyl)quadricyclane with F-/F2. The electron binding energy and proton affinity of 1*- were determined by bracketing experiments to be 0.6 +/- 0.1 eV and 386 +/- 5 kcal/ mol, respectively. These values are in good agreement with values predicted by density functional theory (B3LYP/6-31+G*) and ab initio (CASPT2/6-31+G*) calculations. The experimental heat of hydrogenation of 1, obtained from a thermochemical cycle, was found to be 91 +/- 9 kcal/ mol. This value of deltaH(H2) leads to values of 67 +/- 9 kcal/ mol for the olefin strain energy (OSE) of 1, 172 +/- 9 kcal/ mol for its heat of formation, and 23 +/- 9 kcal/ mol for its pi bond dissociation enthalpy. Since the retro-Diels-Alder reaction of neutral 1 is computed to be highly exothermic, the finding that 1*- apparently does not undergo a retro-Diels-Alder reaction is of particular interest. The B3LYP/6-31+G* optimized geometry of 1 suggests that the bonding in this alkene is partially delocalized, presumably because the highly pyramidalized double bond in 1 interacts with the distal cyclopropane bonds in a manner that eventually leads to a retro-Diels-Alder reaction. The good agreement of the B3LYP and (2/2)CASPT2 values for the heat of hydrogenation and OSE of 1 with the experimentally derived values provides indirect evidence for the correctness of the B3LYP prediction that the equilibrium geometry of 1 lies part way along the reaction coordinate to the transition structure for the retro-Diels-Alder reaction.[1]References
- Heat of hydrogenation of 1,5-dehydroquadricyclane. A computational and experimental study of a highly pyramidalized alkene. Hoenigman, R.L., Kato, S., Bierbaum, V.M., Borden, W.T. J. Am. Chem. Soc. (2005) [Pubmed]
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