Thermochemical and X-ray Crystallographic Investigations of Some (CH)10 Hydrocarbons: Basketene, Nenitzescu’s Hydrocarbon, and Snoutene

Abstract

The enthalpies of formation [ΔH0f (g)] of pentacyclo[4.4.0.02,5.03,8.04,7]dec-9-ene (9, basketene) and pentacyclo[4.4.0.02,4.03,8.05,7]dec-9-ene (14, snoutene) have been determined by measurement of their heats of combustion in a microcalorimeter as 110.2 ± 0.5 kcal·mol−1 and 78.4 ± 0.3 kcal·mol−1, respectively. These values and the strain energies (SEs) [SE(9) = 110.3 kcal·mol−1, SE(14) = 78.4 kcal·mol−1] derived from them were compared with values obtained from MM2/MM3 calculations. The enthalpy of isomerization of 9 to tricyclo[4.2.2.02,5]deca-3,7,9-triene (8, Nenitzescu’s hydrocarbon) was measured by differential scanning calorimetry (DSC) as −20.7 ± 0.3 kcal·mol−1 (384.1 K), corresponding to a strain energy SE(8) of 44.6 kcal·mol−1. The enthalpy of activation for this rearrangement was also determined from the DSC measurements to be 28.6 ± 0.1 kcal·mol−1. The obtained strain energies and the derived heats of isomerization to 9,10-dihydronaphthalene (7) did not correlate in any way either with the activation energies of the thermal isomerizations of these (CH)10 hydrocarbons or with the structural features determined experimentally for basketene (9) and computationally (DFT at the B3LYP/6-311+G* level) for snoutene (14). Compounds 8, 9, 10, and 14 exhibited solid-state phase transitions in a narrow temperature range (−55 to −70 °C), whereas the melting points or rearrangement temperatures varied to a much greater extent (in the 0−126 °C range). Compound 9 formed intermediate plastic phases, 8 had a plastic and a disordered phase, while 10 and 14 both formed phases intermediate between plastic and disordered. Crystal structure determinations based on disordered models could be carried out for 10 and 14. Basketene 9 could be crystallized from solution directly into the ordered phase at low temperatures (−178 °C). The obtained molecular geometry contrasted with results from an older gas-phase diffraction study, but agreed very well with DFT calculations. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)