| Abstract
| - Both C−H bond dissociation energies for cyclobutene were measured in the gas phase (BDE =91.2 ± 2.3 (allyl) and 112.5 ± 2.5 (vinyl) kcal mol-1) via a thermodynamic cycle by carrying out protonaffinity and electron-binding energy measurements on 1- and 3-cyclobutenyl anions. The results werecompared to those for an acyclic model compound, cis-2-butene, and provide the needed information toexperimentally establish the heat of formation of cyclobutadiene. Chemically accurate G3 and W1calculations also were carried out on cycloalkanes, cycloalkenes, and selected reference compounds. Itappears that commonly cited bond energies for cyclopropane, cyclobutane, and cyclohexane are 3 to 4kcal mol-1 too small and their π bond strengths, as given by BDE1 − BDE2, are in error by up to 8 kcalmol-1.
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