Experimental Determination of the Heat of Hydrogenation of Phenylcyclobutadiene.

The heat of hydrogenation of phenylcyclobutadiefle (ΔHohyd = 57.4 ± 4.9 kcal mol-1) was determined via a thermodynamic cycle by carrying out gas-phase measurements on 1 -phenylcyclobuten-3-yl cation. This leads to an antiaromatic destabilization energy of 27 ± 5 kcal mol-1, a difference of 9.6 ± 4.9 kcal mol-1 for the first and second C—H bond dissociation energies of 1 -phenylcyclobutene, and an estimate of 96 ± 5 kcal mol-1 for the heat of formation of cyclobutadiene. These results are compared to G3, G3(MP2), and B3LYP computations and represent the first experimental measurements of the energy of a monocyclic cyclobutadiene. [ABSTRACT FROM AUTHOR]