Experimental determination of the heat of hydrogenation of phenylcyclobutadiene

The heat of hydrogenation of phenylcyclobutadiene (ΔH° _hyd = 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.