Vacuum-UV (10.49 eV) photoionization mass and photoelectron spectroscopy of the three isomers of C3H2, cyclopropenylidene (1), propadienylidene (2), and propargylene (3), are used to test a valence bond promotion energy model for carbene thermochemistry and radical bond dissociation energies. The predicted deviation of DELTA-H(f)[carbene] from bond additivity estimates by the carbene singlet-triplet gap is shown to be valid to within the precision of the measurement and to be in accord with the best ab initio predictions of carbene thermochemistry. The C3H2 system is presented as the best possible test system of molecules because of unique electronic and geometric factors in 1, 2, and 3.
