Ab initio molecular orbital studies of low-energy, metastable isomers of the ubiquitous cyclopropenylidene.

The discovery of cyclopropenylidene in space suggests that other C3H2 isomers may be present and a tentative detection of one such isomer, propargylene (HCCCH), has been reported. Ab initio molecular orbital theory has been used to characterize five low-lying, metastable isomers of cyclopropenylidene. Four are found to be stable minima and their vibrational frequencies are reported, although the stable form of triplet propargylene is not firmly established. Extended calculations including the electron correlation energy, show that the lowest in energy is singlet propadienylidene, followed by propargylene; the singlet and triplet of the latter are too close in energy to allow an assignment of the ground state; triplet propadienylidene is at a significantly higher energy. Rotational frequencies computed to an expected accuracy of +/- 1%-2% do not confirm the tentative detection of propargylene in space, although the discrepancy between theory and the observation is not so great as to unequivocally rule out this possibility.