Structures and Stabilities of C5H2 Isomers:  Quantum Chemical Studies

Five isomers of the carbon-rich molecule C5H2 are investigated computationally, using methods based on the coupled-cluster approximation. All of these structures are related to isomers of C3H2 via substitution of hydrogen by ethynyl or attachment of a C2 fragment to a carbene center. The two most stable forms of C5H2 are linear triplet pentadiynylidene (4) and singlet ethynylcyclopropenylidene (6). Both of these isomers have been observed in the laboratory, as has a thirdthe cumulene carbene pentatetraenylidene (5)which is predicted to lie about 15 kcal/mol above the linear triplet. Two other isomers are also studied: ethynylpropadienylidene (7) and 3-(didehydrovinylidene)cyclopropene (8). Both are found to lie less than 25 kcal/mol above the most stable form of C5H2 and to possess rather large dipole moments. Predictions for the harmonic vibrational frequencies of 12C and mono-13C isotopomers, infrared intensities, and rotational constants are also presented. These should assist efforts to identify these molecules in the laboratory and in the interstellar medium.