Structures and Stabilities of C5H2Isomers: Quantum Chemical Studies

Five isomers of the carbon-rich molecule C 5 H 2 are investigated computationally, using methods based on the coupled-cluster approximation. All of these structures are related to isomers of C 3 H 2 via substitution of hydrogen by ethynyl or attachment of a C 2 fragment to a carbene center. The two most stable forms of C 5 H 2 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 C 5 H 2 and to possess rather large dipole moments. Predictions for the harmonic vibrational frequencies of 12 C and mono- 13 C 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.