Computational studies of C6H2 isomers

High-level ab initio methods are used to characterize nine isomers of C6H2 These include the thermodynamically favored triacetylene form and hexapenta enylidene. which is known to exist in the interstellar medium. Also include d in the survey are three cyclic forms of tetradehydrobenzene and four addi tional structures that have not been investigated previously. Structures, d ipole moments, parameters of rotational Hamiltonians, vibrational frequenci es, intensities, and C-13 isotopic shifts are documented for all forms to a ssist efforts aimed at detecting these species in the laboratory. Perhaps t he best candidate for detection in space is ethynylbutatriehylidene, which has a substantial dipole moment and a thermodynamic stability comparable to that of hexapentaenylidene. Notably, the symmetric disubstituted vinyliden e isomer has a small and perhaps vanishing barrier to rearrangement.