Laboratory Detection of the Ring-Chain Molecule C[TINF]5[/TINF]H[TINF]2[/TINF]

The highly polar ring-chain C5H2, a carbene with a singlet electronic ground state, has been detected with a Fourier transform molecular beam microwave spectrometer in the same diacetylene-neon discharge in which a carbene with the same elemental formula but a linear backbone was recently found. Thirteen a- and b-type rotational transitions between 6 and 27 GHz were measured to 1-2 kHz, and precise values of the three rotational constants and two centrifugal distortion constants were determined. With these, the entire microwave and millimeter-wave spectrum of C5H2 can be calculated to an accuracy of 0.2 km s-1 or better. The identity of the new molecule as C5H2, which is fairly certain because of the close agreement between the measured rotational constants and those calculated ab initio, is conclusively confirmed by detection of the totally deuterated and two partially deuterated isotopic species at exactly the expected isotope shifts.