SIMBAD references

The highly polar ring-chain carbenes HC₄N and HC₆N, formed by substituting either CN or CCCN for a hydrogen atom in cyclopropenylidene (c-C₃H₂), were detected in a supersonic molecular beam with a Fourier transform microwave spectrometer. Seven a- and four b-type rotational transitions of HC₄N and 11 a-type transitions of HC₆N, each with resolved nitrogen nuclear quadrupole hyperfine structure, were measured between 6 and 21 GHz, yielding precise values for the three rotational constants, the leading centrifugal distortion constants, and the quadrupole coupling constants. Like the hydrocarbon carbenes C₅H₂, C₇H₂, and C₉H₂, both new molecules have a planar ring-chain structures and singlet electronic ground states. The strongest lines of HC₄N can be detected with a signal-to-noise ratio exceeding 10 in a total integration time of less than 1 s, but the lines of HC₆N were nearly 100 times weaker.