Laboratory Detection of the Ring-Chain Carbenes HC4N and HC6N

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.