Laboratory Detection of the Ring‐Chain Carbenes HC4N and HC6N

The highly polar ring-chain carbenes HC 4 N and HC 6 N, formed by substituting either CN or CCCN for a hydrogen atom in cyclopropenylidene ( c -C 3 H 2 ), were detected in a supersonic molecular beam with a Fourier transform microwave spectrometer. Seven a - and four b -type rotational transitions of HC 4 N and 11 a -type transitions of HC 6 N, 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 C 5 H 2 , C 7 H 2 , and C 9 H 2 , both new molecules have a planar ring-chain structures and singlet electronic ground states. The strongest lines of HC 4 N 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 HC 6 N were nearly 100 times weaker.