Following a high-level coupled cluster calculation, the rotational spectrum of the bent HC4N singlet
carbene was detected in a supersonic molecular beam by Fourier transform microwave
spectroscopy. The three rotational constants, the leading centrifugal distortion constants, and two
nitrogen hyperfine coupling constants were determined to high accuracy. The rotational constants
agree with those calculated ab initio to better than 0.5%. Like the isoelectronic C5H2 carbene of
similar structure, HC4N was found to have fairly large centrifugal distortion and a large inertial
defect. The calculated dipole moment of HC4N is 2.95 D.