Laboratory detection of four new cumulene carbenes:: H2C7, H2C8, H2C9, and D2C10

Four new cumulene carbenes, H2C7, H2C8, H2C9, and D2C10, were detected in the laboratory by Fourier transform microwave spectroscopy. Like the shorter cumulenes in this series, all four were found to have singlet electronic ground states and linear carbon chain backbones. Rotational and centrifugal distortion constants were determined to high accuracy for each molecule, and the entire radio spectrum of each can be calculated to better than 0.3 km s(-1) in equivalent radial velocity up to 100 GHz. Upper limits of the H2C7, H2C8, and H2C9 column densities have been estimated for the circumstellar shell of the evolved carbon star IRC +/- 10216.