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

Four new cumulene carbenes, H2C7, H2C8, H2C9, and D2C10, were detected in t he laboratory by Fourier transform microwave spectroscopy. Like the shorter cumulenes in this series, all four were found to have singlet electronic g round states and linear carbon chain backbones. Rotational and centrifugal distortion constants were determined to high accuracy for each molecule, an d 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 H2C 7, H2C8, and H2C9 column densities have been estimated for the circumstella r shell of the evolved carbon star IRC +/- 10216.