LABORATORY DETECTION OF THE CARBON CHAINS HC15N AND HC17N

The linear cyanopolyynes HC15N and HC17N were detected in the laborato ry by Fourier transform microwave spectroscopy. Enough rotational line s of each species were measured in the 5-11 GHz frequency range so tha t precise values for the rotational and centrifugal distortion constan ts can be determined and the entire rotational spectra of both molecul es predicted to better than 1 km s(-1) in equivalent radial velocity o ver the range of interest to radio astronomy. Although there is a near ly constant decrement in line intensity from HC3N to HC9N of about sev en, the decrement decreases by at least a factor of 2 on reaching HC17 N, and as a result the lines of HC17N are nearly an order of magnitude stronger than predicted by extrapolation from the shorter cyanopolyyn es. With a molecular weight of 219 amu and a rotational constant of sl ightly more than 50 MHz, HC17N is the longest carbon chain identified to date by high-resolution spectroscopy.