Laboratory detection of the carbon chains HC15N and HC17N

The linear cyanopolyynes HC(15)N and HC(17)N were detected in the laboratory by Fourier transform microwave spectroscopy. Enough rotational lines of each species were measured in the 5-11 GHz frequency range so that precise values for the rotational and centrifugal distortion constants can be determined and the entire rotational spectra of both molecules predicted to better than 1 km s(-1) in equivalent radial velocity over the range of interest to radio astronomy. Although there is a nearly constant decrement in line intensity from HC(3)N to HC(9)N of about seven, the decrement decreases by at least a factor of 2 on reaching HC(17)N, and as a result the lines of HC(17)N are nearly an order of magnitude stronger than predicted by extrapolation from the shorter cyanopolyynes. With a molecular weight of 219 amu and a rotational constant of slightly more than 50 MHz, HC(17)N is the longest carbon chain identified to date by high-resolution spectroscopy.