Laboratory Detection of the Carbon Chains HC[TINF]15[/TINF]N and HC[TINF]17[/TINF]N

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.