MILLIMETER AND SUBMILLIMETER REST FREQUENCIES FOR NaCN ((X)over-tilde1A′): A REMARKABLY ABUNDANT CIRCUMSTELLAR MOLECULE

The pure rotational spectrum of NaCN has been recorded in the millimeter/submillimeter region using direct absorption techniques, the first experimental study of this molecule at frequencies above 40 GHz. The species was produced in the gas phase in an AC discharge by the reaction of sodium vapor with cyanogen. Twelve rotational transitions of NaCN have been measured in the range of 180-530 GHz, extending the previous microwave data to millimeter/submillimeter frequencies. Multiple K-a asymmetry components were recorded in each rotational transition up to K-a = 5 or 6, a total of 131 individual lines. These data have been analyzed with a standard asymmetric top Hamiltonian, combined with prior microwave measurements. Higher-order centrifugal distortion terms were clearly needed to model the millimeter-wave transitions of this floppy molecule. From this revised set of spectroscopic constants, accurate frequency predictions can now be made up to 600 GHz for NaCN for the lower-value K-a components (K-a <= 4). Based on the new laboratory data and past spectral surveys, a revised abundance for NaCN in IRC+10216 has been estimated. For a 5 '' source, the fractional abundance for this molecule was found to be f (NaCN/H-2) similar to 1 x 10(-8), comparable to that of c-C3H2. These new frequency measurements should aid in line identification in surveys in the 0.8 mm band and at shorter wavelengths.