Determination of the HNC to HCN abundance ratio in giant molecular clouds

We have used the rare isotopes of HCN and HNC to measure the relative abundance of these two chemical isomers in four giant molecular clouds. The lines of H/sup 13/CN, HC/sup 15/N, HN/sup 13/C, and H/sup 15/NC are all sufficiently weak to be little affected by saturation. A comparison of intensities measured with different beamsizes indicates that the sources are not resolved even with the 42'' beamsize used in the present work. The abundance ratio (HNC)/(HCN) varies by more than an order of magnitude, ranging from 0.015 to 0.40. These low values (which are similar to those found in warm sources from the common isotopes by Wootten et al.) are not consistent with HNC and HCN production solely from various sources of the precursor ion H/sub 2/CN/sup +/, and they indicate that another source of HCN is required in warm, giant molecular clouds. We suggest that neutral atom-molecule reactions may make an important contribution to the HCN production rate in such regions.