Temperature dependent partition functions and equilibrium constant for HCN and HNC

Ab initio vibration-rotation energy levels are summed to estimate a partition function for the total HCN system. By assigning individual levels to HCN and HNC, separate partition functions are obtained for the isomers. These are used to give a temperature dependent equilibrium constant which suggests that at temperatures typical of cool carbon stars, about 20% of the HCN system is actually HNC. Errors in the partition functions and equilibrium constant are estimated. (C) 2002 American Institute of Physics.