Isomerism and thermodynamics of the HF(g) and HCl(g) association

The thermodynamics of the association of HF(g) and HCl(g) leading to formation of two isomeric complexes HF· HC1 and HC· HF have been studied. This computational study makes use of the molecular parameters from quantum-chemical calculations. The evaluation of the thermodynamics has been carried out for full harmonic vibrational partition function and, simultaneously, for treating torsional vibration as free internal rotation. The characterisation of the latter motion is connected with a study of the effect of choice of the free internal-rotational axis. Attention is also paid to the interplay of both isomeric structures which are present at comparable concentrations at modest and higher temperatures even though the HF · HC1 structure is relatively more populated.