Clarification of the assignment of the electronic spectrum of hydrogen chloride based on ab initio cl calculations

Abstract

Despite a very detailed series of spectroscopic measurements of the HCl molecule it has not been possible heretofore to achieve a satisfactory assignment of its electronic spectrum. In this work similar calculations to those recently published for the isovalent HF system are applied to this problem, with special emphasis on the interplay between Rydberg and valence states found to be the key to understanding the HF spectrum. The results of this large AO basis Cl treatment show that the observed deviations from the normal pattern of Rydberg states expected for a saturated system such as HCl are caused by series of curve crossings between its diffuse states and the valence σ → σ* species leading to among other things a distinctive double minimum in the 2 1Σ + potential curve of this molecule, not suspected in earlier empirical studies of this spectrum. On this basis a new assignment of the lowest-energy absorption line (80888.3 cm −1) in terms of the lowest species involving the first ( 1Σ R1 +) well at short bond distances seems to be appropriate. In general nearly quantitative agreement is found between the present calculated transition and dissociation energies as well as rotational constants and zero point frequencies and the corresponding measured values wherever available, leading to a sound basis for the assignment of the HCl spectrum up to the energy of its first ionization potential. A particularly interesting feature of the calculations is that while pronounced Rydberg—valence mixing is observed in the 3,1Σ + manifolds, no comparable perturbative effects are noted for their counterparts of Π, Δ and Σ − symmetry, so that the latter species do exhibit a quite typical energetic pattern well known for molecular Rydberg series.