Theoretical study of the protonated hydronium radical cation H 40 + as a rydberg molecule. The ground state

Abstract

Ab initio calculations at HF and MP2 (the second-order Møller-Plesset perturbation ) levels with a series of extended basis sets 6-31G*, 6-31G** and 6-31G**R have been carried out for the tetrahedral H 4O + cation and its related species H 4O 2+, H 3O, H 3O +, H 2O, H 2O + and H 2 in order to study it as a Rydberg molecule. The results have shown some evidence of the Rydberg character of the ground state H 4O + including that the calculated ionization potential matches well with the estimate from the modified Rydberg formula. The ground state has been predicted to be unstable with respect to H 3O + +H or H 2O + + H 2. The potential energy curve of the dissociation into H 3O + + H has been calculated and no potential barrier has been found. Analysis of electronic details has also shown some Rydberg character of the ground state H 4O + but it has indicated a strong effect of the 3s Rydberg state interacting with the valence states. The concept of Rydberg-valence state mixing gives a good explanation of the calculated properties of the ground state of the tetrahedral H 4O + presented here.