Pseudo Jahn−Teller Origin of the Proton Tunneling in Zundel Cation Containing Water Clusters

Abstract

The pseudo Jahn–Teller (PJT) origin of the proton transfer barrier in the Zundel cation at different O–O distances and in an H5O2+(H2O)4 cluster is revealed by means of  ab initio calculations of their electronic structures and the adiabatic potential energy curves. The vibronic constants in this approach were estimated by fitting the ab initio calculated adiabatic potential to its analytical expression. It is shown also that the high-symmetry nuclear configurations ofproton-centered water clusters of the type H+(H2O)n (n = 6, 4, 3) are unstable with respect to the low-symmetry nuclear distortions leading to forming the dihydronium cation H5O2+ and the appropriate number of water molecules: H2n + 1On+ →  (n – 2)H2O + H5O2+. The reason for this instability and the subsequent decay is the PJT coupling between the ground and excited electronic states.