Does (H5O2)+− X (X = Ar, Ne, He) complex with C2 symmetry really exist?

Abstract

Different configurations of (H5O2)+− Ar, (H5O2)+− Ne and (H5O2)+− He complexes were calculated using DFT methods and series of Dunning and Pople basis sets. The possibility of the existence of an equilibrium configuration with C2 symmetry for complexes analyzed was in focus of the studies carried out. DFT methods that do not take into account dispersion interactions, as a rule, define (H5O2)+− X (X = Ar, Ne, He) configurations with C2 symmetry as transitional states. At the same time, such DFT methods as B1B95, B98, and WB97X make it possible to determine geometric parameters of these complexes with C2 symmetry in equilibrium configurations. Taking into account dispersion interactions in the framework of D3 Grimme approach and using density functionals, including dispersion interactions (such as M06-2X, wB97XD, etc.), leads to the situation, when configurations of the complexes such as (H5O2)+− X (X = Ar, Ne, He) with C2 symmetry calculated using most of the basis sets turn out to be equilibrium. However, calculations within the framework of MP2, MP4, and CCSD (T) methods determine configurations of complexes with C2 symmetry as transition states, while the experimental data on the IR spectra of (H5O2)+− Ar complex probably point out to the existence of an equilibrium configuration of this complex with C2 symmetry. In addition to structural analysis for equilibrium configurations found, IR spectra were calculated in harmonic and in some cases also in anharmonic approximations. Calculated IR spectra were compared to protonated water dimer IR spectra calculated at the same level of theory in equilibrium configuration and in the transition state.