Is the oxywater radical cation more stable than neutral oxywater?

Ab initio quantum mechanical methods have been applied to the distonic oxywater (H₂OO⁺) and conventional hydrogen peroxide (HOOH⁺) cations. The investigation employed basis sets up to triple‐ζ plus double polarization plus f functions (TZ2Pf) and levels of correlation up to coupled‐cluster including single, double, and perturbatively treated connected triple excitations [CCSD(T)]. The HOOH⁺ cation, which is planar, has both trans (C_2h) and cis (C_2v) conformations, the former predicted to be 8 kcal mol⁻¹ lower in energy. At the highest level of theory, the distonic H₂OO⁺ structure is found to lie 23 kcal mol⁻¹ above the conventional trans form. The barrier separating the oxywater cation from HOOH⁺ is about 33 kcal mol⁻¹, roughly ten times larger than that for the neutral oxywater species. Accordingly, ionization greatly enhances the stability of the nonconventional oxywater structure. Harmonic vibrational frequencies and their infrared intensities are also reported for the H₂O⁺₂ species. Symmetry breaking of Hartree–Fock electronic wave functions is found in HOOH⁺, which adversely affects certain vibrational frequencies due to nearby singularities in related quadratic force constants. This problem is efficaciously overcome via Brueckner methods [BD and BD(T)].