Scratching the surface of the water dication

Abstract

The X̃ 3Σg−, ã 1Δg, and b̃ 1Σg+ states of the water dication, H2O2+, have been investigated using several high-level ab initio methods and a range of basis sets. With Dunning’s augmented correlation consistent polarized valence quadruple-ζ (aug-cc-pVQZ) basis set at the complete active space self-consistent field second-order configuration interaction (CAS-SOCI) level, it is confirmed that the ground and first two excited states of H2O2+ are all of D∞h symmetry, in violation of Walsh’s rules for 6 valence electron AH2 systems. The singlet–triplet splitting (X̃ 3Σg−—ã 1Δg) is predicted to be 53.6 kcal/mol (2.32 eV, 18 700 cm−1), while the X̃ 3Σg−—b̃ 1Σg+ separation is predicted to be 91.1 kcal/mol (3.95 eV, 31 900 cm−1). The vertical double ionization potentials (IPs) from X̃ 1A1 H2O to the X̃ 3B1, 1 1A1, b̃ 1B1, and 2 1A1 states of H2O2+ are predicted within the cc-pVQZ basis to be 40.1, 41.2, 42.6, and 46.1 eV, respectively, in good agreement with recent double-charge-transfer spectroscopic results. The corresponding adiabatic double IPs are 37.0, 39.3, and 41.0 eV to the X̃ 3Σg−, ã 1Δg, and b̃ 1Σg+ states of H2O2+, respectively. The activation barrier to fragmentation of H2O2+ (X̃ 3Σg− H2O2+→3Σ− OH++H+) at the cc-pVQZ CAS-SOCI level is predicted to be 2.1 kcal/mol (0.10 eV, 738 cm−1), and the reaction is exothermic by 126.4 kcal/mol (5.48 eV, 44 210 cm−1), providing a challenge for direct experimental detection of this elusive molecule.