Scratching the surface of the water dication

The (X) over tilde (3)Sigma(g)(-), (a) over tilde (1)Delta(g), and (b) over tilde (1)Sigma(g)(+) states of the water dication, H2O2+, have been invest igated using several high-level ab initio methods and a range of basis sets . With Dunning's augmented correlation consistent polarized valence quadrup le-zeta (aug-cc-pVQZ) basis set at the complete active space self-consisten t field second-order configuration interaction (CAS-SOCI) level, it is conf irmed that the ground and first two excited states of H2O2+ are all of D-in finity h symmetry, in violation of Walsh's rules for 6 valence electron AH( 2) systems. The singlet-triplet splitting ((X) over tilde (3)Sigma(g)(-) -- (a) over tilde (1)Delta(g)) is predicted to be 53.6 kcal/mol (2.32 eV, 18 700 cm(-1)), while the (X) over tilde (3)Sigma(g)(-) - (b) over tilde (1)Si gma(g)(+) separation is predicted to be 91.1 kcal/mol (3.95 eV, 31 900 cm(- 1)). The vertical double ionization potentials (IPs) from (X) over tilde (1 )A(1) H2O to the (X) over tilde B-3(1), 1 (1)A(1), (b) over tilde B-1(1), a nd 2 (1)A(1) states of H2O2+ are predicted within the cc-pVQZ basis to be 4 0.1, 41.2, 42.6, and 46.1 eV, respectively, in good agreement with recent d ouble-charge-transfer spectroscopic results. The corresponding adiabatic do uble IPs are 37.0, 39.3, and 41.0 eV to the (X) over tilde (3)Sigma(g)(-), (a) over tilde (1)Delta(g), and (b) over tilde (1)Sigma(g)(+) states of H2O 2+, respectively. The activation barrier to fragmentation of H2O2+ ((X) ove r tilde (3)Sigma(g)(-) H2O2+ --> (3)Sigma(-) OH+ + H+) at the cc-pVQZ CAS-S OCI level is predicted to be 2.1 kcal/mol (0.10 eV, 738 cm(-1)), and the re action 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. (C) 1999 American Institute of Physics. [S0021-9606(99)31024-2].