The singlet-triplet splitting of the low-lying electronic states of H₂O²⁺ and a comparison with isoelectronic CH₂ and CH₂²⁺

At the MP2/6-31G(d,p) level of theory using complex orbitais, the lowest-lying (bound) singlet state of H₂O²⁺ is the linear ¹Δ_g state 2 with an O-H bond distance of 1.199 Å. 1 is estimated to be 4̃6 kcal mol higher in energy than the lowest-lying ³Σ^-_g state 2 of H₂O²⁺. 1 and 2 are compared to the lowest-lying singlet and triplet states of isoelectronic CH₂ and CHe₂²⁺. Using Walsh diagrams, the trend in the singlet/triplet gap and bending angles is explained by the relative electronegativities between the central and substituent atoms.