Ionization energies of OH−3 isomers

Structures for hydride–water, hydroxide–H2 and double-Rydberg isomers of OH−3 are optimized at the MBPT(2)/6-311++G(d,p) level. While the first two isomers have nearly equal total energies, the double-Rydberg isomer is 1.68 eV less stable. Vertical ionization energies of the isomers are calculated with electron propagator theory and a 6-311++G(2d,2p) basis augmented with extra diffuse functions. The result for the hydride–water complex, 1.50 eV, is in excellent agreement with a recent photoelectron experiment, while the value for the hydroxide–H2 structure, 2.27 eV, is substantially different. Calculations are performed on a double-Rydberg anion with a pyramidal, C3v structure, yielding a vertical ionization energy of 0.43 eV. Corresponding neutral structures are optimized and provide adiabatic ionization energies. Harmonic vibrational frequencies are calculated for the anionic structures.