On the structure and stability of the H2O− ion

Potential energy surfaces of the three lowest bound electronic states (1 2A′, 2 2A′, and 2A″) of H2O− have been investigated by ab initio calculations using highly correlated electronic wave functions. Minima resulting from ion–quadrupole interactions between the O−(2P) and H2(1Σ+) fragments were found for linear 2Σ+ and 2Π O−⋅⋅⋅H2 structures. The corresponding dissociation energies amount to about 0.2 eV. The 2Σ+ (2A′) minimum is separated by a barrier from the H2O+e continuum, while the 2Π (2A″) state is connected via a similar barrier to another local minimum of 2Π symmetry originating from the ion–dipole interaction of OH(X 2Π) and H−(1S). Hence, in accordance with recent experimental findings of de Koning and Nibbering, the H2O− ion is found to be a bound species in ion–quadrupole or ion–dipole cluster structures.