On the structure of the monohydrated superoxide molecular anion, O2-·(H2O). An abinitio molecular orbital study

Abstract

The ground-state potential energy surface of the monohydrated superoxide ion–dipole complex O₂^-·(H₂O) was investigated viaabinitio molecular orbital theory in order to establish firmly the relative energies of stationary points on this surface. Using the configuration interaction (CI) theoretical approach to accommodate the effects of electron correlation, the global minimum on the potential energy surface corresponds to a structure adopting C_s molecular symmetry with one end of the water molecule forming a hydrogen bond with one end of the superoxide. A more symmetric structure adopting C_2v molecular symmetry is shown to be a transition state linking two equivalent forms of the C_s geometry via a water rocking motion. Using a highly flexible triple-zeta basis set with quadratic configuration interaction theory incorporating all single and double electron substitutions [QCISD/6–311++G**], the scaled zero point energies for the C_2v and C_s geometries are 64.6 and 64.1 kJ mol^-1, respectively. The energy barrier to the water rocking motion along the reaction coordinate is 3.9 kJ mol^-1. The frequencies of the symmetric and asymmetric water O–H stretches in the C_2v structure are 3731 and 3765 cm^-1, respectively. The water O–H stretching frequencies in the C_s structure are 3178 cm^-1 for the “hydrogen bonded’' OH and 3937 cm^-1 for the “free’' OH. The geometry of the global minimum of O₂^-·(H₂O) on the equivalent of the first electronically excited potential energy surface of the bare superoxide was also determined using the complete active space self-consistent field (CASSCF) theoretical approach. A vibrational frequency analysis confirms that the excited-state stationary point is a local minimum geometry. The excited-state geometry differs significantly from that in the ground electronic state. The overall molecular symmetry in the excited state remains as C_s, but the water molecule adopts an orientation approximately midway between the ground-state C_s and C_2v configurations.