Effects of ¹⁸O isotopic substitution on the rotational spectra and potential splitting in the OH- OH₂ complex: Improved measurements for ¹⁶OH- ¹⁶OH₂ and ¹⁸OH- ¹⁸OH₂, new measurements for the mixed isotopic forms, and ab initio calculations of the ²A′ - ²A″ energy separation

Rotational spectra have been observed for O 16 H- O 16 H2, O 16 H- O 18 H2, O 18 H- O 16 H2, and O 18 H- O 18 H2 with complete resolution of the nuclear magnetic hyperfine structure from the OH and water protons. Transition frequencies have been analyzed for each isotopic form using the model of Marshall and Lester [J. Chem. Phys. 121, 3019 (2004)], which accounts for partial quenching of the OH orbital angular momentum and the decoupling of the electronic spin from the OH molecular axis. The analysis accounts for both the ground (A2 ′) and first electronically excited (A2 ″) states of the system, which correspond roughly to occupancy by the odd electron in the py and px orbitals, respectively (where py is in the mirror plane of the complex and px is perpendicular to py and the OH bond axis). The spectroscopic measurements yield a parameter, ρ, which is equal to the vibrationally averaged A2 ′ - A2 ″ energy separation that would be obtained if spin-orbit coupling and rotation were absent. For the parent species, ρ =-146.560 27 (9) cm-1. O 18 substitution on the water increases ρ by 0.105 29 (10) cm-1, while substitution on the OH decreases ρ by 0.068 64 (11) cm-1. In the OH- OH2 complex, the observed value of ρ implies an energy spacing between the rotationless levels of the A2 ′ and A2 ″ states of 203.76 cm-1. Ab initio calculations have been performed with quadratic configuration interaction with single and double excitations (QCISD), as well as multireference configuration interaction (MRCI), both with and without the inclusion of spin-orbit coupling. The MRCI calculations with spin-orbit coupling perform the best, giving a value of 171 cm-1 for the A2 ′ - A2 ″ energy spacing at the equilibrium geometry. Calculations along the large-amplitude bending coordinates of the OH and OH2 moieties within the complex are presented and are shown to be consistent with a vibrational averaging effect as the main cause of the observed isotopic sensitivity of ρ.