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 (OH)-O-16-(OH2)-O-16, (OH)-O-16-(OH2)-O-18, (OH)-O-18-(OH2)-O-16, and (OH)-O-18-(OH2)-O-18 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 ((2)A') and first electronically excited ((2)A'') states of the system, which correspond roughly to occupancy by the odd electron in the p(y) and p(x) orbitals, respectively (where p(y) is in the mirror plane of the complex and p(x) is perpendicular to p(y) and the OH bond axis). The spectroscopic measurements yield a parameter, rho, which is equal to the vibrationally averaged (2)A'-(2)A'' energy separation that would be obtained if spin-orbit coupling and rotation were absent. For the parent species, rho=-146.560 27(9) cm(-1). O-18 substitution on the water increases vertical bar rho vertical bar by 0.105 29(10) cm(-1), while substitution on the OH decreases vertical bar rho vertical bar by 0.068 64(11) cm(-1). In the OH-OH2 complex, the observed value of rho implies an energy spacing between the rotationless levels of the (2)A' and (2)A'' 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 (2)A'-(2)A'' 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 rho. (c) 2008 American Institute of Physics.