THE HYDROPEROXYL ANION HO2- - AB-INITIO POTENTIAL-ENERGY SURFACE AND VIBRATIONAL SPLITTINGS FOR PROTON-TRANSFER

We calculate over 1500 ab initio points for the HO2- ground state at t he QCISD(T)/6-311++G(2df,pd) level for a wide range of geometries. We fit these points to a 120-parameter analytic function to obtain a pote ntial energy surface (PES) valid for large amplitude hydrogen motion. We then calculate and assign vibrational energy levels for this PES. T here is intramolecular proton transfer when the hydrogen atom tunnels through a T-shaped transition state separating the two equivalent equi librium geometries. The O-O distances are very different for the equil ibrium and transition state geometries and the proton transfer is trul y multidimensional. When there is nuclear tunneling the vibrational le vels are split and we focus on levels with significant splitting. The barrier to proton transfer is 6058 cm(-1) (4752 cm(-1) with zero-point correction). Significant splittings are observed for relatively low-l ying vibrational levels that may be experimentally accessible. Results are presented for H-O-16-O-16 and H-O-17-O-17. For H-O-16-O-16 the fi rst three levels with splitting greater than 10 cm(-1) are, in order o f increasing energy, (0,2,3), (0,1,5), and (0,3,2) where v(2) is the O -O-H bend quantum number and v(3) is the O-O stretch quantum number. T he (0,4,0) level lies between the (0,2,3) and (0,1,5) levels but the s plitting is only 6 cm(-1) showing that although H-O-O bend excitation is essential, O-O stretch excitation greatly facilitates proton transf er. This is not because it permits tunneling at a smaller O-O distance than that for the equilibrium geometry. Rather, it is because it perm its tunneling at the,larger O-O distance corresponding to the transiti on state geometry. (C) 1996 American Institute of Physics.