A MICROWAVE SPECTRAL AND ABINITIO INVESTIGATION OF O3-H2O

Microwave spectra of O3H2O, O3H218O, O3HDO, and O3D2O have been observed with a pulsed-beam Fabry-Perot cavity Fourier-transform microwave spectrometer. Two tunneling states, designated A1 and A2, are found for the normal and dideuterated isotopic forms, while only one state is observed for the O3HDO isotope. The A1 spectra of O3H2O, O3H218O, and O3D2O as well as the O3HDO spectrum were fit to a Watson asymmetric top Hamiltonian, giving A = 11 960.584(5), B = 4174.036(8), and C = 3265.173(8) in MHz for O3H2O. The A2 states of O3H2O and O3D2O could not be fit to a Watson Hamiltonian. This result, when combined with the large frequency splittings and the observed nuclear spin statistics for the O3H2O and O3D2O isotopic forms, suggests that there is a low barrier to internal rotation of water. The tunneling motion may also involve ozone through a concerted internal rotation of both monomer subunits. Stark effect measurements of a- and c-type transitions for O3H2O give the electric dipole components μa = 1.014(2) D and μc = 0.522(6) D. The dipole and moment of inertia data indicate that the complex has Cs symmetry with water and the unique oxygen of ozone lying in the symmetry plane. This plane bisects the OOO angle of ozone. The distance between the centers of mass of ozone and water is 2.957(2) Å. From the microwave data and ab initio calculations at the MP2 6-31G(d, p) and MP4(SDTQ) 6-31G(d, p) level of theory it is found that the terminal oxygens of ozone are tilted toward one of the nonequivalent hydrogen atoms in water. Furthermore, the calculations reveal that O3 and H2O adopt an orientation within the complex that guarantees maximal stabilization by electric dipole-dipole attraction. © 1991.