Coherent Raman and infrared studies of sulfur trioxide

High-resolution (0.001 cm(-1)) coherent anti-Stokes Raman scattering (CARS) was used to observe the Q-branch structure of the IR-inactive nu(1) symmetric stretching mode of (SO3)-S-32-O-16 and its various O-18 isotopomers. The nu(1) spectrum of (SO3)-S-32-O-16 reveals two intense Q-branches in the region 1065-1067 cm-1, with surprisingly complex vibrational-rotational structure not resolved in earlier studies. Efforts to simulate this with a simple Fermi-resonance model involving nu(1) and 2nu(4) states do not reproduce the spectral detail, nor do they yield reasonable spectroscopic parameters. A more subtle combination of Fermi resonance and indirect Coriolis interactions with nearby states, 2nu(4) (1 = 0, +/-2), nu(2) + nu(4) (1 = +/-1) 2nu(2) (1 = 0), is suspected and a determination of the location of these coupled states by high-resolution infrared measurements is under way. At medium resolution (0.125 cm(-1)), the infrared spectra reveal Q-branch features from which approximate band origins are estimated for the nu(2), nu(3), and nu(4) fundamental modes of (SO3)-S-32-O-18, (SO2O)-S-32-O-18-O-16, and (SOO2)-S-32-O-18-O-16. These and literature data for (SO3)-S-32-O-16 are used to calculate force constants for SO3 and a comparison is made with similar values for SO2 and SO. The frequencies and force constants are in excellent agreement with those obtained by Martin in a recent ab initio calculation. (C) 2001 Elsevier Science.