Electron-diffraction investigations of gaseous sulphur dioxide and trioxide

Abstract

Electron-diffraction intensity data have been collected for gaseous sulphur dioxide (250 K) and rioxide (300 K) at camera lengths of 50 and 25 cm. Least squares refinement led to the following r_a distances, root-mean-square amplitudes of vibration, and estimated standard deviations; for SO₂, r_S–O= 1.431 ± 0.002 Å, r_O‥O= 2.460±0.012 Å, u_S–O= 0.041±0.003 Å, and u_O‥O= 0.053±0.012 Å, for SO₃, r_S–O= 1.418±0.003 Å, r_O‥O= 2.457±0.009 Å, u_S–O= 0.048±0.005 Å and u_O‥O= 0.067±0.010 Å. The corresponding value for the angle in the dioxide is 118.5±1.0°, whilst the trioxide is planar within experimental error. For both molecules, spectroscopic estimates of the vibrational amplitudes have been made from force-constant data, and for SO₂ these are in satisfactory agreement with the diffraction results. For SO₃, however, agreement is poorer and this may indicate the presence of a small amount of S₃O₉ trimer in the diffracting vapour. Mean-square parallel and perpendicular amplitudes of vibration and centrifugal distortion corrections have also been calculated for both molecules and the electron-diffraction r_a parameters converted to equilibrium (r_e), and zero-point-average (r^°_α), values. For SO₂, these structures differ insignificantly from corresponding microwave r_e and r_z estimates.