Electron-diffraction investigations of gaseous sulphur dioxide and trioxide

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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 ra distances, root-mean-square amplitudes of vibration, and estimated standard deviations; for SO2, rS–O= 1.431 ± 0.002 Å, rO‥O= 2.460±0.012 Å, uS–O= 0.041±0.003 Å, and uO‥O= 0.053±0.012 Å, for SO3, rS–O= 1.418±0.003 Å, rO‥O= 2.457±0.009 Å, uS–O= 0.048±0.005 Å and uO‥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 ra parameters converted to equilibrium (re), and zero-point-average (r° α), values. For SO2, these structures differ insignificantly from corresponding microwave re and rz estimates.

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