Infrared Spectrum and Thermodynamic Properties of Gaseous Sulfur Trioxide

Abstract

The infrared spectrum of sulfur trioxide was obtained under prism resolution in the gaseous state and in xenon matrices at liquid‐nitrogen temperature. In both cases a satisfactory vibrational assignment of the monomeric SO₃ spectrum could be made on the basis of a simple D_3h symmetry model. The observed gas phase fundamentals were v₃(e′) = 1391, v₄(e′) = 529, and v₂(a₂″) = 495 cm^—1. The matrix fundamentals were essentially the same as those of the gas phase except that the v₂(a₂″) mode was shifted to 464 cm^—1. A normal coordinate treatment of the SO₃ molecule was carried out and potential constants were determined for both Urey‐Bradley and simplified valence force fields. Three weak absorption bands of S₃O₉ were also detected in the gas‐phase spectrum. From the temperature and pressure variation of the intensities of these bands it was estimated that for the trimerization reaction at 298°K, K_p was approximately 1 atm^—2 and ΔH₂₉₈° was approximately 30 kcal/mole of S₃O₉. The values of the thermodynamic properties of monomeric SO₃ were computed for the ideal gaseous state using the rigid rotor harmonic oscillator approximation at 1 atm from 100° to 1500°K.