The infrared spectrum of sulfur trioxide was obtained under prism resolution in the gaseous state and in xenon matrices at liquidhyphen;nitrogen temperature. In both cases a satisfactory vibrational assignment of the monomeric SO3spectrum could be made on the basis of a simpleD3hsymmetry model. The observed gas phase fundamentals werev3(eprime;) = 1391,v4(eprime;) = 529, andv2(a2Prime;) = 495 cmmdash;1. The matrix fundamentals were essentially the same as those of the gas phase except that thev2(a2Prime;) mode was shifted to 464 cmmdash;1. A normal coordinate treatment of the SO3molecule was carried out and potential constants were determined for both Ureyhyphen;Bradley and simplified valence force fields. Three weak absorption bands of S3O9were also detected in the gashyphen;phase spectrum. From the temperature and pressure variation of the intensities of these bands it was estimated that for the trimerization reaction at 298deg;K,Kpwas approximately 1 atmmdash;2and Dgr;H298deg; was approximately 30 kcal/mole of S3O9. The values of the thermodynamic properties of monomeric SO3were computed for the ideal gaseous state using the rigid rotor harmonic oscillator approximation at 1 atm from 100deg; to 1500deg;K.
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