Sulfur trioxide geometry

Quantum chemical methods are valuable tools for studying atmospheric nucle-ation phenomena. Molecular geometries and binding energies computed using electronic structure methods can be used to determine potential parameters for classical molecular dynamic simulations, which in turn provide information on the dynamics and qualitative energetics of nucleation processes. Quantum chemistry calculations can also be used to obtain accurate and reliable information on the fundamental chemical and physical properties of molecular systems relevant to nucleation. Successful atmospheric applications include investigations on the hydration of sulfuric acid and the role of ammonia, sulfur trioxide and/or ions... 

Carbon dioxide is a molecule with two atoms attached (SN = 2) to the central atom via double bonds. The electrons in each double bond must be between C and O, and the repulsion between these electron groups forces a linear structure on the molecule. Sulfur trioxide has three atoms bound to the sulfur (SN = 3), with equivalent partial doublebond character between sulfur and each oxygen, a conclusion rendered by analysis of its resonance forms. The best positions for the oxygens to minimize electron-electron repulsions in this molecule are at the corners of an equilateral triangle, with O—S—O bond angles of 120°. The multiple bonding does not affect the geometry, because aU three bonds are equivalent in terms of bond order. 

Sulfur trioxide is a liquid at room temperature. The liquid actually consists of S3O9 molecules in equihbrium with SO3 molecules (Figure 22.47). The vapor-phase molecule is SO3, which has a planar triangular geometry. 

Predict the geometry of sulfur trioxide, SO3, and describe the bonding in terms of both the valence bond (VB) and molecular orbital (MO) theories. 

The known thionyl halides include SOX2 (X = F, Cl, and Br Table 13) as well as the mixed compound SOFCl. They are pyramidal in geometry, with the S=0 bond being shorter in the fluoride than the chloride or bromide. Thionyl chloride is prepared industrially by reacting sulfur dioxide or trioxide with sulfur dichloride or sulfur monochloride, or by the reaction of phosphorus trichloride with sulfuryl chloride (equations 54-57). 

---