Linear Triatomic Molecules

Abstract

Triatomic molecules are most frequently bent; less common are the linear and (the quite rare) triangular arrangements. Stable molecular geometries with precise bond angles and bond lengths can be predicted by determining quantum mechanically the molecular energy as a function of atomic positions and then finding the minima in this function. Sophisticated calculations are required for this purpose, however. For large molecules and compounds of heavier elements the computations are still too large even for modern computers to achieve substantial accuracy. Since one of our purposes is to develop qualitative delocalized and localized views of bonding in polyatomic molecules, we must first know their geometries. The concept most often used for this purpose is the so-called “valence state electron pair repulsion” or VSEPR approach developed by Gillespie and Nyholm, and indeed we shall in future chapters utilize this idea extensively. In order to justify the use of this tool, however, we will in the first several examples assume that we know the molecular geometry.