STRAIN ENERGIES OF INORGANIC RINGS

Strain energy, an important concept in organic chemistry, can also be applied to inorganic ring systems. Deviations of bond angles and torsi onal angles from preferred valence values at ring vertices implies a s trained structure with increased energy. Strain energy is the energy d ifference between the energy change for a process as determined by exp eriment and as determined by a model that does not include strain. In practice, the experimental energy change can be approximated by differ ences in energies of products and reactants as obtained from ab initio SCF MO calculations. The bond additivity model can be used to describ e the process that neglects the effects of strain. Strain energies for monocycles O-n and S-n, n = 3-8, will be compared with those for cycl oalkanes. The surprise here is that four-membered rings of sulfur and of oxygen have greater strains than do three-membered rings. Strain en ergies of polycyclic clusters P-4, P-6, P-8, As-4, and As-6 are small compared to those of analogous hydrocarbons. The conventional concepts of strain energy, resonance energy, and average bond energy can be co mbined to rationalize differences in relative energy trends among isom eric structures of C6H6, P-6 and As-6. The rule of additivity of ring strain energies, useful in the estimation of polycyclic hydrocarbons, may also be applicable to polycyclic inorganic clusters. The concepts of average bond energies, resonance stabilization, and strain energy a nd the rules of bond additivity and ring strain energy are useful in u nderstanding properties of inorganic molecules when we know how the in organic parameters differ from the better known organic values.