Geometry of molecules: Part 5. Interatomic distances and electronic structures of some aloukyl-substituted cyclopropanes and cyclopropenes by the IMOA method

The geometric and electronic structures of some alkyl- and vinyl-substituted cyclopropanes and cyclopropenes have been computed by the iterative maximum overlap approach (IMOA). Some MINDO/3 calculations have been carried out for comparison. Asymmetries introduced in the three-membered rings by the substitution are discussed in detail. Comparison of the predicted bond distances in vinylcyclopropane with microwave data indicates the presence of a conjugative HOMO-LUMO interaction between the strained ring and the ethylene moiety. It is stressed that interatomic distances and bond lengths should be distinguished in strained systems. The bond lengths are defined as segments of curves passing through the points of maximum electron density and the relevant nuclei. They are calculated and discussed for some characteristic highly strained rings. The estimated heats of formation and strain energies are in satisfactory agreement with available experimental data.