H-bridged alternatives to the homologues of cyclopropane

Abstract

Alternative H-bridged structures (6, 7 and 8) for homologues of cyclopropane (1) have been calculated using ab initio MO methods at the MP2/6-31G^∗ and MP2/LANL1DZ levels.The energy difference between the classical structure 1 and its bridged analogue 6 decreases in going from C to Pb (1 vs. 6: C 197.0, Si 84.0, Ge 36.0, Sn -1.7, and Pb -45.4 kcal mol^-1). Structure 6C is not a minimum, and 6Pb is the lowest energy minimum that has been located in the present study. The natural bond orbital method was used to understand the nature of bonding in these molecules. Strain energies of three membered rings are estimated using Eq. (1). Isolobal analogy connects 6 to B₃H₉ and (CpCoCO)₃.