Theoretical determination of molecular structure and conformation. 15. Three-membered rings: bent bonds, ring strain, and surface delocalization

Abstract

Energy, geometry, one-electron density distribution, and Laplace concentrations of cyclopropane (l), aziridine (2), oxirane (3), cyclopropyl anion (4), protonated aziridine (5), protonated oxirane (6), halogen-bridged fluoroethyl cation (7), and hydrogen-bridged ethyl cation (8) are investigated and compared with their corresponding alicyclic counterparts. The relationship between three-membered rings (3MR) and a-complexes is demonstrated by analyzing their electron density pattern with the aid of catastrophe theory. A continuous change from 3MRs to .rr-complexes can be observed if the acceptor (donor) ability of a group X interacting with an ethylene unit is increased (decreased). The bend of the 3MR bonds is described by the curvature of the paths of maximum electron density linking the ring atoms. Interpath angles are used to evaluate the strain energy of a 3MR. A value of 75 kcal/mol is found for 1. Strain is partially compensated by stabilizing effects arising from surface delocalization of cr-electrons of the 3MR (48 kcal/mol for 1). Both ring strain and surface delocalization increase in the series 1, 2, 3 with a slight dominance of the former effect. The chemical relevance of theoretical results is discussed with regard to the ability of 3MRs to interact with mystems and to undergo addition reactions with electrophiles and nucleophiles. The observed regioselectivity in ring-opening reactions of oxiranes is explained.