Structural studies of bitetrahedryl molecule C 8H 6, coupled tricyclo[3.1.0.0]hexyl molecule C 12H 14, and coupled bicyclo[1.1.0]butane derivatives

Abstract

The equilibrium geometries of the bitetrahedryl molecule C 8H 6, the coupled tricyclo[3.1.0.0]hexyl molecule C 12H 14, the 1,1′-bi(tricyclo[3.1.0.0]hexane-6,6′-dicarboxylic acid dimethyl ester), and the 1,1′-bi(tricyclo[4.1.0.0]heptane-7,7′-dicarboxylic acid dimethyl ester) have been optimized by the AM1 semi-empirical quantum-mechanical method. As suggested by Ermer, Bell, Schäfer and Szeimies, the central CC bond distance for the coupled tricyclo[3.1.0.0]hexyl molecule is predicted to be longer than that for the bitetrahedryl molecule. Our AM1 calculations predict the central CC distance in the bitetrahedryl molecule to be 0.023 Å less than in the coupled tricyclo[3.1.0.0]hexyl molecule, which is in good agreement with ab initio self-consistent field (SCF) calculations. However, AM1 calculations predict that the shortest CC bond in tricyclo[3.1.0.0]hexyl molecule is the central one, while ab initio SCF calculations predict the adjacent CC bond to be the shortest. X-ray crystal structures of the esters indicate that the shortest CC bond is the central one. AM1 calculations on the esters also predict that the central CC bond is the shortest such bond ever found.