Structure and conformation of some saturated four-membered rings, [formula omitted

Abstract

Complete geometry optimizations of the ground state of three four-membered molecules with structures known from experiment (oxetane. silacyclobutane and thietane) have been carried out by ab initio SCF calculations at the 6-31G level. Additionally, calculations were made for two similar compounds CH 2 CH 2 CH 2 X ⎵ H (X = B, Al) not yet studied experimentally. Although comparison with available experimental data confirms that this level of theory somewhat underestimates both the dihedral angles in these molecules and the barriers to ring planarity, this study fills the gaps previously existing in structural investigations of four-membered ring molecules at a consistent level of theory, making possible the development of a uniform picture of seven monosubstituted heterocyclic compounds together with the parent molecule cyclobutane. Seven structural relationships common to all four-membered rings are systematized and discussed giving emphasis to the changes of C-C bond lengths and angular deformations around tetrahedral carbon and silicon atoms.