Structural properties of some monosubstituted cyclobutanes determined by ab initio calculation

Abstract

The equilibrium geometries of fluorocyclobutane, C 4H 7F, chlorocyclobutane, C 4H 7Cl, cyclobutyl lithium, C 4H 7Li, cyclobutylium, C 4H 7 +, and the cyclobutyl negative ion, C 4H 7 −, have been computed ab initio by Hartree—Fock gradient calculations with basis sets which are double zeta in the valence shells and augmented with d functions on F, Cl, and Li. Two stable conformers with the substituent (or lone pair) in the equatorial and in the axial positions were found in each case, except for C 4H 7 + where the only stable conformation had the hydrogen atom at the carbonium carbon 4.8° out of planarity. For the other materials, the relative stabilities of the two conformers varied with the electron-donating or electron-withdrawing character of the substituent, with E eq— E ax being 11.9 kJ mol −1 for C 4H 7 −, 1.8 for C 4H 7Li, —4.3 for C 4H 7Cl, and —9.1 for C 4H 7F. The ring-puckering angle for the equatorial forms varied only slightly in the direction F & Cl & Li & — while it showed a considerably greater variation in the reverse direction for the axial conformers. The ring CC bonds adjacent to the substituent were shortened slightly for electron-withdrawing substituents and lengthened for electron-donating ones.