Intramolecular sulfur—oxygen interaction: An ab initio conformational study of ( Z)-3-fluorothio-2-propenal

Abstract

Different conformations of ( Z)-3-fluorothio-2-propenal, a simple model of compounds exhibiting intramolecular sulfur—oxygen interaction of 1,5 type, have been studied by the ab initio SCF—MO method. The critical points of the relaxed conformational potential energy surface have been obtained with STO-3G optimized geometries and compared to those of a rigid surface. The antiperiplanar—synperiplanar ( ap— sp) conformation with S⋯O close contact is found to be the most favourable. As calculated by STO-3G, STO-3G* and 6–31G* basis sets, the preferred ap— sp and ap— ap conformations differ in energy by 1.2, 3.8 and 6.5 kcal mol −1, respectively. In accordance with known experimental rules about sulfur—oxygen interaction, the calculated FS⋯O angle (169.6°) is nearly linear. The S⋯O nonbonded distance (2.852 Å), as in most α,β-enone structures, is not extremely short. Bond lengths and bond angles do not differ significantly in the minimum energy conformations with or without S⋯O close contact. In the case of ( Z)-3-fluorothio-2-propenal the sulfur—oxygen interaction can be rationalized qualitatively by electrostatic effects of dipolar character between FS and OC moieties.