Conformational preference in methyl phenyl sulphoxide and in ortho substituted fluorine derivatives: a theoretical approach

Ab initio MO calculations (3-21G*//3-lG* basis set) have been performed for methylphenyl sulphoxide ( 1) ,2-fluoro- (2 ) and 2,6-difluoro- (3 ) derivatives in order to examine the structuralproperties of their conformational ground state (8). The conformational energy profile for rotation around the C,,-S bond has been constructed in the STO-3G* basis set and the minima have been located at the 3-21G* level allowing relaxation of the geometrical parameters of the methylsulphinyl group. For methylphenyl sulphoxide one energy minimum is found and corresponds to the SO bond nearly eclipsed with the phenyl ring (twist angle 7.31” ). Two energy minima are found for compound 2 and in the more stable conformation the SO bond is coplanar with the ring and anti with respect to the ortho fluorine substituent. In the less stable ground state the lone pair of the sulphur atom settles in the nodal plane of the n-electron cloud of the phenyl ring. This conformation is structurally close to the conformational ground state of compound 3. For compound1 the properties of the transition state have also been determined at the 3-21G* level. The energy barrier for internal rotation corresponds to the S-C bond of the methyl sulphinyl group eclipsed with one ortho C-H bond. The origin of the conformational ground- and transition-states in thesemolecules is discussed on the basis of their electronic structure.