An ab initio quantum-chemical study of proton addition to F-, CH3-, and CF3-substituted ethylene derivatives

Protonated forms of the molecules of ethylene derivatives with the general formula C2X2Y2 (X = Y = H (1), F (2), CH3 (3), CF3 (4); X = F, Y = H: cis- (5) and trans- (6)) were calculated by the ab initio MP2/6-31G* method with full geometry optimization. The minima and saddle points located on the po tential energy surface (PES) of the protonated ethylene molecule correspond to the stationary states and transition stares of proton migration, respec tively. The stationary stares are characterized by a nonclassical geometry of carbocations similar to that of re-complexes, whereas the transition sta tes have a classical structure. Unlike 1, the carbocations of molecules 2-6 have the classical structure. The saddle points on the PES of the ethylene derivatives correspond to the structures of the re-complex type, which are the transition states of proton migration between the C atoms of the ethyl ene bond. The barrier to rotation about the C-C bond depends essentially on the substituent nature.