Influence of F and CF3 Groups at the C=C Double Bond on the Structure and Properties of Protonated Forms of Olefin‐Series Compounds: an MP2 ab initio Study

The ab initio MP2 method using the 6−31G* basis set with full geometry optimization was employed to calculate the protonated fluoroolefin molecules (F)i(H)jC=C(F)k(H)l (A) and (CF3)i(F)jC=C(CF3)k×(F)l (B), where i + j = k + l = 2. It is shown that a proton is attached to a carbon atom that is linked to fewer fluorine atoms as substituents. In series A, the proton affinity (PA:rpar; passes through a maximum as the number of fluorine atoms in the molecule increases. The highest PA is found for the 1,1′‐difluoroethylene molecule (182.6 kcal/mole). In series B, the PA decreases monotonically, assuming an anomalously low value for the tetra(trifluoromethyl)ethylene molecule (114.7 kcal/mole). The obtained results are compared with Hartree–Fock calculations; the Hartree–Fock method is inadequate for predicting the structures of the carbocations examined.