A quantum-chemical study of the C2H3F2+ and C2H3Cl2+ isomers and their interconversion. CBS-QB3 proton affinities of difluoroethenes and dichloroethenes

Potential energy surfaces of the C2H3X2+ isomers and proton affinities of d ihaloethenes C2H2X2 (X = Fl Cl) were computed at the B3LYP/6-31++G(d,p), MP 2/6-311++G(d,p), and CBS-QB3 levels. The classical 1,1-dihaloethyl cations CH3CX2+ represent global minima for the C2H3X2+ isomers. Other minima locat ed are classical 1,2-dihaloethyl cations, the chloroethylchloronium (Cl-bri dged) cation, halogen-protonated cis-1,2-, trans-1,2-, and 1,1-dihaloethene s, and ion-dipole complexes of the CH2=CX+ cation with the HX molecule. The classical 2,2-dihaloethyl cations, as well as H-bridged cations, are at fi rst-order saddle points. The fluoroethylfluoronium cation is not at a stati onary point. Transition states were located and activation energies compute d for isomerization (1) of the trans-1,2-difluoroethyl cation to the 1,1-di fluoroethyl cation, (2) of the cis-l,2-difluoroethyl cation to its trans ro tamer, (3) of the chloronium cation to the I,l-dichloroethyl cation, (4) of the cis-1,2-dichloroethyl cation to the chloronium cation, and (5) of the halogen-protonated dihaloethenes to carbon-protonated isomers. Protonation of dihaloethenes at carbon is more favorable than protonation at halogen. T he best estimates at CBS-QB3 for proton affinities (in kcal/mol) are as fol lows: 1,1-C2H2F2, 171.1; cis-1,2-C2H2F2, 152.9; trans-1,2-C2H2F2, 151.9; 1, 1-C2H2Cl2, 176.0; cis-1,2-C2H2Cl2, 159.7; trans-1,2-C2H2Cl2, 162.0.