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 C 2 H 3 X 2 + isomers and proton affinities of dihaloethenes C 2 H 2 X 2 (X = F, Cl) were computed at the B3LYP/6-31++G(d,p), MP2/6-311++G(d,p), and CBS-QB3 levels. The classical 1,1-dihaloethyl cations CH 3 CX 2 + represent global minima for the C 2 H 3 X 2 + isomers. Other minima located are classical 1,2-dihaloethyl cations, the chloroethylchloronium (Cl-bridged) cation, halogen-protonated cis -1,2-, trans -1,2-, and 1,1-dihaloethenes, and ion−dipole complexes of the CH 2 CX + cation with the HX molecule. The classical 2,2-dihaloethyl cations, as well as H-bridged cations, are at first-order saddle points. The fluoroethylfluoronium cation is not at a stationary point. Transition states were located and activation energies computed for isomerization (1) of the trans -1,2-difluoroethyl cation to the 1,1-difluoroethyl cation, (2) of the cis -1,2-difluoroethyl cation to its trans rotamer, (3) of the chloronium cation to the 1,1-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. The best estimates at CBS-QB3 for proton affinities (in kcal/mol) are as follows: 1,1-C 2 H 2 F 2, 171.1; cis -1,2-C 2 H 2 F 2, 152.9; trans -1,2-C 2 H 2 F 2, 151.9; 1,1-C 2 H 2 Cl 2, 176.0; cis -1,2-C 2 H 2 Cl 2, 159.7; trans -1,2-C 2 H 2 Cl 2, 162.0.