Structure and isomerization of arenonium ions of dichlorobenzenes in the gas phase. A theoretical study

Ab initio MP2 calculations of all isomeric arenonium ions (AI) of ortho-, meta-, and para-dichlorobenzenes in the gas phase were carried out with full optimization of geometry with the 6-31G* basis set. The calculated proton affinities depend substantially on the position of geminal center in the corresponding dichlorobenzenonium ion and decrease in the series 1,2-dichloro-4H-benzenonium > 1,2-dichloro-3H-benzenonium > 1,2-dichloro-2H-benzenonium; 1,3-dichloro-4H-benzenonium > 1,3-dichloro-2H-benzenonillm > I,3-dichloro-5H-benzenonium >1,3-dichloro-3H-benzenonium; 1,4-dichloro-2H-benzenonium > 1,4-dichloro-4H-benzenonium. The structures of transition states and activation energies (E-a) of almost all 1,2-shifts of H and Cl atoms in Al were determined. The activation energies of migrations of H atoms are about 6 kcal mol(-1) less than those of migrations of Cl atoms in similar structures. The isomerization routes and relations between the rate constants for isomerization of dichlorobenzenes through AI were established.