ISOMERS OF C2H4CL- STRUCTURES, FREQUENCIES, AND ENERGETICS( )

High level ab initio molecular orbital calculations which include corr elation energy, zero-point energies, and thermal corrections have been used to explore the C2H4Cl+ hypersurface. The 1-chloroethyl cation is the global minimum with the chloronium ion lying 4.3 kcal mol-1 above it at the MP4SDTQ/6-311G(2df,p)//MP2/6-311G(d,p) level. The energy ba rrier for the conversion of the chloronium ion to,the 1-chloroethyl ca tion was calculated to be 27.8 kcal mol-1 at the MP4 level and was fou nd to be sensitive to electron correlation. The transition structure f or interconversion of these two ions has been fully characterized with calculation of vibrational frequencies at MP2/6-311G(d,p). The 2-chlo roethyl cation, which is a minimum with C1 symmetry at the SCF/6-31G(d ,p) level, collapses without a barrier to the 1-chloroethyl cation whe n zero-point energies are included. High-energy isomers, chlorine-prot onated vinyl chloride, and an ion-dipole complex have also been found on the C2H4Cl+ surface. Vinyl chloride protonates on the carbon of the CH2 group and has a proton affinity of 171.4 kcal mol-1. Calculated e nthalpies of formation DELTAH-degrees f,298 for the 1-chloroethyl cati on and the chloronium ion are 198.1 and 202.0 kcal mol-1, respectively . The difference (3.9 kcal mol-1) in DELTAH-degrees f,298 between thes e two isomers compares with experimentally determined differences of 5 .6 and 2 kcal mol-1. Calculated vibrational frequencies and intensitie s are compared with the experimental spectra, and if only one ion cont ributes to these spectra, the calculations favor the chloronium ion.