Isomers on the C2H2Cl+ surface. Calculated enthalpies of formation for unsaturated hydrocarbons and monochloro derivatives

Ab initio molecular orbital calculations are reported for structures at minima and at saddle points on the C 2 H 2 Cl + surface. At SCF/6-31(d,p), the 1-chlorovinyl cation (1), the chlorenium ion (2), the 2-chlorovinyl cation (3), and chlorine-protonated chloroacetylene (4) are all at minima, but inclusion of electron correlation at MP2/6-311G(d,p) eliminates the 2-chlorovinyl cation as a minimum and the transition structure for interconversion of 1 and 2 has a structure similar to 3. At MP4/6-311G(2df,p)//MP2/6-311G(d,p) 1 lies 19.4 kcal mol -1 below 2, and the barrier is 10.7 kcal mol -1 above 2