PROTONATED ETHANE - A THEORETICAL INVESTIGATION OF C2H7+ STRUCTURES AND ENERGIES

The C2H7+ potential energy surface was characterized by high-level ab initio calculations. The effects of electron correlation on geometries and relative energies are substantial. At MP4(SDTQ)/6-311G**//MP2(full)l 6-31G**, the global minimum is the C-C protonated structure 1, 4.4 kcal/mol (corrected to 298 K) more stable than the C-H protonated form 3. The proton affinity of ethane to give 1 (142.5 kcal/mol) is 12.5 kcal/mol greater than that of methane (130 kcal/mol). Methane adds to the methyl cation, leading to 1 without activation energy. Barriers for intramolecular hydrogen interchange are lower than the dissociation energy into the ethyl cation and hydrogen, consistent with the experimental observation that deuterium scrambling is faster than dissociation. C2H7+ loses H-2 by 1,1-elimination in an endothermic (10.6 kcal/mol) process. Three frequencies deduced experimentally for C2H7+ correspond to those computed for 1, but neither 2, the H-2-rotatd C-H protonated form, nor 3 can explain the other set of experimental spectral data. Complexes between H-2 and bridged C2H5+ were located, but they are too weakly bonded to be detected experimentally.