A theoretical study of the H2 elimination from C6H7+:: An example of the role of back-donation in carbocation chemistry

C(6)H(7)(+) has been investigated by using the ab initio method at MP2-FC/6-31G* level and single point calculations on the MP2 geometries at the MP4(SDTQ)-FC/6-31G* level. The most stable structure located corresponds to the benzenium ion C(6)H(7)(+) in which a proton is bonded to one of the carbon atoms of the benzene ring and an extension of the delocalization has taken place to include the attached proton, the pi-electron density distribution being basically allylic in agreement with experimental findings. Two transition structures (TSs) for hydrogen scrambling were found on the C(6)H(7)(+) PES. The first one corresponds to the peripheral migration of a proton in the benzenium ion and determines an energy barrier of 7.8 kcal/mol at MP2 level (10.8 at MP4/MP2 level) in good agreement with experimental data. The second scrambling TS corresponds to the interchange of the two hydrogen atoms bonded to the same carbon atom in the ring of the benzenium ion and presents an energy barrier of 59.2 kcal/mol at MP2 level (62.6 at MP4/MP2 level). The Hz-elimination is an endoergic process that proceeds through a very late TS and an intermediate very close in energy to it that finally renders the products, C(6)H(5)(+) + H(2), without any energy barrier. The activation energy for this process (70.3 and 70.1 kcal/mol at MP2 and MP4/MP2 levels, respectively) is practically its endoergicity (71.1 kcal/mol at MP2 level and 70.8 kcal/mol at MP4/MP2 level) in accordance with the experimental finding of null energy release as relative motion between the fragments. A configuration analysis of the wave function of C(6)H(7)(+) along the reaction coordinate for H(2)-elimination clearly shows that the back-donation from the NHOMO of C(6)H(5)(+) to the LUMO of H(2) plays a fundamental role in the bonding structure of the benzenium ion. The H(2) elimination takes place through deactivation of the back-bonding at an early stage in the process.