Multiprotonation of Benzene: A Theoretical Study

Abstract

The lower-energy portion of the potential energy surfaces of the multiprotonated benzenes are thoroughly studied at the MP2(full)/6-311++G(d,p) computational level. It is shown together with its monoprotonation results, which are well studied both experimentally and theoretically, that benzene admits di- and even triprotonation whereas the tetra one destabilizes it by opening the benzene ring. In particular, the search of the potential energy surface of the monoprotonated benzene reveals few subtle structures related to the two protonation channels, viz., C6H6 + H+ ⇌ C6H7+ and C6H6 + H+ → C6H5+ + H2, reported in the present work for the first time. In the former reaction, the benzenium cation may exist in two conformers. The most stable conformer is the canonical C2v conformer involving σ-type of bonding. The second one, bicyclo[3.1.0] hexenyl cation, is less stable by 18.14 kcal/mol. The former conformer is separated from the van der Waals complex by the transition structure [BzH+]tr, wherein the excess proton is at a distance of ∼3.18 Å away from the nearest carbon atom of benzene. The other reaction channel related to the H2 loss is underlied with two new structures. It is also found that the affinity of benzene to bind two excess protons is larger than its proton affinity by 36.9 kcal/mol. Seven lower-energy stable structures are identified in the present work on the potential energy surface of diprotonated benzene together with the transition and second-order saddle structures governing the single proton and diproton migration over the benzene ring. The lower energy portion of the triprotonated benzene consists of three minimum-energy structures, which demonstrates that the triprotonation of benzene must proceed simultaneously. An attempt has been made to characterize the vibrational spectra of protonated benzenes, which might facilitate their experimental detection.