Ab initio reaction path energetics for the CX dissociations of C6H5X+ with X = H, F, Cl, and Br

The energetics of the CX dissociations of C6H6+, C6H5F+, C6H5Cl+, C6H5Br+, and C10H8+ are investigated with various ab initio quantum chemical simulators. The primary focus is the determination of the energetics as a function of the CX bond length at the MP2 level for the C6H5X+ dissociation paths leading to the lowest energy products [C6H5+((1)A(1)) + X]. The bending force constants along this path are determined at either the HF or MP2 level. The absolute energetics of the various reactants and products are also studied at the MP2 and B3LYP levels including the dissociation of C10H8+. A combination of CCSD(T)/6-31G* and MP2/6-311G(2df,2p) calculations are employed in the determination of higher level estimates for the key reactants and products in the dissociation of the benzene cation. The key dissociation paths for this dissociation are also studied at a higher level (CASPT2). For the halogen-producing dissociations the energetics are investigated for each of the three separate orientations of the radical p-orbital in the halogen atom. The various results obtained illustrate the increasing strength of the long-range interactions for the progression from H to F to Cl to Br. Correspondingly, one expects the transition state to be more and more phase space theory like for this same progression. For the C-Br dissociation it seems very unlikely that short-range interactions will play any role in determining the reactive flux. In contrast, short-range repulsions are again expected to play an important role for the C-H dissociations in both C6H6+ and C10H8+. The present results also indicate the dominant importance of the singlet phenyl cation channel to the dissociation dynamics. (C) 1997 Elsevier Science B.V.