Energetic and electron density analysis of hydrogen dissociation of protonated benzene

Abstract

The electronic structure of the benzenium cation, [C6H7]+, the simplest intermediate of electrophilic aromatic substitution reactions, was analyzed in terms of the properties of electron densities obtained from multiconfigurational quantum theoretical methods. The indirect C-H coupling constants and the physical contributions to their values were calculated and rationalized in terms of the electron delocalization between the quantum topological atoms in the molecule. The evolution of the electronic structure for the intramolecular proton migration and for the dissociation into [C6H6]+ + H, or [C6H5]+ + H2 was also studied. The potential energy surface for intramolecular H migration has six equivalent transition states and two equivalent two-fold saddles, whereas each dissociation process occurs without the presence of any transition state. The calculated energy barriers of 9.5, 80.3 and 72.4 kcal mol(-1) for the intramolecular proton migration, H and H2 eliminations, respectively, agree with experimental reports. The quantitative chemical descriptors based on the electron density of the benzenium cation provide insight on the nature of the chemical bond, including electron delocalization, and its evolution during chemical transformations of the molecule.

Application of Electron Delocalization Indicators in the Study of Electrophilic Aromatic Substitution Reactions

“…The study in reference [49] showed a relationship between the Fermi contact contribution of calculated coupling constants J C H and the electron delocalization index (C,H) . The computation of the average J C H yielded a value of 30 Hz in good agreement with the experimental value of 26 Hz.…”

“…For instance, while the equilibrium structure of the complex [C 6 H 6 ... Br 2 ] has the Br 2 molecule close to the rim of the aromatic ring, the system [C 6 H 6 ... NO + ] has the electrophile in top of the center of the benzene molecule [70]. Figure 11 and it has been demostrated that they correspond to saddle points in the PES of C 6 H 7 + [49]. In spite of the fact that the structures in Figure 11 are not minima of the PES of C 6 H 7 + , the topological analysis of these systems [49] along with that of the benzenium cation in subsection 3.2.1 provides information about the changes in the electronic delocalization along the prototype mechanism of an EAS (Figure 3).…”

“…Figure 11 and it has been demostrated that they correspond to saddle points in the PES of C 6 H 7 + [49]. In spite of the fact that the structures in Figure 11 are not minima of the PES of C 6 H 7 + , the topological analysis of these systems [49] along with that of the benzenium cation in subsection 3.2.1 provides information about the changes in the electronic delocalization along the prototype mechanism of an EAS (Figure 3). The migration of a proton from the ipso to the ortho carbon reduces considerably the bond order of the C-C bond between the ortho and meta carbons whereas it increases the delocalization and the value of ( r ) at the bcp between the ipso-ortho and the meta-para carbons as shown in Figure 11(a).…”

“…The NMR experimental studies of the benzenium ion are complemented by the examination of the electron delocalization of this species as determined by the topology of ( r ) at the MRMP2/ 6-311++G(d,p)//CASSCF(6/6)/6-311++G(d,p) level of theory [49]. The study in reference [49] showed a relationship between the Fermi contact contribution of calculated coupling constants J C H and the electron delocalization index (C,H) .…”

Photodissociation Dynamics of ClCN at Different Wavelengths

“…[1][2][3][4][5][6][7] Most rearrangement reactions also involve at least one disruption of a bond. [8][9][10][11][12][13] Such bond dissociations are mainly of "chemical" nature and are characterized by electron redistribution [14][15][16][17] through availability of more favorable bonding partners or through external driving forces such as environmental conditions (solvent or temperature). [18][19][20] Furthermore, many atmospheric reactions are also of this type.…”