Kinetic Energy Release and Position of Transition State during Intramolecular Aromatic Substitution in Ionized 1-Phenyl-1-(2-pyridyl)ethylenes

Article abstract of DOI:10.1021/ja00536a034

The loss of substituents (X = H, CH3, Cl, Br, I) from the molecular ions of ortho-substituted 1-phenyl-1-(2-pyridyl)ethylenes 1a-f and of the isomeric 1-phenyl-1-(3-pyridyl)- and 1-phenyl-1-(4-pyridyl)ethylenes 2 and 3 has been investigated.Cyclic fragment ions a are formed from the ortho-substituted 1-phenyl-1-(2-pyridyl)ethylene molecular ions by an intramolecular aromatic substitution reaction.The energetic requirements of this reaction have been studied in dependence from the dissociation energy of the C-X bond by measurements of the ionization energies, appearance energies, and kinetic energies released during the reaction.The activation energy ε_h of the process varies only slightly with the dissociation energy of the C-X bond cleaved during the reaction, whereas the entalpy of reaction changes from positive (endothermic) to very negative (exothermic) values in the reaction series 1a-f.Consequently the reverse activation energy ε_r ranges from small to very large values in this series.This trend in ε_r is not followed by the kinetic-energy release.A large kinetic-energy release and energy partitioning quotient q = 0.7 - 1.0 is only observed for endothermic or thermoneutral processes, while a small kinetic-energy release and q ca. 0.2 is associated with exothermic reactions in spite of a large ε_r.This behavior has been correlated to the position X_o^* of the transition state on the reaction coordinate according to Miller's quantification of the Hammond postulate.The release of ε_r as kinetic energy is only observed for reactions with "symmetrical" or "late" transition states (X_o^* > 0.4) while most of ε_r remains as internal energy in the products of reactions with "early" transition states (X_o^* < 0.4).