Excited States with the Hydrogen Bond in the Reaction of Aromatic Dianhydrides with Diamines

摘要

Polycondensation of aromatic dianhydrides and diamines is widely used for the synthesis of heat-resistant polymers, polyimides [1]; however, the mechanism of its elementary steps has not been adequately studied. An important problem to be solved concerns the role of the hydrogen bond in proceeding of this reaction. It is expedient to consider this problem in terms of electron excitation in the elementary chemical reaction [2] with the use of specific mechanisms of the electron—proton effect [3, 4]. It is suggested that the interaction of the π-electron systems of the diamine (donor) and dianhydride (electron acceptor) in the excited state through hydrogen bonding leads to a considerable decrease in the energy of the electronic level and its displacement into the thermal energy range (less than 1 eV). The optical charge transfer (CT) transitions for complexes of aromatic diamines and dianhydrides are observed in the visible spectral range 13 000-21000 cm"1 [5]. Recently [2], we put forward the idea that the reaction states have the significance of equilibrium electronically excited states with energies close to the experimental activation energies E_a. For the reaction of pyromellitic dianhydride (PM) with m-phenylenediamine (m-PD), the activation energy is 6—8 kcal/mol [6]. In this context, a problem arises to theoretically estimate the energy of equilibrium excited states and, simultaneously, to determine the role of the electron—proton effect, as a factor of chemical catalysis, due to the interplay of the electron donor—acceptor interaction and hydrogen bond that occurs in electron excitation events.