Abstract
The 7-azaindole (7AI) dimer is found to experience excited state double proton transfer (ESDPT) reaction to become its tautomer dimer in the first singlet excited state. The hydrogen transferred 7AI dimer, which is in the first singlet excited state, subsequently relaxes to its ground state. The 7AI tautomeric dimer is not stable in the ground state and soon converts to the original reactant, the normal 7AI dimmer, through ground state reverse proton transfer (GSRPT) reaction. The cyclic reaction is viewed as the model of photoinduced mutation in the deoxyribonucleic acid (DNA) helix. The analogous cyclic reaction is also happened to the 7AI/alcohole, 7AI/carboxylic acid and other complexes, where the additional molecule in the complex serves as catalyst and the reaction mainly locates in 7AI. To explore the feasibility of the cyclic reaction in the DNA, the three and four nitrogen atoms contained azoheterocyclic molecules and their acid complexes are investigated. The first singlet excited states, the first triplrt state and the ground state of monomers and complexes are computed by the CASSCF method with 6-311G++** and 6-31G++** basis sets. The B3LTP method accompanied by cc-pVTZ and 6-311G++** basis sets is also applied to calculate the first triplet states and the ground state of the monomers and complexes. All the energies of the first singlet excited states of the tautomeric monomers and complexes are lower than those of the normal monomer and complexes. The energies of the ground states of all the tautomeric monomers and complexes are also found to be higher than those of the normal monomers and complexes. Thus, the ESDPT and GSRPT reactions are suggested to occur in all systems. The first triplet states are not involved in the cyclic reaction. The energies of the first singlet excited states increase as the number of nitrogen atom contained in the azoheterocycic molecules increases. On the contrast, the energies of the ground states decrease as the number of nitrogen atoms contained decreases. The hydrogen bonding energies between azoheterocyclic molecules and formic acid are about -11 kcal/mol for the ground states and -12 kcal/mol for the first singlet excite states in the normal complexes. In the case of tautomeric complexes, the hydrogen bonding energies are about -14.5 kcal/mol for the ground states and -10 kcal/mol for the first singlet excited states.