The molecular symmetry and electronic spectroscopy of 7-azaindole dimer: Its proton-transfer channels

The potential-energy surfaces for the proton transfer in the doubly hydrogen-bonded dimer of 7-azaindole in its lowest excited electronic states were examined. The dimer with C 2 h symmetry in its lowest excited electronic states, 2 A g and 1 B u , undergoes concerted double-proton transfer via transition states of the same symmetry placed at energies 4.55 and 4.70 kcal ∕ mol higher, respectively. This suggests that the activation barriers for the double-proton transfer, if any, are lower than 1 kcal ∕ mol . Emission from the dimers resulting from the double-proton transfer involves a Stokes shift of 5605 cm − 1 , as theoretically estimated from the 0-0 components of the absortion and emission transitions of the dimer. Surprisingly, however, the calculations suggest that the green emission cannot arise from the 2 A g state generated by a double-proton transfer, because this structure possesses an imaginary frequency. In the 7-azaindole dimer of C s symmetry, the first excited electronic state, a ′ , lies 4.9 kcal ∕ mol below 1 B u . This excited state a ′ can be the starting point for single-proton transfers giving a zwitterionic form that can dissociate into the protonated and deprotonated forms of 7-azaindole, the former being electronically excited. This situation of lower symmetry is consistent with the mutational scheme proposed by Goodman [ Nature (London) 378 , 237 ( 1995 ) ].