Ab initio calculations (4-31G basis set at CIS and CIS-MP2 levels) were carried out to investigate the nature of the double proton-transfer process of the 7-azaindole base-pair in both S0 and S1 states. The result is in agreement with the stepwise mechanism recently observed for S1, and reveals a diffuse transition state for the reaction in S0. The stepwise nature of the phototautomerization originates from a localized electronic excitation in one part of the pair. Compression of the internal hydrogen bonds is crucial for the occurrence of the reaction in both states.
Douhal, A., Guallar, V., Moreno, M., & Lluch, J. M. (1996). Theoretical study of molecular dynamics in model base pairs. Chemical Physics Letters, 256(4-5), 370-376. https://doi.org/10.1016/0009-2614(96)00457-5