Theoretical study of molecular dynamics in model base pairs

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.