Arresting intermediate states in a chemical reaction on a femtosecond time scale: proton transfer in model base pairs

A new concept to study the dynamics of chemical reactions in real time is presented using the phenomenon of Coulomb explosion to arrest and directly interrogate the intermediates in a chemical reaction. This technique is employed to study the tautomerization reaction of the model DNA base pair 7-azaindole, providing direct mass spectral evidence of a stepwise versus a concerted mechanism for the proton transfer upon excitation into the S1 state.