H-bonded N-heterocyclic base-pair dhototautomerizational notential barrier and mechanism:: The 7-azaindole dimer

A theoretical analysis of the double proton transfer (PT) in a hydrogen-bonded N-heterocyclic base pair is presented. The calculated (time-dependent density functional theory) double PT barrier calculated for the concerted process of the 7-azaindole C-2h dimer in the first excited singlet electronic state S-1 conforms well to the kinetic data and the photophysical evidence reported in this article. The calculated PT energy barrier of 4.8 kcal/mol height, and the corresponding zero point energy value, yield for the S-1 state an activation energy barrier of 0.3 kcal/mol. This finding implies that the double PT concerted process is almost barrierless, confirming previous experiments. Upon N-H deuteration of the 7-azaindole dimer, the theoretical excited-state activation energy for the double deuterium transfer is determined to be 1.4 kcal/mol, in agreement with experiment, which in low-temperature spectroscopy is shown to negate excited-state double-deuteron transfer.