Theoretical investigation on the water-assisted excited-state proton transfer of 7-azaindole derivatives: substituent effect.

A systematic study on the first excited-state proton transfer (ESPT) in 2RAI-H2O (R = OH, OCH3, CN, NO2, CHO) complexes in solution were investigated at the TD-M06-2X/6-31 + G(d, p) level. The double proton transfer occurred in an asynchronous but concerted protolysis fashion no matter which substituent R was used at C2 position in pyrrole ring in the 7AI-H2O complex. The specific vibrational-mode of ESPT in the model systems was confirmed and contributed to promote the reaction rate by shortening the reaction path. The substituent effects of different groups on the ESPT thermodynamics and kinetics were discussed. It was obvious that the geometries, barrier height, asynchrony, and specific vibration-mode of excited-state proton transfer changed with the different substituent groups. Graphical Abstract The distance between two neighboring heavy atoms such as N1-O11 (R1) and O11-N8 (R2) distances played an important role in the proton transfer reaction. The sum of the N1-O11 and O11-N8 distances in the reactant of 2RAI-H2O (R=H, OH, OCH3; CN, CHO, NO2) complexes is in the range of 5.542 Å~5.692 Å and changes along with the substituent group at C2 position in the pyrrole ring. The ESDPT barrier height and the sum of the N1-O11 and O11-N8 distances have a good correlation.