Excited-State Double-Proton Transfer in the 7-Azaindole Dimer in the Gas Phase.2.Cooperative Nature of Double-Proton Transfer Revealed by H/D Kinetic Isotopic Effects

摘要

The dispersed fluorescence (DF) spectra of the 7-azaindole dimer (7AI_2) and deuterated dimers 7AI_2-hd and 7AI_2-dd,where hd and dd indicate the deuteration of an imino proton and two imino protons,have been measured in a supersonic free jet expansion.The undeuterated 7AI_2-hh dimer exhibits only the tautomer fluorescence,but both the normal and tautomer fluorescence have been detected by exciting the origins of 7AI_2-h~*d,7AI_2-hd~* and 7AI_2-dd in the S_1-S_0 region,where h~* and d~* indicate the localization of the excitation on 7AI-h or 7AI-d moiety.The DF spectra indicate that 7AI_2-h~*d and 7AI_2-hd~* undergo excited-state proton/ deuteron transfer (ESPDT),while excited-state double-deuteron transfer (ESDDT) occurs in 7AI_2-dd.The H/D kinetic isotopic effects on ESDPT have been investigated by measuring the intensity ratios of the normal fluorescence to the tautomer fluorescence.The ESPDT rate is about l/60th of the ESDPT rate,and the ESDDT rate is about 1/12th of the ESPDT rate,where ESPDT rate is an average of the rates for 7AI_2-h~*d and 7AI_2-hd~*.The observed H/D kinetic isotope effects imply that the ESDPT reaction of 7AI_2 has a "cooperative" nature;i.e.,the motion of the two moving protons strongly couples each other through the electron motions.The difference in the estimated ESPDT reaction rates,9.8 x 10~9 and 6.9 x 10~9 s~(-1) for 7AI_2-h~*d and 7AI_2-hd~*,respectively,is consistent with the concerted mechanism rather than the stepwise mechanism.