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.
The dispersed fluorescence (DF) spectra of the 7-azaindole dimer (7AI2) and deuterated dimers 7AI2-hd and 7AI2-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 7AI2-hh dimer exhibits only the tautomer fluorescence, but both the normal and tautomer fluorescence have been detected by exciting the origins of 7AI2-h*d, 7AI2-hd* and 7AI2-dd in the S1-S0 region, where h* and d* indicate the localization of the excitation on 7AI-h or 7AI-d moiety. The DF spectra indicate that 7AI2-h*d and 7AI2-hd* undergo excited-state proton/deuteron transfer (ESPDT), while excited-state double-deuteron transfer (ESDDT) occurs in 7AI2-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 1/60th 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 7AI2-h*d and 7AI2-hd*. The observed H/D kinetic isotope effects imply that the ESDPT reaction of 7AI2 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 109 s(-1) for 7AI2-h*d and 7AI2-hd*, respectively, is consistent with the concerted mechanism rather than the stepwise mechanism.
Sakota
Kenji
K
Department of Chemistry, Faculty of Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan.
Sekiya
Hiroshi
H
D003903:Deuterium / Q000737:chemistry*; D041961:Deuterium Exchange Measurement; D019281:Dimerization; D005740:Gases / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry*; D007211:Indoles / Q000737:chemistry*; D007554:Isotopes / Q000737:chemistry; D007700:Kinetics; D015394:Molecular Structure; D011522:Protons
C023422:7-azaindole dimer; D005740:Gases; D007211:Indoles; D007554:Isotopes; D011522:Protons; D006859:Hydrogen; D003903:Deuterium