H-bonded N-heterocyclic base-pair phototautomerizational potential barrier and mechanism: The 7-azaindole dimer

doi:10.1073/pnas.0306721101

H-bonded N-heterocyclic base-pair phototautomerizational potential barrier and mechanism: The 7-azaindole dimer

^†Departamento de Quúmica Física Aplicada, C-2-203, Universidad Autónoma de Madrid, Canto Blanco, E-28049 Madrid, Spain; and ^‡Institute of Molecular Biophysics and Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32306-4390

Contributed by M. Kasha, November 7, 2003

Abstract

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 S₁ 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 S₁ 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.

Footnotes

↵ § To whom correspondence should be addressed. E-mail: kasha{at}chem.fsu.edu.
Abbreviations: PT, proton transfer; ESDPT, excited-state double PT; 7AI, 7-azaindole; PE, potential energy; QM, quantum mechanical; CI, configuration interaction; DFT, density functional theory; ZPE, zero point energy.