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Abstract
Excited state hydrogen (ESHT) and proton (ESPT) transfer pathways in the solvent clusters of 6-azaindole 6AI-S3,4 and 2,6-diazaindole 26DAI-S3,4 (S=H2O, NH3) were computationally explored to understand the fate of photo-excited biomolecules. The ESHT energy barriers in (H2O)3 complexes (39.6-41.3 kJmol-1) were decreased in (H2O)4 complexes (23.1-20.2 kJmol-1). Lengthening the solvent chain reduced the barrier because of the relaxed transition states geometries with reduced angular strains. Replacing the water molecule with ammonia drastically decreased the energy barriers to 21.4-21.3 kJmol-1 in (NH3)3 complexes and 8.1-9.5 kJ mol-1 in (NH3)4 complexes. The transition state was identified as Ha atom attached to the first solvent molecule. The formation of stronger hydrogen bonds in (NH3)3,4 complexes resulted in facile ESHT reaction than that in the (H2O)3,4 complexes. The ESPT energy barriers, in 6AI-S3,4 and 26DAI-S3,4 are found to range between 40-73 kJmol-1. The above values were significantly higher than that of the ESHT processes and hence are considered a minor channel in the process. The energetics of ESHT and ESPT explored in this study would be of great importance to study the photochemistry of N-rich biomolecules in the presence of various protic environments.
Supplementary materials
Title
Solvent Mediated Excited State Hydrogen Transfer in 6-Azaindole - S3,4 and 2,6-Diazaindole - S3,4 Clusters (S= H2O, NH3)
Description
In the present article, we have investigated excited state hydrogen and proton transfer pathways in 6-azaindole-S3,4 and 2,6-diazaindole-S3,4 (S= H2O, NH3) complexes to understand the underlying aspects of hydrogen atom relay between the donor and acceptor groups in the photo-excited biomolecules through a solvent wire. We have investigated the variation of ESHT and ESPT energy barriers by (a) lengthening the solvent wire, (b) changing the solvent characteristics and (c) substitution with a N atom. We have performed relaxed potential energy surface calculations in the electronic S1 (ππ*) state to find the hydrogen atom transfer barriers for eight solvent clusters. . Our results reveal that severe geometrical constrains is the reason for the higher energy barriers in the S3 complexes, which is significantly reduced upon lengthening of solvent chain length. We have also reported the effect of extra N on the photophysical processes of azaindole derivatives for the very first time.
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