Excited state hydrogen atom transfer pathways in 2,7-diazaindole – S1-3 (S = H2O and NH3) clusters

The photoinduced tautomerization reactions via hydrogen atom transfer in the excited electronic state (ESHT) have been computationally investigated in 2,7-diazaindole (27DAI) – (H2O)1-3 and 27DAI – (NH3)1-3 isolated clusters to understand the role of various solvent wires. Two competing ESHT reaction pathways originating from the N(1)–H group to the neighbouring N(7) (R(1H-Sn-7H)) and N(2) (R(1H-Sn-2H)) atoms were rigorously examined for each system. Both one- and two-dimensional potential energy surfaces have been calculated in the excited state to investigate the pathways. The R(1H-Sn-7H) was found to be the dominant route with reaction barriers ranging from 26–40 kJmol−1 for water clusters, and 14–26 kJmol−1 for ammonia clusters. The barrier heights for ammonia clusters were found to be nearly half of the that observed for the water systems. The lengthening of the solvent chain up to two molecules resulted in a drastic decrease in the barrier heights for R(1H-Sn-7H). The barriers of the competing reaction channel R(1H-Sn-2H) were found to be significantly higher (31–127 kJmol−1) but were observed to be decreasing with the lengthening of the solvent wire as in the R(1H-Sn-7H) pathway. In both the reactions, the angle strain present in the transition state structures was dependent upon the solvent chain's length and was most likely the governing factor for the barrier heights in each solvent cluster. The results have also affirmed that the ammonia molecule is a better candidate for hydrogen transfer than water because of its higher gas-phase basicity. The results delineated from this investigation can pave the way to unravel the excited-state hydrogen atom transfer pathways in novel N–H bearing molecules. © 2021 Elsevier B.V