Deciphering the Differential Origin of Hydrogen Bonds in the Normal and Tautomer Forms of 7‐Azaindole:Piperidin‐2‐one Hydrogen‐Bonded Complex: Excited‐State Double Proton Transfer

Abstract

AbstractThe excited‐state double proton transfer (ESDPT) reaction in the intermolecular hydrogen‐bonded complex 7‐azaindole:piperidin‐2‐one is investigated by quantum chemical calculations with particular emphasis on the H‐bonding interactions within the system. The natural bond orbital (NBO) calculations show that the X─H···Y H‐bonds in the studied complexes, namely, 7‐azaindole:piperidin‐2‐one (N‐form) and 7H‐pyrrolo[2,3‐b]pyridine:3,4,5,6‐tetrahydropyridin‐2‐ol (T‐form, proton transferred tautomer) are aptly described by hyperconjugative charge transfer effect ( is non‐bonded lone‐pair on Y‐atom). That the hyperconjugation effect outplays the rehybridization effect is invoked to account for the observed red‐shifting H‐bonds in harmony with the Bent's rule. The differential nature of interactions underlying the origin of X─H···Y H‐bonds in the complexes (N‐form and T‐form) is explored from atoms‐in‐molecules (AIM) calculations. The H‐bonds in the N‐form are found to have a primarily electrostatic origin (closed‐shell interaction) whereas the relatively stronger H‐bonds in the T‐form are assisted by contribution from shared‐shell (covalent) interaction. The nonoccurrence of the double‐proton transfer reaction in the S0 state is argued from the large barrier for the transformation N‐form → T‐form (N‐form constitutes the global minimum on the S0‐PEC), whereas the stability order is reversed together with a marked reduction of the energy barrier in the S1‐PEC accounting for the ESDPT process.