Theoretical evidence of barrier-free proton transfer in 7-azaindole-water cluster anions

Abstract

Water clusters of 7-azaindole (7AI) and its radical anion with up to three water molecules have been investigated by B3LYP and MP2 methods. While the adiabatic electron affinities (AEAs) of 7AI(H(2)O)(n=0,1) and the most stable configuration of 7AI(H(2)O)(2) were calculated to be negative, the AEAs of 7AI(H(2)O)(3) were found to be positive, consistent with the experimental observation that the cluster anions of 7AI(-)(H(2)O)(n) start to appear continuously in mass spectra when n > or = 3. However, some high-energy configurations of 7AI(H(2)O)(2) were found to have potential for capturing excess electrons and forming stable anions. The B3LYP approach was shown to systematically overestimate the AEA due to its insufficient description of buckling of conjugated ring induced by electron attachment. The computational results show that the activation energy of proton transfer in 7AI(-)(H(2)O)(n) decreases as the number of water molecule increases. For n = 3, electron attachment was found to induce a barrier-free proton transfer from water to 7AI(-), resulting in the formation of a neutral radical of protonated 7AI solvated by a water cluster of hydroxyl anion, OH(-)(H(2)O)(2). The protonated structures were found to be lower in energy than the fully tautomerized structures where the tautomeric 7AI radical anion is solvated by a neutral water cluster. In addition, the tautomeric structures were found to be kinetically unstable with respect to the reverse transformation to the protonated structures. These results indicate that the protonated configuration of 7AI(-)(H(2)O)(3) is the major species detected in molecular beam experiments. This conclusion was further confirmed by the calculations of vertical detachment energies of cluster anions. The van der Waals structures of 7AI(-)(H(2)O)(3), in which the water molecules locate over the 7AI conjugated ring and point their O-H bonds toward the pi-electron cloud, were explored as well. Comparison of the protonation energies for DNA base anions and 7AI anion suggests that analogous proton-transfer reactions might occur in the water clusters of DNA base anions with only few water molecules.