Single- versus Multi-Proton-Coupled Rydberg-State Electron Transfer in Amine Clusters

Abstract

Amino fragments (−NH2) are well-known to exist widely in biological systems and their protonated forms are inclined to trap electrons and form Rydberg radicals (−NH3•) in the electron-excess systems. Taking CH3–NH3+ as a mimicking group of the protonated alkylamine side-chain of lysine, ab initio calculations indicate that the proton/electron cooperatively transfer from CH3NH3 to CH3NH2 via a single-proton-coupled Rydberg-state electron transfer (ET) mechanism with an Rydberg-orbital channel for ET outside the −NHn hydrogens and a N–H+ → N proton migrating pathway. Besides, in big amine clusters, CH3NH3·(NH3)n·NH2CH3 (n = 1–3), the proton/electron transfer along an amine wire is stepwise and every step takes place via the similar single-proton-coupled Rydberg-state ET mechanism with low energy barrier (<4.0 kcal/mol). When a water chain, (H2O)n (n = 1–3), lies between CH3NH3 and NH2CH3 as a bridge, the energy barriers (8.5–15.0 kcal/mol) of proton/electron cooperatively transfer between CH3NH3 and NH2CH3 are raised significantly as compared to these of the pure amine wires (<4.0 kcal/mol). We attribute this fact to the combined effects of the proton binding energies and electron affinities of CH3NH2 and H2O. Interestingly, different from the amine-wire case, movement of the solvated electron along the water-wire can promote two or three protons synchronously moving at the same direction. This process can be described in terms of a multi-proton-coupled solvated-ET mechanism.