title: Effect of metal ions on radical type and proton-coupled electron transfer channel: sigma-radical vs pi-radical and sigma-channel vs pi-channel in the imide units.
abstract: The mechanism of proton transfer (PT)/electron transfer (ET) in imide units, and its regulation by hydrated metal ions, was explored theoretically using density functional theory in a representative model (a nearly planar and cisoid complex between uracil and its N(3)-dehydrogenated radical, UU). In UU (sigma-radical), PT/ET normally occurs via a seven-center, cyclic proton-coupled sigma-electron sigma-channel transfer (PC(sigma)E(sigma)T) mechanism (3.8 kcal/mol barrier height) with a N(3)-->N(3') PT and an O(4)-->O(4') ET. Binding of hydrated metal ions to the dioxygen sites (O(2)/O(2') or/and O(4)/O(4')) of UU may significantly affect its PT/ET cooperative reactivity by changing the radical type (sigma-radical <--> pi-radical) and ET channel (sigma-channel <--> pi-channel), leading to different mechanisms, ranging from PC(sigma)E(sigma)T, to proton-coupled pi-electron sigma-channel transfer (PC(pi)E(sigma)T) to proton-coupled pi-electron pi-channel transfer (PC(pi)E(pi)T). This change originates from an alteration of the ordering of the UU moiety SOMO/HDMO (the singly occupied molecular orbital and the highest doubly occupied molecular orbital), induced by binding of the hydrated metal ions. It is a consequence of three associated factors: the asymmetric reactant structure, electron cloud redistribution, and fixing role of metal ions to structural backbone. The findings regarding the modulation of the PT/ET pathway via hydrated metal ions may provide valuable information for a greater understanding of PT/ET cooperative mechanisms, and an alternative way for designing imide-based molecular devices, such as molecular switches and molecular wires.
authors.affiliation: The Center for Modeling & Simulation Chemistry, Institute of Theoretical Chemistry, Shandong University, Jinan 250100, People's Republic of China.
authors.forename: Xiaohua
authors.initials: X
authors.lastname: Chen
authors.forename: Dianxiang
authors.initials: D
authors.lastname: Xing
authors.forename: Liang
authors.initials: L
authors.lastname: Zhang
authors.forename: Robert I
authors.initials: RI
authors.lastname: Cukier
authors.forename: Yuxiang
authors.initials: Y
authors.lastname: Bu
mesh_terms: D004579:Electron Transport; D007094:Imides / Q000737:chemistry*; D007477:Ions / Q000737:chemistry; D008670:Metals / Q000737:chemistry*; D008958:Models, Molecular; D011522:Protons; D011789:Quantum Theory
chemical_list: D007094:Imides; D007477:Ions; D008670:Metals; D011522:Protons