Effect of metal ions on radical type and proton‐coupled electron transfer channel: σ‐Radical vs π‐radical and σ‐channel vs π‐channel in the imide units

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 (σ‐radical), PT/ET normally occurs via a seven‐center, cyclic proton‐coupled σ‐electron σ‐channel transfer (PC σ E σ 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 (σ‐radical ↔ π‐radical) and ET channel (σ‐channel ↔ π‐channel), leading to different mechanisms, ranging from PC σ E σ T, to proton‐coupled π‐electron σ‐channel transfer (PC π E σ T) to proton‐coupled π‐electron π‐channel transfer (PC π E π 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. © 2009 Wiley Periodicals, Inc. J Comput Chem 2009