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
✍ Scribed by Xiaohua Chen; Dianxiang Xing; Liang Zhang; Robert I. Cukier; Yuxiang Bu
- Publisher
- John Wiley and Sons
- Year
- 2009
- Tongue
- English
- Weight
- 554 KB
- Volume
- 30
- Category
- Article
- ISSN
- 0192-8651
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✦ Synopsis
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 (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