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Metal-organic charge transfer can produce biradical states and is mediated by conical intersections
Oksana Tishchenko, Ruifang Li, Donald G. Truhlar and Nicholas J. Turro
Proceedings of the National Academy of Sciences of the United States of America
Vol. 107, No. 45 (November 9, 2010), pp. 19139-19145
Published by: National Academy of Sciences
Stable URL: http://www.jstor.org/stable/25748640
Page Count: 7
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The present paper illustrates key features of charge transfer between calcium atoms and prototype conjugated hydrocarbons (ethylene, benzene, and coronene) as elucidated by electronic structure calculations. One- and two-electron charge transfer is controlled by two sequential conical intersections. The two lowest electronic states that undergo a conical intersection have closed-shell and open-shell dominant configurations correlating with the 4s² and 4s¹3d¹ states of Ca, respectively. Unlike the neutralionic state crossing in, for example, hydrogen halides or alkali halides, the path from separated reactants to the conical intersection region is uphill and the charge-transferred state is a biradical. The lowest-energy adiabatic singlet state shows at least two minima along a single approach path of Ca to the π system: (i) a van der Waals complex with a doubly occupied highest molecular orbital, denoted ϕ²₁, and a small negative charge on Ca and (ii) an open-shell singlet (biradical) at intermediate approach (Ca/C distance ≈2.5–2.7 Å) with molecular orbital structure ϕ₁ϕ₂, where ϕ₂ is an orbital showing significant charge transfer form Ca to the π-system, leading to a one-electron multicentered bond. A third minimum (iii) at shorter distances along the same path corresponding to a closed-shell state with molecular orbital structure ϕ²₂ has also been found; however, it does not necessarily represent the ground state at a given Ca/C distance in all three systems. The topography of the lowest adiabatic singlet potential energy surface is due to the one- and two-electron bonding patterns in Ca-π complexes.
Proceedings of the National Academy of Sciences of the United States of America © 2010 National Academy of Sciences