Electron Transfer

Diabatic Trapping in BMA
The Mechanistic Spectrum of IET

"Intramolecular Electron Transfer in Bis(methylene) Adamantyl Radical Cation: A Case Study of Diabatic Trapping"

Lluís Blancafort, Patricia Hunt, and Michael A. Robb (J. Am. Chem. Soc.; 2005; 127(10) pp 3391 - 3399.)
DOI: 10.1021/ja043879h

Two different mechanistic processes in intramolecular electron transfer chemistry have been studied with the complete active space self consistent field quantum chemical method for a model bis(hydrazine) radical cation. These correspond to:

a chemical electron transfer, where a reaction intermediate or a transition structure exist with the charge localized on the linker, and
a non-adiabatic electron transfer, where the bridge remains neutral.

These processes coexist on the same potential energy surface. They are characterized by very different reaction co-ordinates and are thus distinct elements of the mechanistic spectrum of intramolecular electron transfer in organic radical cations. The energetically favoured chemical electron transfer process involves conventional reaction paths. In contrast, the non-adiabatic electron transfer process involves an unconventional reaction path, which connects reactant and products via an un-avoided (i.e. real) crossing seam (i.e. an (n-1)-dimensional intersection, where n is the number of vibrational degrees of freedom of the system) between two different adiabatic potential energy surfaces.

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"Potential Energy Surface Crossings and the Mechanistic Spectrum for Intramolecular Electron Transfer in Organic Radical Cations"

Lluís Blancafort, Franck Jolibois, Massimo Olivucci, and Michael A. Robb (J. Am. Chem. Soc.; 2001; 123(4) pp 722 - 732)
DOI: 10.1021/ja003359w

The structure of the potential energy surface for the intramolecular electron transfer (IET) of four different model radical cations has been determined by using reaction path mapping and conical intersection optimization at the ab initio CASSCF level of theory. We show that, remarkably, the calculated paths reside in regions of the ground state energy surface whose structure can be understood in terms of the position and properties of a surface crossing between the ground and the first excited state of the reactant. Thus, in the norbornadiene radical cation and in an analogue compound formed by two cyclopentene units linked by a norbornyl bridge, IET proceeds along a concerted path located far from a sloped conical intersection point and in a region where the excited state and ground state surfaces are well separated. A second potential energy surface structure has been documented for 1,2-diamino ethane radical cation and features two parallel concerted and stepwise paths. In this case a peaked conical intersection is located between the two paths. Finally, a third type of energy surface is documented for the bis(methylen)adamantane radical cation and occurs when there is effectively, a seam of intersection points (not a conical intersection) which separates the reactant and product regions. Since the reaction path cannot avoid the intersection, IET can only occur non-adiabatically. These IET paths indicate that quite different IET mechanisms may operate in radical cations, revealing an unexpectedly enriched and flexible mechanistic spectrum. We show that the origin of each path can be analyzed and understood in terms of the one-dimensional Marcus-Hush model.

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