physics

Outer sphere electron transfer

Outer sphere electron transfer is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Outer sphere electron transfer rather than just read about it. In short: Outer sphere refers to an electron transfer (ET) event that occurs between chemical species that remain separate and intact before, during, and after the ET event. In contrast, for inner sphere electron transfer the participating redox sites undergoing ET become connected by a chemical bridge.

Key takeaways

Outer sphere electron transfer belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Outer sphere electron transfer to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Outer sphere electron transfer from memory before moving on to harder problems.

Reference excerpt

Outer sphere refers to an electron transfer (ET) event that occurs between chemical species that remain separate and intact before, during, and after the ET event. In contrast, for inner sphere electron transfer the participating redox sites undergoing ET become connected by a chemical bridge. Because the ET in outer sphere electron transfer occurs between two non-connected species, the electron is forced to move through space from one redox center to the other.

Marcus theory The main theory describing the rates of outer sphere electron transfer was developed by Rudolph A. Marcus in the 1950s, for which he was awarded the Nobel Prize in Chemistry in 1992. A major aspect of Marcus theory is the dependence of the electron transfer rate on the thermodynamic driving force (difference in the redox potentials of the electron-exchanging sites). For most reactions, the rates increase with increased driving force. A second aspect is that the rate of outer sphere electron-transfer depends inversely on the "reorganizational energy." Reorganization energy describes the changes in bond lengths and angles that are required for the oxidant and reductant to switch their oxidation states. This energy is assessed by measurements of the self-exchange rates (see below). Outer sphere electron transfer is the most common type of electron transfer, especially in biochemistry, where redox centers are separated by several (up to about 11) angstroms by intervening protein. In biochemistry, there are two main types of outer sphere ET: ET between two separate biological molecules or fixed distance electron transfer, in which the electron transfers within a single biomolecule (e.g., intraprotein).

Examples

Self-exchange Outer sphere electron transfer can occur between chemical species that are identical except for their oxidation state. This process is termed self-exchange. An example is the degenerate reaction between the tetrahedral ions permanganate and manganate:

[MnO4]− + [Mn*O4]2− → [MnO4]2− + [Mn*O4]− For octahedral metal complexes, the rate constant for self-exchange reactions correlates with changes in the population of the eg orbitals, the population of which most strongly affects the length of metal-ligand bonds:

For the [Co(bipy)3]+/[Co(bipy)3]2+ pair, self exchange proceeds at 109 M−1s−1. In this case, the electron configuration changes from Co(I): (t2g)6(eg)2 to Co(II): (t2g)5(eg)2. For the [Co(bipy)3]2+/[Co(bipy)3]3+ pair, self exchange proceeds at 18 M−1s−1. In this case, the electron configuration changes from Co(II): (t2g)5(eg)2 to Co(III): (t2g)6(eg)0.

Iron-sulfur proteins Outer sphere ET is the basis of the biological function of the iron-sulfur proteins. The Fe centers are typically further coordinated by cysteinyl ligands. The [Fe4S4] electron-transfer proteins ([Fe4S4] ferredoxins) may be further subdivided into low-potential (bacterial-type) and high-potential (HiPIP) ferredoxins. Low- and high-potential ferredoxins are related by the following redox scheme:

Because of the small structural differences between the individual redox states, ET is rapid between these clusters.

Worked examples

Example 1 — a first encounter with Outer sphere electron transfer

Start with the simplest possible case. Write down what Outer sphere electron transfer claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Outer sphere electron transfer before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Outer sphere electron transfer ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Outer sphere electron transfer

In research: Outer sphere electron transfer appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Outer sphere electron transfer in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Outer sphere electron transfer is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electron, Physical chemistry, so understanding it makes those chapters shorter.
In everyday life: Look for Outer sphere electron transfer outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Outer sphere electron transfer in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Outer sphere electron transfer means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Outer sphere electron transfer out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Outer sphere electron transfer in simple terms?

Why does Outer sphere electron transfer matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Outer sphere electron transfer?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Outer sphere electron transfer.