Biradicals are molecular species with two electrons occupying degenerate or nearly degenerate molecular orbitals with a pure open shell configuration. I can't seem to see a difference between this and a dianion (2-), yet in the literature there seem to be preferences among the terms. I can't tell if this is simply a semantic preference or if there is a meaningful difference. If a pi-conjugated small molecule (e.g. perylene diimide, etc.) is doubly reduced by a sacrificial reductant to its 2- state, would it be accurate to refer to this species as either a dianion or birdaical? Are the terms interchangeable, or is one term correct and the other incorrect? If one term is favored, why?
Dear Daniel Powell ,
Biradicals and dianions are completely different terms. As you said, a biradical is an open-shell species with two unpaired electrons, and it can be either a singlet or a triplet. On the other hand, a dianion is a species containing two extra electrons (the number of electrons exceeds the total nuclear charge in two units). There may be cases in which it may coincide that a dianion is also a biradical, but that is not compulsory.
Take the examples of benzene and oxygen. The first one, in its neutral ground state, is a neutral closed-shell molecule. If the dianion is formed, it can form an open-shell triplet biradical, an open-shell singlet biradical or even a closed-shell singlet! (I'm pretty sure the triplet biradical will be the most stable configuration). Oxygen works the other way around, it is a triplet biradical in its neutral ground state that can be a singlet biradical or a closed-shell in excited states, but the peroxide dianion is a closed-shell system. Please see the attached file.
Hope you find it helpful
- Badges
- Science topic
- Similar topics
- Engineering
- Electrical Engineering
- Electrodes