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Context: a retinal molecule the isomerizes from all-trans to 13-cis form, then "reisomerizes thermally".

I tried googling it but couldn't find a simple, straightforward explanation.

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The term "thermally" is used here to contrast with a photochemical isomerization. In the chemistry of vision, 11-cis retinal absorbs light and is isomerized to the all-trans isomer. This is a photochemical process because it involves the absorption of a photon to promote one electron into an antibonding orbital. In this excited state, the geometry can rearrange to the more stable all-trans form before the electron relaxes back to the ground state.

In a "thermal" process, no photons are absorbed, no electrons are promoted, and no excited states are needed. The molecule takes advantage of ambient or supplied kinetic energy to reorganize.

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