The very thermally stable, but photochemically sensitive radical, 4,5-bis(trifluoromethyl)-l,3,2-dithiazolyl has been prepared, isolated, and fully characterised including a gas phase structure, and found t o be paramagnetic in the liquid state at room temperature; the X-ray structure of benzo-l,3,2-dithiazolyl has been obtained for comparison.

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The new thermally stable paramagnetic liquid 5-t-butyl-l,3,2,4-dithiadiazolyI has been isolated in the dark and quantitatively photochemically isomerised to the paramagnetic liquid 5-t-butyl-2,3,1,4-dithiadiazolyl (non-systematic numbering for ease of comparison).Thermally stable, neutral, free-radical, non-metal compounds can be categorised by the state of lowest energy in which they remain substantially paramagnetic. (i) Paramagnetic solids: t radicals which are paramagnetic in all states. These include the sterically hindered radicals such as diphenylpicrylhydrazyl (DPPH),3 (C6C15)3C,4 and the non-sterically hindered 0 2 and C102.5 (ii) Paramagnetic liquids: radicals which are diamagnetic dimers in the solid state but are extensively dissociated upon melting, e.g. But2N0,6 (CF3)2N0,7 and C F 3 m C F 3 . g (iii) Paramagnetic gases: radicals which are t There are a number of examples of ionic paramagnetic solids, e.g.

Die durch Cycloaddition von (I) und (I I) erhaltenen Verbindungen (IIIa) und (IIIb) sowie (IIIc) werden zu den Radikalen (IVa) ‐ (IVc) reduziert, die sich thermisch zu den Verbindungen (V) isomerisieren.

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