The sterically hindered hydroxylamine TH is known as an anhydrous crystalline solid and as a crystal containing one water for every three TH units . In both cases, there are closed hydrogen bond networks, involving three TH units for the anhydrous structure and six for the trient-or 0.33-hydrate. In these structures, the hydroxylamine adopts a conformation where the hydrogen points to the open side (o) of the sterically hindered substituted nitroxyl unit and prefers to bind to the oxygen of a neighbouring hydroxylamine or water. To close the strained trimeric ring in the anhydrous crystal, the last TH binds to the N atom (instead of O) of the first unit. In the 0.33-hydrate, the N-binding is taken over by water molecules. There is thus a clear numerical preference for OH• • • O coordination and closed ring structures among TH units in the solid state. In this context, it is interesting to look at the structure of the TH-TH dimer, because calculations suggest an OH• • • N contact instead in the absence of further molecules (Fig. , bottom).