Ab initio calculations on the rearrangement aminonitrene ¿ trans-diimide are reported. It is shown that a bimolecular hydrogen exchange is much easier than a unimolecular 1,2 hydrogen shift: the reactions have activation energies of 4 and 94 kcal/mol, respectively. The hydrogen exchange starts from dimers which have a fairly large stabilization energy. Infrared and Raman frequencies of these dimers are calculated in order to facilitate experimental studies. Some striking similarities are indicated between diimide chemistry and the photochemistry of formaldehyde. It is shown that results from one field are transferable to the other one. From this, some new experiments are suggested.