4-iodonitrosobenzene. Structural and spectroscopic studies of the monomeric solid and of previously unreported dimers

4-Iodonitrosobenzene has been isolated as the (E)-azodioxy dimer for the first time. Its solid-state structure has been identified by X-ray crystallography and characterised by IR, MS and C-13 NMR techniques. In CDCl3 or CD2Cl2 solution dynamic equilibria exist between monomeric 4-iodonitrosobenzene and its (Z)- and (E)-azodioxy dimers. Thermodynamic data (Delta G*, Delta H* and Delta S*) for these equilibria have been calculated from low temperature H-1 NMR spectra. Criteria for the dimerisation tendencies of all types of C-nitroso compounds are proposed.