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 13C NMR techniques. In CDCl3 or CD2C12 solution dynamic equilibria exist between monomeric 4-iodonitrosobenzene and its (Z)- and (E)-azodioxy dimers. Thermodynamic data (ΔG⊖, ΔH⊖ and ΔS⊖) for these equilibria have been calculated from low temperature 1H NMR spectra. Criteria for the dimerisation tendencies of all types of C-nitroso compounds are proposed.