NMR and computational study of monomer-dimer equilibrium of aromatic dinitroso compounds

Aromatic C-nitroso compounds can exist as monomers or Z- and E-azodioxides (dimers). The preferred form in solution at ambient temperature is monomeric while lowering of temperature or crystallization favors dimerization. Azodioxides undergo solid-state photodissociation to corresponding monomers under cryogenic conditions, which again dimerize by warming. Aromatic C- nitroso compounds with two or more nitroso groups could be used as building blocks for supramolecular structures that could be disassembled or reassembled by external stimuli such as UV radiation or heat. In the present work, we studied solution-state monomer-dimer equilibrium of several new aromatic dinitroso compounds that differ in spacer between two aromatic rings (Scheme 1). In order to observe formation of dimers, we recorded 1H and COSY NMR spectra in chloroform-d5 at ambient and low temperatures. Inspection of NMR spectra revealed that by lowering of temperature new signals appear in the spectra that could be assigned to dimers. Signals of dimers disappear after warming the solution to room temperature. The NMR spectra of monomers and dimers were also modelled using density functional theory. Solution-phase (SMD) optimizations, followed by GIAO calculations, were done at the ωB97-XD/6-311G(d, p) level of theory. In case of dimers, multiple conformers had to be considered in order to obtain a good agreement (MUE < 0.1 ppm) between calculated and experimental spectra.