Study of the monomer‐dimer equilibrium of nitrosobenzene using multinuclear one‐ and two‐dimensional NMR techniques

The monomer‐dimer equilibrium of nitrosobenzene in CDCI 3 solution was studied in the temperature range ‐50 to 30 °C using 13 C, 15 N and 17 O NMR spectroscopy. Decreasing temperature favours the dimeric azodioxy species, and below ca 10 °C both cis and trans dimers were detected in addition to the monomer. The relative abundance of the trans dimer is low (⩽6%) at all temperatures and this is the first report of its existence in solution. The monomer and dimer species were characterized by their 13 C and 15 N shifts, and exchange between them was measured quantitatively by 13 C 2D‐EXSY spectra. Exchange occurred exclusively between the cis ‐dimer and monomer and between the trans ‐dimer and monomer, activation energies, Δ G ≠ (298.15 K) being 65.8±0.1 and 70.0±0.3 kJ mol −1 , respectively.