Solvent Effects on the Nitrogen NMR Shielding of 2-Methyl-2-nitrosopropane and its Azodioxy Dimer

Abstract

High-precision 14N NMR shielding data are reported for 2-methyl-2-nitrosopropane and its azodioxy dimer in a variety of solvents. A range of about 30 ppm, as a function of solvent, is observed for the nitroso nitrogen atom. This contrasts with the corresponding shielding range for the dimer, which is about 6 ppm. The effects of solvents on the nitrogen shielding of the nitroso group are largely controlled by the polarity of the solvents. In conjunction with molecular orbital calculations incorporating the solvaton model, the increasing polarity of the solvents is found to enhance the migration of electric charge from the nitrogen to the oxygen atom of the nitroso group. The reverse trend was observed for nitrogen atoms doubly bonded to carbon rather than to oxygen. Hydrogen bond effects on the nitrogen shielding are surprisingly small for the nitroso group in comparison with those found for O-nitroso groups in covalent nitrites. This difference is probably due to steric effects caused by the bulky alkyl group directly attached to the nitrogen atom of the monomer studied. In the majority of solvents used, the monomer–dimer equilibrium is displaced heavily towards the monomer form. Aqueous solutions are a notable exception, where the dimer still largely predominates at low concentrations.