Structural investigations of C-nitrosobenzenes. Part 2.1 NMR studies of monomer–dimer equilibria including restricted nitroso group rotation in monomers

Abstract

Energy barriers associated with rotation of nitroso groups in monomeric nitrosobenzene and eleven monomeric substituted nitrosobenzenes in solution have been measured by total ¹H NMR bandshape analysis. ΔG ^‡ (298.15 K) values for the rotations are in the range 31–41 kJ mol^–1. The values for 4-substituted compounds correlate well with the Hammett σ_p⁺ parameter for the substituent. The experimental energy barrier for nitrosobenzene is compared with theoretical calculations. Monomer–dimer equilibria of these compounds in solution have been investigated, with ΔH, ΔS and ΔG data calculated for the dissociation of both Z- and E- azodioxy dimers to monomers. Kinetic data, based on time-dependent and two-dimensional exchange spectroscopy (2D-EXSY) NMR measurements, have been obtained for the dissociation of 3-methylnitrosobenzene and 3,5-dimethylnitrosobenzene dimers. Energy profiles for the ground and transition states of both dimers and monomers (including exchange between both their rotameric forms) are discussed.