Organic Host Encapsulation Effects on Nitrosobenzene Monomer–Dimer Distribution and C–NO Bond Rotation in an Aqueous Solution

[Image: see text] The intermolecular (monomer–dimer equilibrium) and intramolecular (C–NO and C–NMe(2) rotations) dynamics of 4-nitrosocumene (1a) and 4-(N,N-dimethylamino)nitrosobenzene (1b), respectively, were found to be controlled by the medium (water) and the host environment (organic capsules and cavitands). The ability of water to shift the equilibrium toward the dimer appears to result from dipolar stabilization of the polar dimer structure and has a resemblance to water’s known ability to favor organic cycloaddition reactions. In an aqueous medium, a range of organic hosts selectively include only the nitrosocumene monomer 1a. Encapsulation in the octa acid duplex (OA(2)) selects two 1a monomers rather than a dimer structure. Octa acid encapsulation also results in more restricted intramolecular C–N rotations of the guest 1b.