Paramagnetic relaxation reagents nuclear magnetic resonance studies of preferential solvation

Abstract: The interactions between neutral paramagnetic relaxation reagents (PARR's) and certain aromatic compounds have been studied by 1H and 13 C spin-lattice relaxation time measurements. In media such as cyclohexane and carbon tetrachloride, Cr(acac)3 becomes preferentially solvated by aromatic solutes. The solvation is significantly suppressed in a more interacting solvent like dichloromethane. Paramagnetic induced chemical shifts of the aromatic outer sphere ligand indicate in addition to relaxation data, a preferential orientation caused by dipole-dipole interactions. For benzene or for several alkylated benzenes which have small or no permanent dipole moments, the interaction is electrostatic, i.e. of a dipole-dipole induced type and where the easily polarizable aromatic ring is preferred in the solvation sphere.Carbon tetrachloride is shewn to have a specific PARR interaction. If co-ordination number, solution structure etc., are to be determined using weakly interacting substrates, this solvent should be avoided.A multivariate statistical approach is also reported, where 13 C electron-nuclear relaxation data and induced shifts of monosubstituted arcmatics have been related to different physical descriptors. Most of the variance in relaxation and shift data is best described by the dipole moment. The results support a dipole-dipole interaction as the preferred solvation mechanism.The preferential solvation of several organic substrates with the diamagnetic Co(acac)3 is studied by varying the substrate concentration in cyclohexane. By the use of 59 Co shift, it is shown that proton donating solutes such as chloroform and methanol have a specific solvation. The order of preference is close to that obtained in Cr(acac)3 solutions.