Probing mixed solution systems using NMR diffusion studies

Abstract

Diffusion studies can be used to assist in identification of different compounds in a mixture. However, since the diffusion coefficient of a species is primarily dependent on its effective hydrodynamic radius, it is particularly difficult to resolve compounds with similar size and structure (such as isomers) on the basis of diffusion. For a molecule in an isotropic freely diffusing system, diffusion studies provide valuable information regarding the interactions of the molecule with its environment. Differential solution interactions between species in certain solutions can afford possibilities for resolution of similar molecules with respect to diffusion. Isomers, particularly those with similar hydrodynamic radii, allow the effects of isomerism and differential solute-solvent interactions on diffusion to be investigated. Nuclear magnetic resonance (NMR) diffusometry is a powerful technique for studying the motion of species in solution. The motion of molecules can provide interesting information regarding the molecular interactions and structuring that occur in solution. Here NMR was used to measure the self-diffusion coefficient (D) of three sets of ortho-, meta- and para-disubstituted benzene isomers. The isomers of dihydroxybenzene (DHB), diaminobenzene (DAB) and difluorobenzene (DFB) were studied in water-monohydric-alcohol systems (i.e., methanol (MeOH), ethanol (EtOH), 1-propanol (1-PrOH), and tert-butanol (t-BuOH)), at a range of mole fractions for each solvent system. It was found that the different positions and functional groups had a significant effect on the diffusion of the isomers. While in aqueous solution the DHB isomers had the same diffusion coefficient, it was found that in water-monohydric-alcohol systems the diffusion of the ortho-substituted isomer differed from that of the meta- and para-substituted isomers. The DAB isomers also had similar diffusion coefficients in aqueous solution, however unlike with the DHB isomers, in the water-monohydric-alcohol solvents the resonances of all three DAB isomers were differentiable via diffusion (i.e., ortho > meta > para). The DFB isomers were found to be differentiable in both aqueous and water-monohydric-alcohol solvent systems; however, the order of diffusion coefficients was the opposite (i.e., para > meta > ortho) of that found for DHB and DAB. In all three cases the differentiation was found to be greater at higher concentrations of monohydric-alcohols. While there is immediate usefulness to mixture analysis by the resolution (with respect to diffusion) of similar molecules such as isomers using solvent modulation, the similar structure of the three sets of disubstituted benzene isomers allow a comparison of solvent interactions on diffusion. Solute-solvent interactions are also of great importance to liquid chromatography, where the separation of similar molecules can be difficult. The effect of ring position and the different functional groups on the interaction of isomer solutes with the mixed solvent has been examined here to gain further insight into the solution molecular dynamics. Mixed solvent systems are interesting in that they offer the opportunity for adjustments to solvent properties, sometimes with only slight changes in concentration ratios. Previous studies have shown that dimethyl sulfoxide (DMSO) and monohydric-alcohols preferentially form dimers with one another as opposed to with the same species. Here the dynamics of the systems have been investigated using NMR diffusometry. The binary mixtures of DMSO-MeOH, -EtOH, -1-PrOH, -2-propanol (2-PrOH), -1-butanol (1-BuOH) and t-BuOH were studied using 1H chemical shift, self-diffusion measurements and their experimentally determined effective Stokes radii. The systems displayed non-linear diffusion and chemical shift behaviours across the concentration ratios. The results have been discussed in relation to intermolecular interactions and solution structuring.

Date of Award	2014
Original language	English