New Techniques in Diffusion-Ordered NMR Spectroscopy

Access to files

•   FULL-TEXT.PDF (pdf)

Abstract

The analysis of mixtures is a subject that spans several different analytical techniques. In NMR, a powerful technique for this analysis is Diffusion-Ordered NMR Spectroscopy (DOSY), in which signals from different chemical species can be distinguished by their different diffusion coefficients. DOSY NMR allows an analysis of mixture components and their interactions in a non-invasive way and is proving to be an accurate and time effective method for looking at mixtures.An in-depth analysis of DOSY NMR is presented using the commercial mixture “monoacetin”. The chemically cognate species in this mixture produce complex and overlapping multiplet signals in 1H NMR that are difficult to assign and interpret. A previous analysis of this mixture used 1H NMR together with Liquid Chromatography (LC) and Gas Chromatography (GC) to identify the components present, but failed to provide complete assignments of all the signals. Here, the possibility of using DOSY as an alternative to hyphenated techniques is examined, and it is shown that a full analysis of the spectrum of “monoacetin” is possible with careful selection of experimental parameters and processing techniques, without recourse to chromatography. DOSY NMR can be ineffective when signals overlap and/or diffusion coefficients are similar. Many methods have been proposed to overcome these problems, and some of these are presented here. In order to increase resolving power, it is possible to gather further information about a mixture and incorporate this into diffusion experiments as another dimension. This creates a 3D dataset that can be analysed using a multiway method, such as PARAFAC, to extract the component spectra. This method is explored for the mixture “monoacetin” that has been partially separated by high-performance liquid chromatography. Resolution of two out of four components was achieved from poor HPLC separation; the decomposition obtained the component spectra, diffusional decay and HPLC elution profile for these components. Improved HPLC separation should result in further resolution.Diffusion coefficients of different mixture species can be manipulated by changing the matrix in which they diffuse: Matrix-Assisted DOSY (MAD). Previous techniques have involved either improving resolution in the diffusion domain or aiming to improve resolution in chemical shift. A method is presented here that simultaneously addresses both problems in a chemically-selective way, using lanthanide shift reagents. The chemically-selective binding of the LSR to mixture components can both enhance chemical shift dispersion and increase diffusion resolution in DOSY. This neatly deals with the two main drawbacks of the DOSY experiment, and is demonstrated using a mixture of an alkane, alcohol and aldehyde. The manipulation of a molecule’s electrostatic charge through pH control has been investigated, where small ions with a high charge density would be highly solvated, resulting in a change in D. The effect, however, was not measurable and so the indirect effect of pH on the interaction of charged species with the cationic micelle CTAB is presented, where an increase in resolution between of mixture of aspirin and salicylic acid is achieved.Although DOSY NMR is a powerful tool for mixture analysis, in recent years it has been used for studying molecular interactions. An example of this is presented here where species aggregate under specific conditions, a process identified by DOSY NMR.

Bibliographic metadata

Institutional metadata

Record metadata