2D and 3D DOSY methods for studying mixtures of oligomeric dimethylsiloxanes

Abstract

Diffusion-ordered spectroscopy (DOSY) is a powerful method for the NMR analysis of many types of mixture without the need for physical separation, and requires only relatively standard spectrometer hardware. The principal requirements for high resolution analysis using DOSY, that the basic NMR spectrum be well-resolved and that it have good signal-to-noise ratio, pose a dilemma where multiple chemically similar species with NMR-active heteronuclei are involved. Generally the 1 H spectrum of such a mixture has good sensitivity but relatively poor chemical shift resolution, while for heteronuclei the situation is reversed. The dilemma is illustrated for the case of a mixture of cyclic dimethylsiloxanes, and the results of a range of 1 H 2D DOSY, 29 Si 2D DOSY and 1 H { 29 Si} 3D DOSY-HMQC experiments are compared. In selecting the most appropriate technique to use for a given sample it is necessary to balance conflicting requirements for speed, resolution and accuracy, and to consider the balance between systematic and random errors. Results are presented for three different concentrations of a mixture of cyclic siloxanes containing between 4 and 20 monomer units. The diffusion coefficients measured show an inverse half power law relationship with molecular mass.

Constant time gradient HSQC–iDOSY: practical aspects

“…Many of the most powerful of these extend the experimental data into a further spectral dimension, giving 3D DOSY experiments. This can be achieved either by concatenating a diffusion-weighting pulse sequence with a conventional 2D NMR sequence, as in DOSY-NOESY [5] and DOSY-HMQC, [6] or by incorporating diffusion weighting internally, [7] where the parent pulse sequence can accommodate an extra diffusion delay , as in the COSY-iDOSY [8] and HMQC-iDOSY [9] pulse sequences. Recently, a new experiment has been described by Vitorge and Jeannerat, [10] which incorporates diffusion encoding into the constant evolution time T of a constant-time (CT) gradient HSQC pulse sequence.…”

Improving the accuracy of pulsed field gradient NMR diffusion experiments: Correction for gradient non-uniformity

“…Gradient non-uniformity has long been known to be a problem in diffusion measurement by NMR [6,16,17]; a clear explanation of its effects, and one effective route to compensating them, was presented some time ago by Damberg, Jarvet and Gräslund [17] (abbreviated below as DJG). This paper gives an extended analysis, describes an alternative and slightly more general approach to compensating for non-uniform gradients that has proved very effective [8,15,16,[18][19][20][21][22][23][24][25][26], and examines some of their practical consequences.…”

Tetrapropylammonium Occlusion in Nanoaggregates of Precursor of Silicalite-1 Zeolite Studied by 1H and 13C NMR

“…Careful PFG NMR enabling in the pulse sequence to measure accurately diffusion, known under the acronym DOSY NMR, has been proven to be a valuable tool to study diffusion of a variety of molecules, especially chemical objects with different dimensionality [35,36]. Application of 29 Si DOSY NMR has been performed successfully on some aqueous silicate solutions for studying silicate speciation and spectral assignment [37][38][39]. Recently, few 1 H PFG NMR studies have been conducted in typical precursor sols providing useful insights on dynamic behavior of organocations with respect to silicated nanoparticles [4,27,28,[40][41][42].…”