Constant time gradient HSQC–iDOSY: practical aspects

Abstract

An improved constant time gradient HSQC-iDOSY pulse sequence is presented, and the corresponding form of the Stejskal-Tanner equation is derived. The pulse sequence is particularly well suited to the problem of analysing mixtures of chemically cognate species, where the high spectral resolution afforded by 1H-13C correlation methods is needed for DOSY experiments to give good diffusion resolution. Its use is illustrated for a mixture of rutin and its aglycone quercetin.

Spectral Aliasing: A Super Zoom for 2D-NMR Spectra. Principles and Applications

“…We used such techniques to make a quick study of the two candidate structures generated by LSD. We also compared the experimental 13 C chemical shifts with values determined using the NMRShiftDB [6] database and estimates from ChemBioDraw. Unsurprisingly the experimental spectrum matches the calculated ones for Fig.…”

“…This compound has previously been described in Podocarpus macrophylla D. Don (Podocarpaceae) from Japan, [1] but this is the first time it has been isolated in an African plant. The classical set of 1 H, { 1 H}- 13 C, DEPT-135, COSY, HSQC and HMBC experiments were recorded but structure determination was hindered by signal clustering due to the non-degenerated dimeric nature of the compound, causing serious assignment problems. The enlargement of the HMBC spectrum shown in Fig.…”

“…COSY experiments being unreliable in significantly separating signals, we have recourse to an indirect carbon dimension. Exploiting the high gyromagnetic ratio of protons, HMQC and HSQC experiments are especially sensitive and partially compensate for the low natural abundance of 13 C. When considering the extreme resolution attainable using spectral aliasing, the HSQC sequence is advantageous because signals do not show the elongation observed in HMQC spectra.…”

“…We therefore combined these two long delays into a Constant-Time HSQC to obtain a HSQC-iDOSY experiment [12] which has been recently improved by Morris and coworkers. [13] …”

“…In the direct proton dimension, it is quite fine because it stems from the FID, but it is much coarser in the indirect carbon dimension to avoid exceedingly long experimental times. This asymmetry is especially pronounced in heteronuclear 1 H- 13 C experiments such as HSQC and HMBC spectra where the resolution is typically close to 1 Hz/pt in the proton dimension but often over 100 Hz/pt along the 13 C chemical shift axis. If we compare NMR spectra with digital images, a typical HSQC is like the middle image in Fig.…”