Correlating Oxygen-17 NMR Parameters to Structure

One objective of our work is to discover the relationships between NMR parameters and local structure. Our work on 17O has been particularly fruitful because the strong covalent bonds formed between oxygen and network forming cations, as well as the low coordination number of oxygen in many of these linkages result in a local electronic structure and, specifically, an electric field gradient that can be interpreted in terms of local structure. Armed with the experimental and computational work of previous workers in the field, we have taken advantage of recent experimental NMR methodological developments for quadrupolar nuclei to build up more refined predictive models for oxygen in a T—O—T' linkage, and more recently, a T—O—H linkage, where T is a tetrahedrally coordinated cation.

T—O—T linkage

In symmetric T—O—T linkages the nature of the coordinating cations is the primary factor in determining the size of the oxygen electric field gradient (Cq).

BridgingOxygen

Si—O—Si linkage

The primary geometrical structural factors that influence the bridging oxygen electric field gradient are the briding oxygen T—O—T angle and the T—O distance.

BOCqEtaDependence2

These trends were experimentally confirmed through measurements of the 17O quadrupolar coupling parameters in crystalline silica polymorphs, such as coesite,

CoesiteFragment

using techniques like Dynamic-Angle Spinning, which not only spectroscopically resolves the crystallographically inequivalent oxygen site, but also provides separated anisotropic lineshapes from which the quadrupolar coupling parameters can be determined.

CoesiteDASSpectrum

T—O—H linkage

The same approach has been taken to examine 17O quadrupolar coupling parameters in the hydroxyl groups of organic molecules. Here we have used a simple methanol molecule as our model system. In this model the relevant structural parameters are the C—O—H angle, C—O and O—H distance, and the presence (or absence) of hydrogen bond donors and acceptors. In glycosides the C—O—H angles range from 105-115°, and the C—O and O—H distance from 1.39-1.43Å and 0.80-1.00Å, respectively. Over this range, the largest variation in 17O efg occurs with changes in C—O—H angle and O—H distance. Variations due to changing C-O distance account for less than 1%. In figure below are the variation in the efg components with C—O—H angle and O-H distance for the methanol model. Remarkably, this model predicts no significant variation in 17O efg over the range of C—O—H angles and O—H distances found in the methyl glucosides sites measured here.

SugarFig2

To investigate this prediction we examined oxygen in a glycosidic linkage and hydroxyl groups in a series of 17O labeled carbohydrates.

SugarFig1

Below is a plot of the principal components of the quadrupolar coupling tensor for each hydroxyl measured. Because the sign of Cq is unknown, the principal components were calculated assuming both signs for each site. Experimental variations are less than 10%. For the four sites, β-D-Glc-O2 and α-D-Glc-O6 are acting as both donor and acceptor, α-D-Gal-O4 acts only as donor, and α-D-Glc-O4 acts as neither donor nor acceptor.

SugarFig3

Overall, the structural invariance of the 17O quadrupolar couplings observed here for the hydroxyl group will be advantageous in future NMR studies of oriented samples (e.g., membranes), and in correlation experiments with other NMR interactions such as 1H—17O or 13C—17O dipolar and J couplings.



References and Related Resources from our Lab