Heteronuclear correlations characterization

The development and application of multidimensional solid state homo- and heteronuclear correlation (HETCOR) NMR techniques have lead to an increasingly important role in structure solution of zeolitic materials and have had many practical applications in the detailed structural characterization of completely siliceous zeolites[6,7] and AlPOs.[8-ll] However, HETCOR NMR is not readily applicable to aluminosilicates... 

Recently, the two-dimensional 13C H heteronuclear correlation (HETCOR) NMR method, using the frequency-switched Lee-Goldberg (FSLG) H decoupling sequence55 at high MAS rates, has been developed, in order to provide intermolecular and spatial distance information. The HETCOR spectrum often has multiple proton cross peaks for each carbon, and these cross peaks can be extremely helpful for assigning the spectrum. Thus, this method can be used to characterize the structure of polymers in solids.55 71... 

Heteronuclear correlation (HETCOR) is an experiment establishing a correlation between the chemical shift of proton and that for another nucleus (i.e. C, P, Nor Si). The experiment relies on the heteronuclear dipolar interactions and uses a CP to transfer magnetization from protons to a rare spin.4,6 We have used 2D HETCOR experiment to characterize support/counter-cation/cluster interaction in the supported mesoporous catalysts.21... 

The study of the composition of a mixture is an extremely common problem in analytical and bioanalytical chemistry. While chromatography and solvent extraction are commonly employed to simplify the analysis prior to characterization of the constituents, NMR has provided a series of tools that help in unravelling the components of complex samples, when a previous separation of the pure compounds is not feasible or complete. Thus, TOCSY, NMR diffusometry (DOSY, among all) and heteronuclear correlation experiments are widely used to this purpose, for example, for the characterization of small molecules in biologically relevant samples, such as in metabolomics,1 plant extracts analysis,2 food quality control,3 4 to name a few cases. 

A survey of literature gave over 20 papers where 2D NMR experiments was successfully applied in quantification of complex samples. The selection of the used experiments was also quite diverse all typical 2D homo- and heteronuclear correlation experiments were among them. Many of the authors also compared the 2D NMR results against ID NMR quantification or quantification with other analytical technique. In overall, 2D NMR quantification offered a better linearity and accuracy in these comparisons due to better resolved peaks. Additionally, the approach often facilitated analysis of minor components that were not distinguishable from ID NMR spectra. The main part of the papers that demonstrated 2D NMR quantification focused on the analysis of natural products like animal and plant metabolites. Some articles were also found where quantitative 2D NMR was applied to quality control in food industry, and on characterization of the products of oil industry. The following sections give a short summary of the quantitative 2D NMR on the aforementioned topics. 

As previously mentioned, the highest theoretically possible sensitivity could be achieved when the spin states of large y nuclei are initially perturbed and their magnetization is detected. Therefore, among many possible heteronuclear correlation schemes, those exploiting excitation and detection of sensitive nuclei (e.g. H) are of great importance. Such methods are characterized by two-fold coherence... 

Structural characterization of the dihydrides of Rh and Ir phosphine complexes with properly modified 2D NMR methods (such as COSY, HSQC, HMQC, and NOESY) was reported, which, in particular, can address heteronuclear correlations involving such nuclei as and ° Rh [27,63]. Dynamic transformations can be addressed with ID and 2D NOESY/EXSY methods. The exchange between the dihydrides and pH2 can be studied efficiently as it leads to the permanent replenishment of hyperpolarization of the dihydride complex. Ligand (e.g., CO, PPhj) exchange has been addressed as well. 

Figure 11.5 Solution-state NMR analysis of silicone breast implant materials can be utilized to characterize chemical species associated with degradation, (a) The chemical shifts measured in Si NMR allow one to correlate distinct NMR signals with changes in specific chemical functionalities present in the system, i.e., cyclic oligomers denoted D in the figure, (b) 2-dimensional Si- H HSQC heteronuclear correlation experiments dramatically increase sensitivity of the Si NMR spectrum (as illustrated by the cross-peaks indicating bonding between H and Si resonances that are not observable in the direct Si NMR spectrum (vertical spectrum). Reproduced with permission from [24] Copyright Elsevier (2003).

Conformations of mycothiol bimane (MSmB) were studied by H and 13C NMR using rotational nuclear Overhauser effect spectroscopy (ROESY) and heteronuclear single quantum correlation (HSQC) methods with expansions of the anomeric region <2003JOC3380>. NMR characterization of iV-acetyl-L-(Y)-cysteinyl monobimane and peracetylated MSmB was also published (Section 12.10.15.4) <2002JA3492>. 

Smaller diameter probes reduce sample volumes from 500 to 600 pi typical with a 5 mm probe down to 120-160 pi with a 3 mm tube. By reducing the sample volume, the relative concentration of the sample can be correspondingly increased for non-solubility limited samples. This dramatically reduces data acquisition times when more abundant samples are available or sample quantity requirements when dealing with scarce samples. At present, the smallest commercially available NMR tubes have a diameter of 1.0 mm and allow the acquisition of heteronuclear shift correlation experiments on samples as small as 1 pg of material, for example in the case of the small drug molecule, ibu-profen [5]. In addition to conventional tube-based NMR probes, there are also a number of other types of small volume NMR probes and flow probes commercially available [6]. Here again, the primary application of these probes is the reduction of sample requirements to facilitate the structural characterization of mass limited samples. Overall, many probe options are available to optimize the NMR hardware configuration for the type and amount of sample, its solubility, the nucleus to be detected as well as the type and number of experiments to be run. 

The development of carbon-13 NMR during the last eight years has been characterized by a continual increase in the sensitivity and quality of spectra. A reduction in measuring time - equivalent to an enhancement in sensitivity has been achieved mainly by cryomagnet technology. The efficiency with which NMR information can be obtained has been substantially improved by new computer-controllable pulse sequences for one-and two-dimensional NMR experiments. A selection of these new methods, in particular, those used for multiplicity analysis and homo- or heteronuclear shift correlations, is presented in chapter 2 of this edition. 

Furthermore, the allohimachalane (see Section 13.11.9.1) <1999T14623> as well as boletunones A and B, highly functionalized sesquiterpenes from the fruit body of the mushroom Boletus calopus <20040L823>, have been characterized by 2D-NMR (heteronuclear single quantum correlation (HSQC), HMBC, and 111-COSY). The structure of a drimen-ll,12-acetonide, isolated from Maya s herb, was deduced by means of 111 and 13C NMR, distortionless enhancement by polarization transfer (DEPT), COSY, NOESY, HSQC, and HMBC analyses <2005MRC339>. 

The vast literature associated with flavanoid chemistry precludes a discussion here but two valuable reviews have been published. The first reviews a number of spectroscopic techniques used for flavonoid analysis, with a strong emphasis on NMR spectroscopy (plus also mass spectrometry, vibrational spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, X-ray crystallography, and circular dichrosim (CD)) . The second review deals with NMR methods that have been successful in the characterization of phenolic acids and flavonoids from plant extracts that have not been separated or isolated as single components. The emphasis of the article is 2-D NMR methodology and a variety of experiments such as total correlated spectroscopy (TOCSY), COSY, nuclear Overhauser enhancement spectroscopy (NOESY) and heteronuclear multiple quantum correlation (HMQC) are discussed . 

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