2D/-resolved NMR

The proton-decoupled proton spectra allowed a distinction to be made between homo-and heteronuclear spin couplings, and Laurie and coworkers also demonstrated nulling of residual solvent resonances during the 2D /-resolved NMR of uridine in aqueous solution, wrote software for 45° tilting of the 2D spectra, and developed experimental protocols for multiple data-acquisition and processing, and a method for acquisition of the 2D /-resolved spectra in phase-sensitive mode. Lately, the 2D /-resolved technique has been less used, as it yields little evidence for spectral assignments. 

Key Words Quantitative 2D NMR, 2D -resolved NMR, COSY, TOCSY, HSQC, 2D INEPT, Natural products, Metabolomics, Oil fractions. Food analysis. 

D /-resolved NMR has been applied to metabolomics to enhance resolution of the overlapping resonances and facilitate identification of the sample compo-nents. Viant used this approach to achieve quantitative ID F2-projections which were analysed using principal component analysis to evaluate metabolites citrate, taurine and alanine in embryogenesis of fish Oryzias latipes. In comparison to conventional NMR quantification the author stated that the likelihood that a... 

A report where quantitative 2D NMR was applied to differentiation of beer brands was published by Khatib et al. They used 2D /-resolved NMR to improve resolution of components present in 2-butanol extracts of several beer brands. With application of principal component analysis they distinguished six lagers from each other based on their amino acid contents. They also discussed how the amount of compounds that have an important effect to the taste, like tyrosol, can be estimated with the method. 

Pozsgay V, Nese S, Coxon B. Measurement of interglycosidic /ch coupling constants of selectively labeled oligosaccharides by 2D /-resolved NMR spectroscopy. Carbohydr Res 1998 308 229-238. 

The two frequency axes may consist of a diverse assortment of pairs of fundamental NMR parameters. Examples might include chemical shift on one axis and a frequency axis for scalar couplings on the second as in the 2D /-resolved NMR experiments. Both axes may be proton chemical shift, in which responses may be correlated by scalar (/) couphng as in the COSY experiment [46—48], by dipolar relaxation pathways as in the NOESY [35, 36, 49—51] and ROESY [35, 36, 52, 53] experiments, or by chemical exchange pathways as in the EXSY experiment [54—59]. Other examples may involve chemical shift on one axis and a multiple quantum frequency on the second axis. Examples here would include proton double [60 62] and zero quantum spectroscopy [63—67], C—INADEQUATE [68, 69], etc. The available axes in a 2D NMR experiment may also be used for hetero-nuclear chemical shift correlation, e.g. H—or H— N, where the respective nucHde pairs are correlated via their one-bond ( /xh) or multiple bond ("/xh) hetero-nuclear couphngs [14, 16, 17, 23—27, 29—31, 70—72]. 

Figure 3.1 The various time periods in a two-dimensional NMR experiment. Nuclei are allowed to approach a state of thermal equilibrium during the preparation period before the first pulse is applied. This pulse disturbs the equilibrium ptolariza-tion state established during the preparation period, and during the subsequent evolution period the nuclei may be subjected to the influence of other, neighboring spins. If the amplitudes of the nuclei are modulated by the chemical shifts of the nuclei to which they are coupled, 2D-shift-correlated spectra are obtained. On the other hand, if their amplitudes are modulated by the coupling frequencies, then 2D /-resolved spectra result. The evolution period may be followed by a mixing period A, as in Nuclear Overhauser Enhancement Spectroscopy (NOESY) or 2D exchange spectra. The mixing period is followed by the second evolution (detection) period) ij.

Heteronuclear two-dimensional /-resolved spectra contain the chemical shift information of one nuclear species (e.g., C) along one axis, and its coupling information with another type of nucleus (say, H) along the other axis. 2D /-resolved spectra are therefore often referred to as /,8-spectra. The heteronuclear 2D /-resolved spectrum of stricticine, a new alkaloid isolated by one of the authors from Rhazya stricta, is shown in Fig. 5.1. On the extreme left is the broadband H-decoupled C-NMR spectrum, in the center is the 2D /-resolved spectrum recorded as a stacked plot, and on the right is the con tour plot, the most common way to present such spectra. The multiplicity of each carbon can be seen clearly in the contour plot. 

In 2006, Milosavljevic and co-workers64 reported a study of the complete 4H and 13C NMR assignment of a new triterpenoid saponin, leucantho-side-A (13), from Cephalaria leucantha L. In the course of determining the structure and assigning the spectra, the authors made extensive use of the normal ensemble of 2D NMR experiments in use for the characterization of natural product structures HSQC, HMBC, DQF-COSY, TOCSY, and NOESY. The authors supplemented the aforementioned list of experiments with 2D /-resolved, DINE-(Double INEPT-Edited)-HSQC, and INADEQUATE spectra. The authors made no mention of the use of the connectivity information derived from the 1,1-ADEQUATE spectrum in the assembly of the triterpene nucleus of the molecule but reported extensive tabulations of the 1,1-ADEQUATE correlations that were used to sequence and assign the saccharide resonances of the tri- and di-saccharide sub-units, 14 and 15, respectively, linked to the triterpene nucleus. 

Fig. 10.13. 2D J-resolved NMR spectrum of santonin (4). The data were acquired using the pulse sequence shown in Fig. 10.12. Chemical shifts are sorted along the F2 axis with heteronuclear coupling constant information displayed orthogonally in F . Coupling constants are scaled as J/2, since they evolve only during the second half of the evolution period, t /2. 13C signals are amplitude modulated during the evolution period as opposed to being phase modulated as in other 13C-detected heteronuclear shift correlation experiments.

Figure 11.6 2D /-resolved solid-state NMR spectrum of [(=SiO)Ta(=CH Bu)(CH2 Bu)2], 10% C enriched at the R positions ( ). Traces extracted along the 8i dimension of the 2DJ-resolved spectrum at different carbon chemical shift frequencies 31,47, 95 and 247 ppm. (Reprinted from Reference [18].)... 

In recent years new NMR techniques offering broad applications in stereochemical analysis have come into use. A prominent example is 2D-NMR (both 2D-resolved and 2D-correlated spectroscopy), which has been extensively applied to biopolymers (149-151). Its use with synthetic polymers has, until now, been limited to but a few cases (152, 153). A further technique, cross-polarization magic-angle spinning spectroscopy (CP-MAS NMR) will be discussed in the section on conformational analysis of solid polymers. 

After 2D Fourier transformation J-Resolved spectra usually contain a distortion along the horizontal line leading through the centre of the matrix. In order to get rid of this distortion and to separate chemical shifts from homonuclear J-couplings, the whole matrix is tilted. With 2D WIN-NMR a Tilt command is available which automatically adjusts the corresponding parameters (Tilt factor) and performs a tilt operation. 

The most recent developments in 2D NMR of solids are the heteronuclear chemical shift correlation spectroscopy (421), 2D exchange NMR, which enables very slow molecular reorientations to be monitored (422), and heteronuclear. /-resolved 2D NMR (423). 

We have characterized (8e-i) this intermediate using solid-state 13C CP/MAS NMR, 2H NMR, and two dimensional (2D). /-resolved 3C NMR spectroscopy in the course of dehydration of isobutyl alcohol and /er/-butyl alcohol in HZSM-... 

Further heating results in the selective transfer of the l3C label from the CH2 group of IBSE into the CH groups of two nonequivalent isobutyl fragments (lines at 30.5 and 31.5 ppm) and to a lesser extent into the CH3 group (the line at 19.2 ppm) of IBSE. The lines at 30.5 and 31.5 ppm were attributed to the CH group according to their 2D. /-resolved 13C MAS NMR spectra (8i) (insert, Fig. 7D). 

Laurie was one of the first to apply two-dimensional (2D) NMR to carbohydrates. With students Subramaniam Sukumar and Michael Bernstein, and visiting scientist Gareth Morris, he demonstrated and extended the application of many of the directly observed 2D NMR techniques of the time. These included the homo- and hetero-nuclear 2D /-resolved techniques, delayed proton /-resolved NMR that allowed broad resonances to be suppressed, for example, those of dextran in the presence of methyl /Lxvlopyranoside. proton-proton chemical shift correlation spectroscopy (COSY), nuclear Overhauser enhancement spectroscopy (NOESY), proton-carbon chemical shift correlation (known later as HETCOR), and spin-echo correlated spectroscopy (SECSY). Trideuteriomethyl 2,3,4,6-tetrakis-<9-trideuterioacetyl-a-D-glucopyranoside served as a commonly used model compound for these studies. 

In the 1970s, we were all impressed by the ability of the proton 2D /-resolved technique to provide (after tilting of the twice-Fourier-transformed data matrix) a proton-decoupled proton spectrum comprised of a singlet at each of the 1H chemical shifts, and resolved /-multiplets by taking slices parallel to the F1 axis. These /-multiplets displayed all of the coupling constants for a resonance (no chemical shift) and were remarkable in that they were sharper than the spin multiplets in ID H NMR spectra, due to the refocusing effect of the 180° pulse in the /-resolved sequence. 

The ID and 2D MAS NMR and powder X-ray diffraction have successfiilly been used to resolve the temperature dependent phase transformations of ZSM-11, to assign their space groups and isolate the temperature range of disorder. 

Interglycosidic 3/hcoc values have also been measured by non-selective, 2D /-resolved H NMR of tri-, tetra-, and penta-saccharide fragments of the O-specific polysaccharide of Shigella dysenteriae type 1 substituted with 13C at C-l of the galactopyranose residues.85... 

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