Correlated 2D NMR

The two-dimensional H- H chemical shift correlated (2D NMR, COSY-45) data for isomeric 5P-cholestane-3a,7a,12a, 25-tetrol and 5P-cholestane-3a,7P, 12a,25-tetrol were acquired at a sweep width of 3200 Hz using a standard pulse sequence (collection of 256 free induction decays, FlDs) as described previously (28-30). A transform size of 2K x 2K data points was obtained after zerofilling. 

Chemometric techniques have also been used to identify grapevine cultivars and clones based on the H and NMR of polyphenols observed in extracts/ Extractions from crushed grape seeds and vines obtained in France were analyzed using HMBC and HMQC (heteronuclear multiple quantum correlation) 2D NMR... 

The assignments shown in Fig. 17B are made based on the connectivities shown above and on consideration of the magnetic equivalency of the methylene protons. The H NMR assignments have been extended to 13C NMR assignment using the l3C- H heteronuclear shift-correlated 2D NMR technique (13C- H COSY).199... 

The 15N signal assigned to 4,5-dimethyl-2-phenylisothiazolium perchlorate 57f was identified also by its l3C 1H-hetero-correlated 2D NMR spectrum. The 15N signal is located at —173 ppm and the estimated coupled constant to the C-3atom (12.2Hz), Cipso Ph (13.3 Hz) and H-3 (3.3 Hz) could be identified (92JPR25). 

DP Burum, A Bielecki. An improved experiment for heteronuclear-correlation 2D NMR in solids. J Magn Reson 94 645-652, 1991. 

Expected correlations between H NMR methyl group signals and 13C NMR signals in H-13C shift correlation 2D NMR spectra via two and three bonds for brassinosteroids... 

Part of the H-,3C long-range shift correlated 2D NMR spectrum (HMBC) of (20R)-hydroxy-3,24-diepicastasterone (107)... 

The basic aim of heteronuclear correlation 2D NMR is, as in the homonuclear experiment, to indicate which nuclei are coupled to one another. In heteronuclear NMR the correlation information is doubly useful since it provides chemical shifts for two different nuclear species in a single experiment. In the discussion which follows, the nuclei concerned will be assumed to be protons and carbon-13, although similar experiments have been reported for 31p 1h,53,54 15n-1h55 and correlation. 

Apphcation of 2D NMR methods overcame this obstacle and elucidated various LCC linkages in preparations isolated from softwoods, HWs, and pulps [21,25,29,48,50,51]. A HSQC correlation 2D NMR technique allowed for the first time direct detection of phenyl glycoside (Structure A, Fig. 1) and benzyl ether (Structure C, Fig. 1) LCC linkages [25,29]. In contrast to the common belief [2,14,17,18], no benzyl ester (a-ester) LCC linkages were detected in these studies. However, y-ester LCC moieties (Structure B, Fig. 1)... 

Figure 4 The NMR pulse sequence for the H-detected (inverse detection), correlation 2D NMR spectrum...

All the 2D-NMR experiments have the same basic format which can be divided into four units preparation, evolution, mixing, and detection periods. For an A-B correlated 2D-NMR experiment, the preparation period usually starts with a delay during which the spins are allowed to return to equilibrium this is followed by a pulse or a cluster of pulses and delays that transfers the magnetization from B to A nuclei, called coherence transfer, just before the evolution period. The evolution period (tj) is a variable time delay, increased in a stepwise manner from an initial value of zero to a final value ni x lAw (related with spectral window swl in the indirectly detected dimension of nucleus A), provides the key to the generation of the second dimension. The mixing period serves to transfer the magnetization back from A to B nuclei. Then the detection ((2) period is needed to collect the FID (Free Induction Decay) data. It produces a spectrum similar to the one obtained from a ID experiment of the detected nucleus. Each value in the 2D sequence is repeated nt times and np data points (FID s) are stored (np is related to the acquisition time at and sw is the direct detection dimension spectral window of nucleus B). 

The presence of three NMR active nuclei in many fluoropolymers provides the possibility of performing a number of permutations of heteronnclear correlated 2D-NMR experiments. heteronnclear correlated experiments (with either H or... 

FIGURE 24.5 Comparison of H- F correlation 2D-NMR experiments of C2F5(VDF>2l in acetone-dg at 25°C H- F HMQC NMR spectrum and (b) hetero-COSY NMR... 

Like their H C counterparts, inverse-detected F C experiments are preferred to direct-detected C F experiments. In principle, H C correlated experiments can be readily adapted to collect F C chemical shift correlated 2D-NMR spectra by taking into account the differences between the C—F and C—H couplings when setting the coherence transfer delays and the different resonance frequencies of F and H. However, their application to the study of fluoropolymers is not widespread. In practice, many other factors must be considered in order to obtain good quality correlated 2D-NMR spectra from fluoropolymers. The high... 

Quinting and Cai [62] carried out high-resolution C-NMR and proton NMR measurements to determine the tacticity of poly(n-butyl methacrylate) (PBMA) with particular focus on the peak assignments for the n-butyl side chain. Free-radical and anionic PBMA were examined, with the former being predominantly syndiotactic and the latter isotactic. Proton NMR resonances for the n-alkyl chain of these polyacrylics show a combination of effects from configurational sensitivity and homonuclear scalar interactions. A combination of J-resolved proton NMR and proton- C-heteronuclear correlated 2D-NMR spectra was used to characterise the long-range chemical shift effects due to tacticity. 

The heteronuclear shift correlated 2D-NMR spectrum of a cyclic peptide, cyclo(Pro-Phe-Gly-Phe-Gly), is presented in Figure 5.53. The proton chemical shifts (0-5 ppm) are on the horizontal axis while the chemical shifts (20-70 ppm) are on the vertical axis. The cross peaks establish the interconnection between the chemical shifts of protons and the chemical shifts of carbon atoms to which those protons are attached. 

Figure 5.53. Heteronuclear shift correlated 2D-NMR spectrum of a cyclic peptide, cyclo(Pro- Phe-Gly- Phe-Gly).

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