J-resolved 2D-NMR

Figure 24.13a shows the ID-NMR spectra of the poly(tetrafluoroethylene-co-propylene) poly(TFE-P) the complex peak patterns were mainly attributed to couplings which made peak assignment difficult. They obtained a F J-resolved (Figure 24.14) and F COSY (Figure 24.15) 2D-NMR spectra which helped with the assignments. In the J-resolved 2D-NMR spectrum, if the data are skewed about the /i =0 axis by 45° in frequency space, a spectrum with pure F chemical shift in the/2 dimension and pure Tpp coupling in the/i dimension is obtained. A projection of this 2D-NMR spectrum onto the/2 axis produces a pure chemical shift spectrum (with homonuclear couplings removed). This broadband homonuclear decoupled spectmm is shown across the top of the 2D-NMR plot in Figure 24.14, and is reproduced in Figure 24.13b below the normal F ID-NMR spectrum. The F broadband homonuclear decoupled spectmm shows eight resolved peaks numbered from 1 to 8 (-110.3(1), -112.6(2), -113.1(3), -115.2(4), -116.0(5),...

FIGURE 24.14 homonuclear J-resolved 2D-NMR spectrum at room temperature. 

Figure 1 2D "C J-resolved MAS NMR spectrum of the products of a catalytic conversion of methanol to gasoline on zeolite H-ZSM-5 [9]. Methanol was enriched to 30% in the C isotope.

Fast spinning MAS spectra, even in the case of J-resolved 2D experiments do not allow to distinguish between the presences of one or two chemically equivalent nuclei since under fast spinning MAS conditions the situations is comparable to liquid state NMR i.e. chemical shift anisotropies are averaged out. However, under static conditions, the chemical shift tensors of two chemically equivalent nuclei do not coincide and both nuclei have in general dilferent chemical shifts at certain orientations of the molecules with respect to the magnetic field axis, i.e. they are magnetically inequivalent. Thus under static conditions, the spin system may be described as an AB or AX system, instead of an A2 spin system. 

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. 

Figure 3.5 Partial 2D J-resolved H NMR spectrum of poly(acrylonitrile) in DMSO-dg. The multiplets are effectively rotated into the vertical dimension. The large peak at 3.06 ppm arises from water, and the uneven ridge marked x is partly an artefact...

Figure 5.43. Gated decoupler method for recording 2D J-resolved C-NMR spectra. The upper portion shows the pulse sequence while the lower portion represents the effect of the pulse sequence on magnetization of a CH group decoupling during the preparation period provides nuclear Overhauser enhancement. The position of the two magnetization vectors in (d) will be dependent on 11 the sum of the two vectors which is detected will depend both on and the magnitude of the coupling constant, J. The signals are then said to be J-modulated. ...

Figure 5.44. The spin-flip method for recording 2D J-resolved C-NMR spectra. The application of the 180° pulse as well as 180° pulse in the center of the evolution...

There are basically three main types of 2D NMR experiments J-resolved, shift correlation through bonds (e.g., COSY), and shift correlations through space e.g., NOESY). These spectra may be of homonuclear or heteronuclear type involving interactions between similar nuclei (e.g., protons) or between different nuclear species (e.g., H with C). 

Three-dimensional NMR spectra, like 2D NMR spectra, may be broadly classified into three mtyor types (a) 3D J-resolved spectra (in which the... 

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 2.6 2D J-resolved solid-state NMR spectrum of Hf( CH2Bu )3/Si02 (soo) (a) and of Zr("CH2Bu )3/Si02-(8oo) (b) trace extracted along the tB dimension of the 2D J-resolved spectrum at S = 106 and 93 ppm, respectively.

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. 

A similar structural study was made with angustifoline. The H and C NMR spectra of the alkaloid in four different solvents were fully assigned by resorting to 2D H- H and COSY and 2D J resolved spectra. 

Figure 10 Schematic representation of nontilted 2D J-resolved experiment. Projection onto the chemical shift axis recovers the high resolution ID NMR spectrum.

Figure 14 Partial 2D J-resolved NMR spectrum for Alloc-Asp-derivatized oxazoli-dinone attached via its side chain carboxyl to SCAL-linked aminomethylpolystyrene swollen with DMF-d7.

In individuals with a deficiency in methylacetoacetyl CoA thiolase (MACT), the conversion of 2-methylacetoacetyl-CoA into acetyl-CoA and propionyl-CoA is inhibited and an accumulation of abnormal catabolic products is observed in the urine. Williams etal. have successfully used both ID and 2D H NMR spectroscopy to investigate the urinary metabolites of two unrelated patients with this disorder. The urine spectra from both patients clearly showed the presence of both 2-methyl-3-hydroxybutyrate and tiglylglycine, which is characteristic for MACT deficiency due to the build-up of metabolites close to the position of enzyme deficiency. However, at 360 MHz, the H NMR spectra are quite crowded and difficult to interpret fully. This difficulty was overcome by the use of the 2D JRES H NMR experiment, which resolved all the proton-proton couplings into the second dimension, allowing much clearer spectral interpretation. In addition to the spin-echo and 2D H J-resolved NMR spectra, 2D H COSY NMR spectroscopy was used to determine the spin-spin coupling connectivities between the protons in the various urinary components. 

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