Homonuclear decoupling. NMR

The mechanical properties of PLA rely on the stereochemistry of insertion of the lactide monomer into the PLA chain, and the process can be controlled by the catalyst used. Therefore, PLAs with desired microstructures (isotactic, heterotactic, and S3mdiotactic) can be derived from the rac- and W50-Iactide depending on the stereoselectivity of the metal catalysts in the course of the polymerization (Scheme 15) [66]. Fundamentally, two different polymerization mechanisms can be distinguished (1) chain-end control (depending on stereochemistry of the monomer), and (2) enantiomorphic site control (depending on chirality of the catalyst). In reality, stereocontrolled lactide polymerization can be achieved with a catalyst containing sterically encumbered active sites however, both chain-end and site control mechanisms may contribute to the overall stereocontrol [154]. Homonuclear decoupled NMR analysis is considered to be the most conclusive characterization technique to identify the PLA tacticity [155]. Homonuclear... 

Fig. 25 Homonuclear decoupled NMR spectrum of the methine (—C//(CH3)-) region of heterotactic PLA P = 87%) obtained from ROP of rac-lactide with [L2Mg2(p-OBn)2]2 (L = (Z)-4- [2-(dimethylamino)ethylamino](phenyl)methylene -3-methyl-l-phenyl-pyrazol-5-one) at 0 °C for 48 h (600 MHz, CDCI3) [70]...

Conditions monomeninitiater ratio 100 1( 1 BnOH if required) solvent toluene, temperature 80°C. Determined from H NMR analysis. Determined from GPC analysis using THF as the solvent and reference to polystyrene standards. Determined from the analysis of the methane region of the H homonuclear decoupled NMR spectrum. 

FIGURE 8.15 Left methine resonances in the NMR spectra of poly(lactide) samples (a) 1, (b) 2, and (c) 3. The peak at about 68.95 ppm comprises resonances from Hi, iis, sii, and sis core stereosequences. Right methine resonances in the homonuclear decoupled NMR spectra of poly... 

Figure 1.8. Homonuclear decoupling of the CH protons of 3-aminoacrolein (CDaOD, 25 C, 90 MHz), (a) H NMR spectrum (b) decoupling at Sh = 8.5 (c) decoupling at = 7.3. At the position of the decoupled signal in (b) and (c) interference beats are observed because of the superposition of the two very similar frequencies...

As with any other homonuclear decoupling, the transverse magnetization of the 13C , for example in the evolution time, is perturbed by the decoupling pulse, resulting in an additional precession virtually around the z axis. Since the adiabatic decoupling is applied during the entire evolution time of the 13C , a non-linear frequency shift rather than a phase shift appears in the spectrum. This is termed the Bloch-Siegert (frequency) shift in the NMR literature in honour of their discovery of the phenomenon. 

The UV spectrum (/Imax 218, 237, 260,299,325, and 338 nm) of tubingensin B (380) was very similar to that of tubingensin A (379), indicating the presence of a similar carbazole framework. Comparison of the H-NMR spectrum of tubingensin B (380) with that of tubingensin A (379) indicated that these two alkaloids are structural isomers with a similar 2,3-disubstituted carbazole substructure. This was also supported with homonuclear decoupling and COSY experiments. In addition,... 

The PLA tacticity (and stereocontrol mechanism) can be assessed by analysis of the NMR spectrum of the PLA. This can be carried out either by analysis of the homonuclear NMR spectrum, at the tetrad level, or by analysis of the I3C H NMR spectrum (carbonyl region). An illustration showing the assignment of the tetrads in the methane region of the homonuclear decoupled 1H H NMR spectrum is included in Fig. 8. 

Fig. 8 Methine region (homonuclear decoupled 1H NMR spectrum) of moderately isotactic PLA (Pi = 0.69) Table 2 rac-Lactide tetrad probabilities based on Bemoullian statistics ...

Heteronuclear correlation (HETCOR) spectroscopy is a standard two-dimensional NMR technique. The resolution of 31P 1H CPMAS spectra is generally not sufficient to unequivocally detect the HP042 ions and the apatitic OH- ions in bone or dentin samples.15 In this regard, the breakthrough came from the first application of 31P 1H HETCOR to HAp and bone samples,24 where the correlation peak at 0.2 and 3 ppm of the 1H and 31P dimensions, respectively, has been established as the spectral marker of apatitic OH- ions. To enhance the spectral resolution in the XH dimension, 1H homonuclear decoupling was employed during the fi... 

Although we might be able to get all the H-H coupling information through a long series of homonuclear decoupling experiments, there is a much simpler way the 2D NMR technique known as homonuclear shift correlation spectroscopy, or COSY. 

The first and most direct evidence for the existence of the M 3Sp- and M 4Sn-cores in mammahan MTs came from the homonuclear " Cd NMR decoupling studies on " Cd reconstituted Cd7-MT-2. Evidence for a similar cluster organization involving the tetrahedral M°S4 centers was also obtained for Co - and Fe -containing MT derivatives. For C07-MT such evidence has come indirectly from the temperature dependence of isotropically shifted proton resonances (between -50 and 300 ppm) of the Cys residues of the Co4-cluster in the C-terminal domain. This was further confirmed by MCD, ESR (see Electron... 

If one is truly pushing the limits of NMR detection or has only an extremely time-sensitive sample, yet another strategy is preferred. Here a 1-D proton spectrum may be all that is realistically attainable. Possible additional experiments include a 1-D version of TOCSY and a 1-D version of COSY, also known as a homonuclear decoupling experiment. This applies to sample quantities down to hundreds of nanograms, which is currently the practical limit of NMR detection using commercially available equipment. 

A. Bielecki. A. C. Kolbert and L H. Levitt, Frequency-switched pulse sequences - homonuclear decoupling and dilute spin NMR in solids. Chem. Phys. Lett., 1989, 155, 341-346. 

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