Decoupling, noise

Figure 2.62 The31P NMR spectrum of mer-RhCljfPMej, with random noise decoupling of the protons. (Reproduced with permission from J. Chem. Soc., Dalton Trans., 1973, 704.

Proton noise decoupled 13C-NMR spectra of equimolar mixtures of the cyclic hexamer and metal thiocyanates showed that the signals of the carbonyl carbon and two methine carbons gave downfield shifts upon the addition of metal thiocyanates, while those of the three methylene carbons of the tetrahydropyran ring gave upfield... 

Figure 3. NMR-DEPT spectra of maitotoxin in CD3CN-D2O (1 1) A, methyls and methines appear as positive peaks and methylenes as negative peaks B, only methines appear C, no quarternary carbons appear and D, a conventional noise-decoupled spectrum.

As stated earlier, since tt]/ = yff2yr and since the gyromagnetic ratio of proton is about fourfold greater than that of carbon, then if C is observed and H is irradiated (expressed as C H ), at the extreme narrowing limit Ti, = 198.8% i.e., the C signal appears with a threefold enhancement of intensity due to the nOe effect. This is a very useful feature. For instance, in noise-decoupled C spectra in which C-H couplings are removed, the C signals appear with enhanced intensities due to nOe effects. 

Carbon-13 nuclei, due to their low natural abundance, do not interact with each other in a molecule, though they are affected by adjacent protons. In practice, such couplings are removed by irradiation of the whole spectrum as it is recorded, in a technique known as proton noise decoupling. This means that practical NMR spectra exhibit one unsplit signal for each type of carbon atom present in the sample. 

Proton-noise decoupled and single-frequency off-resonance decoupled carbon-13 NMR spectra were determined for the CTC Working Standard (Figure 13). 

In the 1H noise-decoupled 75.5 MHz 13C NMR spectrum of 72, the signals of the sp-hybridized carbon atoms C15 and C. 15 are found at 98.3 and 97.3 ppm. This is in the expected region for substituted alkynes and the chemical shifts agree very well with those of other didehydrocarotenoids. As can be seen in Table 21, the 15,15 -triple bond leads to an upheld shift of ca 22 ppm for the directly connected C14 and C. 14. The chemical shifts of the other carbon atoms of the polyene chain are also affected a downheld shift is observed for the odd carbon atoms and a (shght) upheld shift for the even carbon atoms, both decreasing with increasing distance from the central part. 

Figure 5. Proton noise-decoupled 22.6-MHz C-13 NMR spectrum of the hydrogenated 1,4-polyisoprene sample. Perdeuteriohenzene solution at 25°C with TMS as internal reference. Approximately 5000 pulses with an acquisition time of 0.7 sec and a flip angle of 30°.

The proton noise-decoupled 13c-nmr spectra were obtained on a Bruker WH-90 Fourier transform spectrometer operating at 22.63 MHz. The other spectrometer systems used were a Bruker Model HFX-90 and a Varian XL-100. Tetramethylsilane (TMS) was used as internal reference, and all chemical shifts are reported downfield from TMS. Field-frequency stabilization was maintained by deuterium lock on external or internal perdeuterated nitromethane. Quantitative spectral intensities were obtained by gated decoupling and a pulse delay of 10 seconds. Accumulation of 1000 pulses with phase alternating pulse sequence was generally used. For "relative" spectral intensities no pulse delay was used, and accumulation of 200 pulses was found to give adequate signal-to-noise ratios for quantitative data collection. 

Figure 1 shows the proton noise-decoupled C-NMR spectrum of a polytetrahydrofurein (polytetramethylene ether glycol, PTMEG) dissolved in THF. In this spectrum the carbons numbered 1, 2 and 3 which cure a to the oxygen appear at lower field them the 6-carbons labeled as 4, 5 and 6. The carbon atoms in the polymer are clearly resolved from the corresponding carbons of the THF monomer. The fact that carbons 3 and 4 near the hydroxyl end-groups can be easily identified shows the excellent resolution of this technique. 

Schaefer and Natusch have shown that for many synthetic high polymers in solution the NOE factors and relaxation times of carbon atoms in or near the main chains eire similcir (.2. In such cases the relative peak areas in the spectra obtained by the noise-decoupled and fast pulsing technique can be used as a good approximation for quantitative microstructure euialysis. However for our investigation of the polymerization of cyclic ethers we are frequently interested in the quantitative measurements of monomers and oligomers as well as the concentrations of the continuously growing polymeric species. Therefore, the assumption of Schaefer and Natusch is not applicable. 

Figure 12. H Noise decoupled 13C-NMR spectrum of a mixture of acetals from racemic 3-mcthylcyclohex-anone and (R)-butane-2,3-diol66.

C- NMR was measured in the mode of 1H noise decoupling without NOE, using D2O sealed up in a capillary tube. All UV and NMR measurements were performed under precise temperature control. 

C NMR spectra of poly(3-methyl-1-butene) and poly(4-methyl-1-pentene) were determined with a Varian CFT-20 spectrometer operated at ambient probe temperature ( 35° C) using 20-30% solutions of polymer in deuterated chloroform. Spectra were obtained utilizing off-resonance coupling and white noise decoupling techniques for both poly(3-methyl-l-butene) and poly(4-methyl-l-pentene). 

Carbon -13. Use of 13C in NMR developed slowly because of the low natural abundance of this isotope. Another complication was the occurrence of 13C- H coupling involving the many protons normally present in organic compounds. The latter problem was solved by the development of wide-hand proton decoupling (noise decoupling). With a natural abundance of only 1.1%, 13C is rarely present in a molecule at adjacent positions. Thus, 13C-13C coupling does not introduce complexities and in a noise-decoupled natural abundance spectrum each carbon atom gives... 

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