Noise decoupling technique

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). 

Tentative assignments of /2-carbon vinyl ether resonances in acetylated kraft lignin preparations have previously been made (S3), Values of 144.3-144.4 ppm (trans) and 141.5-142.0 ppm (cis) in CDCI3 were reported. However, a rather low-field (20 MHz) instrument was used along with the conventional proton noise-decoupled technique, so extreme overlap with quaternary carbons was unavoidable. 

Figure 3.16. Proton noise decoupling techniques (a) single-frequency decoupling (b) noise decoupling. Irradiation of He in (a) results in the collapse of only one carbon, Cc, to a singlet. Proton noise decoupling (b) results in all the carbon atoms appearing as singlets.

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. 

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. 

With a chromatographic technique capable of routinely yielding preparative fractions, quantitative and C FT NMR was the major spectroscopic tool used for chemical characterization. The established utility of and C NMR for characterization of coal products is documented well. Unfortunately, high-resolution C FT NMR is not quantitative normally under operating conditions used typically. (It should be noted that quantitative FT NMR measurements also are not obtained routinely. The problem of variable spin lattice relaxation times (Ti s) is present also in FT NMR. In addition, the greater signal intensity of NMR in comparison with C FT NMR poses an additional potential problem of detector linearity in the FT NMR receiver.) For C FT NMR, variable spin lattice relaxation times (Ti s) and nuclear Over-hauser effects (a result of pseudo random noise decoupling) usually... 

NMR spectroscopy as a routine tool. The double-irradiation field B2 was traditionally centered at about 8 5 of the H range. To cover all the H frequencies, B2 was modulated with white noise, so the technique often was called noise decoupling. 

Early work on the application of NMR conventional techniques to the study of solvent swollen crosslinked polymers was reported by Stenlicht and co-workers [92] and Schaefer [93]. The first detailed work on the chlorome-thylated polystyrene resins used in solid phase synthesis appears to be by Manatt and co-workers [94]. They employed a 0.095% crosslinked species supplied by the Dow Chemical Co. and chloromethylated this to varying degrees. noise decoupled C NMR spectra were recorded in the usual... 

A more recent demonstration of the usefulness of high resolution deuterliim NMR as an analytical technique Is Illustrated In Figure 5. Here deuterltim resonances were used to characterize the radical- and cation-derived products obtained by anodic oxlda tion of the [2,2-d2] butyrate Ion ( ). The assignment of these resonances was made on the basis of the 1 1 relationship between the chemical shifts of and H, and the deuterium label distribution was determined from the proton noise-decoupled NMR spectra. As can be seen from Figure 5, propene was labeled In the terminal olefinic carbon but not in the central olefinlc carbon atom, while propane turned out to be deuterium-labeled exclusively at Cj. 

HC-NMR spectrum of Neu5Ac is shown in Fig. 13. This proton noise-decoupled spectrum shows 11 intense singlets. Assignment of the resonances was made by comparison with spectral data of carbon atoms in similar chemical environments of model compounds and by various proton decoupling techniques (Bhattacharjee et al. 1975). The relatively low intensities for the carbonyl carbons and for C2 are due to the restricted nuclear Overhauser enhancement as a result of the absence of proton substituents at these carbon atoms. In addition, the spectrum shows several small resonances (see Table 11) belonging to the aNeuSAc anomer which is present for 9% in the equilibrium mixture (Jaques et al 1977). 

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