Decoupling techniques

From the evidence discussed above, the framework of methylkasugaminide is determined to be methyl 2,4-diamino-2,3,4,6-tetradeoxyhexo-pyranoside in which hydrogens at C-4 and C-5 are axial-axial and hydrogens at C-l and C-2 are not in axial-axial relation. The structure was definitely proved by the application of the spin decoupling technique and, moreover, the relative relations of all hydrogens were confirmed. 

Therefore the discussions of the RGA INA, and decoupling techniques will be quite brief I include them, not because they are all that useful, but because they are part of the history of multivariable control. You should be aware of what they are and of their limitations. 

FTIR and NMR. FTIR spectra of the O-alkylated extracts were obtained (KBr pellets) using a Nicolet lODX spectrometer. The O-methylated extract was prepared using C-enriched methyl iodide (99% C) to enhance the NMR signal of the methyl carbon in the extract. The spectrum was obtained on a pyridine solution of the extract using a Varian XL 300 NMR Spectrometer. Proton decoupling techniques were used and the data was collected in a quantitative mode. 

Spin-lattice relaxation times were measured by the fast inversion-recovery method (24) with subsequent data analysis by a non-linear three parameter least squares fitting routine. (25) Nuclear Overhauser enhancement factors were measured using a gated decoupling technique with the period between the end of the data acquisition and the next 90° pulse equal to eibout four times the value. Most of the data used a delay of eibout ten times the Ti value. (26)... 

C NMR Spectroscopy. 13C NMR measurements were carried out using a JEOL JMN-GSX 400 spectrometer for quantitative analysis in order to examine the amount of bound phenol in the phenolated SEL s. The analysis was conducted in DMSO-dg by using gated decoupling technique. 

NMR instruments are now equipped to facilitate spectral interpretation. Thus, it is now possible to nullify the spin-coupling effect between neighbouring nuclei. This spin decoupling technique stems from the belief that a nucleus in resonance does not always conserve the same spin state with time. It alternates between different states and this effects the neighbouring nuclei. 

By this method, assignments of the signals are, in most cases, readily made by employing first-order analysis with the aid of a decoupling technique, the effectiveness of which is exemplified by the various spectra (see Fig. 3) of D-glucopyranose85 134. 

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). 

As was illustrated for the methyl and carbonyl signals in the 13C NMR spectrum of acetone (Fig. 1.10), 13C— H spin-spin coupling vanishes when proton broadband decoupling is applied. Proton broadband decoupling is the most important decoupling technique used in routine 13C NMR, simplifying 13C — H multiplet systems to spectra of up to z singlet lines for z nonequivalent 13C nuclei of a sample. 

The potential of carbon-13 NMR in the analysis of keto-enol tautomerism has been demonstrated for 2,4-pentanedione (acetylacetone) and dimedone [293]. Quantitative evaluation of equilibrium concentrations is possible by application of the inverse gated decoupling technique illustrated in Fig. 2.23. 

In a recent 13C-NMR study by Halasa et al. (31), the addition of n-BuLi to 1,3-butadiene at DP = 10-12 (Fig. 7) was studied in the absence of polar modifiers. The vinyl carbon at DP 12 showed the usual (=CH2) olefinic position at 110-114 ppm, structure 16, and a new and unusual absorption at 90-100 ppm appeared. The latter resonance was shown by a gated decoupling technique to be split into a doublet. This observation indicates that this carbon is a methine carbon and could be the y-carbon in structure 15 or the y-carbon in structure 16. Since the methine y-carbon atom in structure 15 is an olefinic carbon, the assignment of the... 

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