Noise decoupling, nmr

Fig. 1 a-c. Simulated proton noise decoupled NMR spectra of a polyketide-derived moiety a at natural abundance b enriched from [1,2" C2] acetate c after cleavage or rearrangement of an originally intact acetate unit... 

Austin (89) is clearly biosynthetically related to andibenin B and is formed from the same key intermediate (86). An interesting possibility was that the tetracyclic intermediate (91) involved in austin biosynthesis (Scheme 24) is not formed directly from cyclisation of (86) but is formed via the same bicyclo-farnesyl intermediate (87) involved in andibenin biosynthesis (Scheme 22). To test this, [6- C, 6-2H3]mevalonate was fed to 5-day old cultures and the resultant enriched austin isolated and its H, noise decoupled nmr spectrum determined. This showed clear isotopically shifted signals corresponding to the incorporation of two and mainly three deuteriums into the 12-, 13- and 14-methyl groups. The result for the 12-methyl group excludes the possibility of the involvement of (87) in austin biosynthesis. 

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. 

Figure 5. a, NMR spectra (at 100 MHz, in CCI under pressure) of a mixture of gaseous propane, propene, and cyclopropane, b, Proton noise-decoupled NMR spectra (at 15.4 MHz, in CCl under pressure) of some of the reaction products obtained from the electrolysis of the [2,2-d,] butyrate ion Expansion, proton-coupled deuterium resonances. (Reproduced from Ref. 6. Copyright 1980, American Chemical Society.)... 

A Varian T-60 H, NMR, and Carey-14 UV spectrometers were employed to detect the presence of the CTC. High resolution noise decoupled NMR spectra were obtained by use of a Varian XL-100-15 Spectrometer, and Nicolet Technology TT-100 Data System. All spectra were taken in deuteroacetone. 

Figure 5.6. The noise decoupled NMR spectrum of 4-chloroethylbenzene in HCCh solvent.

Figure 6.10 252 MHz proton noise decoupled NMR spectrum at 125 °C of 97 3 wiv ethylene-propylene copolymer in 1,2,4-trichlorobenzene and perdeutrobenzene. Internal standard hexamethyldisoloxane.

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.

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. 

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... 

Comparison with the complete 13C NMR signal assignments of steroids enabled the interpretation of the 13C NMR spectra of structurally related cardenolides and sapogenins [596, 597]. Besides single-frequency off-resonance decoupling and low power noise decoupling, spectra of specifically deuterated compounds were used as additional aids for the signal identifications. The 13C chemical shifts are collected in Table 5.12 and the... 

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