Proton noise decoupling

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... 

Figure 4.7 Pulse schemes representing separation of decoupling effects from the nOe during X nucleus acquisition. The decoupler is programmed to produce noise-modulated irradiation or composite pulse decoupling at two power levels. Suitable setting of the decoupler may produce either (a) nOe only, (b) proton decoupling only, or (c) both nOe and proton decoupling.

Proton-decoupled 13C-NMR spectra were recorded on a Varian XL-300 operating at 75.4 MHz. Approximately 250 mg of the sample was dissolved in 3 ml of deuterated chloroform. 13C chemical shifts were referenced internally to CDCL (77 ppm). A delay of 200s was used to ensure relaxation of all the carbon nuclei and 1000 transients were collected to assure a good signal-to-noise ratio. 

For illustration, the proton decoupled 13C NMR spectra of quinoline, obtained from one CW scan and one pulse, respectively, are compared in Fig. 2.18. A further signal noise enhancement of up to 40% arises from application of the QD technique outlined in Section 2.5.4.1 (Fig. 2.18(c)). 

C spin-lattice relaxation times of individual nuclei can also be measured by PFT 13C H experiments using a 90°, r, 90", r,... pulse train and noise modulation of the proton decoupling frequency. This method is known as progressive saturation [43] and is based on the following concept. 

Using single-frequency and noise-modulated resonance and off-resonance proton decoupling, 7] relaxation time measurements, relaxation reagents like Gd (fod)3 and specifically deuterated compounds, all the carbons in retinal isomers, the model compounds a-and /i-ionone, and vitamin A and its isomers [165, 555-557] were assigned. The olefinic ring carbons (C-5 and C-6) could be identified on the assumption that the 13C relaxation times are dominated by intramolecular dipole-dipole interactions with neighboring protons and that the same rotational correlation time characterizes the interactions for both carbons. Consequently the ratio of T/s for C-5 and C-6 can be estimated from eq. (5.1)... 

The proton decoupled 29Si spectrum of tetram-ethylsilane (TMS) is shown at the top of Figure 6.9 with the proton coupled spectrum for comparison as an inset. TMS is the obvious choice for a 29Si reference compound and we set it at zero ppm. The proton-coupled spectrum is quite interesting because the 29Si nucleus is coupled to 12 equivalent protons in TMS. First order rules predict a multiplet with 13 peaks. There are 9 peaks clearly visible and 11 with a little imagination we do not see the full 13 peaks because the outer ones are too weak and are lost in the noise. 

FIGURE 54. 29Si NMR spectra (IGD) of the 3-deuterio derivative 39 under conditions of proton noise decoupling (a) and with selective proton decoupling of (CH3)3Si protons (b). The line due to silicon from the 2HC(3)—O—Si fragment is the line that does not exhibit any splitting in spectmm b. Reproduced by permission of John Wiley Sons, Ltd from Reference 167... 

Fig. 5. Fourier transform i C spectra of 1,1-dimethylcyclooctane at various temperatures. ) Protons are noise decoupled. Only a partial assignment of carbon resonances has been made...

Resonance spectra, employing noise-modulated proton decoupling, have permitted identification of signals due to each carbon atom in the steroid skeleton (up to C28) at the natural abundance of this isotope. Chemical shifts covering a span of 200 p.p.m. are found. Data are reported for a range of sterols and steroid hormones. 

---