Continuous-wave decoupling

Answer The spectrum was obtained with continuous wave decoupling. The -decoupler was set to the low-frequency, high-field, end of the spectrum, thereby decoupling the high-field methyl protons more efficiently than the lower field methylene and methine protons. 

Continuous wave decoupling, CW decoupling. The application of a single frequency of RF to a sample for the purpose of selectively irradiating (saturating) a particular resonance, thus perturbing other resonances from spins that interact with the spins corresponding to the irradiated resonance. 

The noise-decoupled natural-abundance n.m.r. spectra of several pyrimidine and purine ribo- and 2 -deoxyribo-nucleosides have been measured at 18.24 MHz and fully assigned. " The use of off-resonance H continuous-wave decoupling as an aid to the assignment of signals was discussed. Natural-abundance N n.m.r. spectra have also been recorded for several naturally occurring nucleosides (in DMSO) and nucleotides (in water) by Roberts s group, who also studied the effects of protonation on the N chemical shifts, which enabled the site of protonation to be detected directly. ... 

Figure 5 Portions of the C n.m.r. spectra of asperlin from CHjCOjNa (a) continuous-wave decoupled spectrum (in benzene)-, (b) proton-noise-decoupled spectrum (in benzene) (c) continuous-wave decoupled spectrum (in dioxari)...

Give an exact descnption of mstrumentation—magnetic field strength. Continuous Wave (CW) or Founer Transformed (FT), pulse sequence, decoupling, etc. 

I assume that you are conversant with basic principles of XH or proton NMR spectroscopy as applied to small molecules. In particular, I assume that you understand the concepts of chemical shift (8) and spin-spin coupling, classical continuous-wave methods of obtaining NMR spectra, and decoupling experiments to determine pairs of coupled nuclei. If these ideas are unfamiliar to you, you may wish to review NMR spectroscopy in an introductory organic chemistry textbook before reading further. 

Selective decoupling experiments are sometimes still useful in correlating specific peaks via spin coupling and deducing bonding networks, and continuous wave NOE can occasionally be helpful in establishing spatial relationships. Generally, however, correlations and their implications for molecular structure are best carried out via 2D techniques. 

The most efficient way to speed up spin diffusion is the so-called r.f.-driven spin-diffusion experiment [15, 19] where the chemical-shift differences are removed by r.f. irradiation. For small chemical-shift differences, r.f.-driven spin diffusion can be implemented by applying a continuous-wave r.f. field to the S-spins which can theoretically be described by a transformation into a tilted rotating frame (see Appendix B). To zeroth-order average Hamiltonian theory the chemical-shift differences are removed (fl, — fty = 0 for all spins i and j) and the dipolar-coupling frequencies are scaled by a factor s = -1/2. The scaled-down (or ideally vanishing) chemical-shift difference allows one to keep the zero-quantum line narrow by decoupling the protons. This results in fast spin-diffusion rates. Furthermore, the rate constants are now determined by the S-spin coupling network, and the proton spins need not be considered for the data analysis. 

Lineshape and resolution tests on other nuclei follow a similar procedure to that above. Not all nuclei available with a given probe need be tested and typically only tests for inner and outer coil observations on multinuclear probes are required. This means the second test will often involve carbon-13 for which two samples are in widespread use the ASTM (American Society for Testing and Materials) test sample (40% p-dioxane in deuterobenzene also used for the sensitivity test) or 80% benzene in deuteroacetone. In either case on-resonance continuous-wave (CW) decoupling of protons should be used as this provides improved results for a single resonance relative to broadband decoupling. Rather long (30-40 s) acquisition times will be required for a well shimmed system. 

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