Decoupling, broadband

Aue W P, Kharan J and Ernst R R 1976 Homonuclear broadband decoupling and two-dimensional J-resolved NMR spectroscopy J. Chem. Phys. 64 4226-7... 

Decoupling (Section 13 17) In NMR spectroscopy any process that destroys the coupling of nuclear spins between two nuclei Two types of decoupling are employed in NMR spectroscopy Broadband decoupling removes all the H- C couplings ojf resonance decoupling removes all H- C couplings except those between directly bonded atoms... 

In C NMR spectroscopy, three kinds of heteronuclear spin decoupling are used In proton broadband decoupling of C NMR spectra, decoupling is carried out unselectively across a frequency range which encompasses the whole range of the proton shifts. The speetrum then displays up to n singlet signals for the n non-equivalent C atoms of the moleeule. 

Figure 1.9. NMR spectra of a mixture of ethanol and hexadeuterioethanol [27 75 v/v, 25 °C, 20 MHz], (a) H broadband decoupled (b) without decoupling. The deuterium isotope effect Sch - d on chemical shifts is 1.1 and 0.85 ppm for methyl and methylene carbon nuclei, respectively...

Figure 2.4. CH multiplicities of a-pinene (1) [hexadeuterioacetone, 25 C, 50 MHz], (a) h broadband decoupled NMR spectrum (b) DEPT subspectrum of CH (c) DEPT subspectrum of all C atoms which are bonded to H (CH and CH3 positive, CH2 negative) (d) an expansion of a section of (c). Signals from two quaternary C atoms, three CH units, two CH2 units and three CH3 units can be seen...

Figure 2.9. NMR spectra of 3,4-dimethoxy-p-methyl-p-nitrostyrene (9) [CDCI3, 25 C, 20 MHz], (a, b) H broadband decoupled, (a) complete spectrum with CH3 quartets at Sc = 14.1 and 56.0 (b, c) decoupled and coupled partial spectrum of benzenoid and alkene carbon atoms, (c) obtained by gated decoupling...

The pulse sequence which is used to record CH COSY Involves the H- C polarisation transfer which is the basis of the DEPT sequence and which Increases the sensitivity by a factor of up to four. Consequently, a CH COSY experiment does not require any more sample than a H broadband decoupled C NMR spectrum. The result is a two-dimensional CH correlation, in which the C shift is mapped on to the abscissa and the H shift is mapped on to the ordinate (or vice versa). The C and //shifts of the //and C nuclei which are bonded to one another are read as coordinates of the cross signal as shown in the CH COSY stacked plot (Fig. 2.14b) and the associated contour plots of the a-plnene (Fig. 2.14a and c). To evaluate them, one need only read off the coordinates of the correlation signals. In Fig. 2.14c, for example, the protons with shifts Sh= 1.16 (proton A) and 2.34 (proton B of an AB system) are bonded to the C atom at c = 31.5. Formula 1 shows all of the C//connectivities (C//bonds) of a-pinene which can be read from Fig. 2.14. 

The main contribution to the spin-lattice relaxation of C nuclei which are connected to hydrogen is provided by the dipole-dipole interaction (DD mechanism, dipolar relaxation). For such C nuclei a nuclear Overhauser enhancement of almost 2 will be observed during H broadband decoupling according to ... 

No further information is required to identify this compound from its NMR spectra 15. Conditions CDCI3, 25 °C, 20 MHz. (a) Proton broadband decoupled spectrum (b) NOE enhanced coupled spectrum (gated decoupling) (c) expanded section of (b). 

Conditions (CD3)2CO, 25 °C, 200 MHz H), 50 MHz ( C). (a) //NMR spectrum with expanded sections (b,c) C NMR partial spectra, each with proton broadband decoupled spectrum below and NOE enhanced coupled spectrum above with expanded multiplets at 6c = 76.6 and 83.0. 

Conditions CDCI3, 25 °C, 100 MHz ( C), 400 MHz H). (a-e) C NMR spectra (a,b) //broadband decoupled spectra (c,d) NOE enhanced coupled spectra (gated decoupling) with expansion (e) of the multiplets in the sp shift range (f) //NMR spectrum with expanded multiplets. 

In the //broadband decoupled C NMR spectrum, 15 carbon signals can be identified, in agreement with the molecular formula which indicates a sesquiterpene. The DEPT experiments show that the compound contains four quaternary C atoms, three CH units, seven CH units and a CH3... 

The sample prepared is not particularly pure, so instead of the 30 signals expected, 33 signals are observed in the // broadband decoupled C NMR spectrum. Only by pooling information from the DEPT experiment and from the reliable analysis be obtained, as shown in Table 51.1. Here the AB systems of the geminal CH2 protons are assigned. 

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