Carbon spinning

Table 7.58 Carbon-Carbon Spin Coupling Constants 7.108...

The transfer of magnetization from the proton spins to the carbon spins occurs now when the Hartmann-Hahn condition, Eq. (2), is fulfilled. 

Figure 2 Schematic representation of carbon spin-lattice relaxation time, T,c, and spin-spin relaxation time under CPMAS or DDMAS condition, T2C, as a function of correlation times.

The advantage of 13C NMR over proton NMR is that much greater range of chemical shifts is exhibited by 13C (and most other nuclei), some 200 ppm in contrast to 10 ppm for H. Moreover, there is no carbon-carbon spin coupling because of the low chemical abundance of the 13C nucleus, 1.1 per cent... 

D 1H-13C CT-HSQC/HMQC identifies proton-carbon spin pairs in both sugar and base. The constant time evolution removes the peak splitting due to 13C-13C coupling. The proton and carbon resonances are subsequently connected by... 

The TROSY effect was also used in a relayed HCCH-COSY experiment to correlate adenine H2/H8 resonances in uniformly 13C-labeled RNA molecules [49], and significant sensitivity over the existing HCCH-TOCSY version was reported. Magnetization is transferred simultaneously in an out-and-back manner from H2 and H8 to the three aromatic carbon spins, C4, C5 and C6, establishing thus the connectivity within the adenine base spin system. 

One-bond carbon-carbon spin-spin coupling constants are included in a review by Krivdin and KalabinR". They are analyzed in terms of hybridization, substitution effects, lone pair effects and steric effects as well as respective applications to structural determination. The carbon-carbon spin-spin coupling constants between carbons that are separated by more than one bond were reviewed by Krivdin and DeUa and are discussed in terms of experimental techniques, the effects of hybridization, substituent effects, steric effects and respective additivity patterns. 

In this experiment a series of selective 180° pulses serves to individually label selected carbon spins prior to the series of C- and H-pulses used for refocusing and polarization transfer. Therefore the frequency of any of these initial 180° pulses is set either on-resonance to the resonance of carbon i (fi), or set off-resonance (/off-res.) which simply inverts (label —) or not inverts (label -f) the corresponding spin polarization respectively. In the example below three target spins with resonance frequencies f, fi and /3 are chosen. A series of three selective pulses has to be applied and at least four experiments have to be performed with the frequencies of the selected pulses set as shown in the acquisition scheme below and four separately... 

Table 6.48 Carbon-Carbon Spin Coupling Constants. 6.93...

---