Decoupling of protons

Most 13C spectra are run on Fourier-transform NMR (FT-NMR) spectrometers using broadband decoupling of proton spins so that each chemically distinct carbon shows a single unsplit resonance line. As with NMR, the chemical shift of each 13C signal provides information about a carbon s chemical environment in the sample. In addition, the number of protons attached to each carbon can be determined using the DEPT-NMR technique. 

The heteronuclear COSY experiment investigates the connectivities imparted by coupling paths between two different nuclei, most commonly those between and The experiment is facilitated by the fact that one-bond CH coupling constants are much larger than the two or three bond (CCH or CCCH) coupling constants. Decoupling of protons from is accomplished... 

Fig. 2.42. I3C NMR spectra of D-camphor in tetradeuteriomelhanol at 15.08 MHz (a). /-modulation of aliphatic carbon signals depending on the decoupling delay z, a verification of Fig. 2.41 (b) proton broadband decoupled spectrum (c-e). /-modulated spin-echo experiments with z = 4, 6, and 8 ms for CH multiplicity analysis (f-g) spectra with off-resonance (0 and gated decoupling of protons (g) for comparison.

Selective probe heads are used for optimal sensitivity for a particular nucleus. Sensitivity of a selective H probe head is normally greater than that of a switchable probe head with indirect observation. With a selective X-nucleus (a nucleus other than proton) probe head, decoupling of protons is normally possible. Because of their limited usefulness, selective probe heads are rare in NMR laboratories. Other probe heads are also available, for example, those for triple resonance experiments and experiments utilizing pulsed-field gradients. In addition to their suitability for 2-D experiments, the gradients are particularly suitable for solvent suppression (20). 

Spectra are usually recorded with complete decoupling of protons by broadband noise decoupling. The general scheme of the sequence for a routine spectrum is illustrated in Fig. 5.4.5a and a set of recording values are listed in Table 5.4.2. 

FIG. 2. Simultaneous decoupling of protons and F from - C. Upper trace normal fully coupled C spectrum at 25 MHz of 2.2,3.3-tetrafiuoropropanol. CHFjCFjCH OH. Lower trace with wide-band H. F decoupling. The total spectral width is 4000Hz. By courtesy of JEOL (U.K.) Limited. 

The second refocusing period again is a spin echo in which chemical shifts are focused by the 180° pulses. The spin roles, however, are reversed in the second set, so that I magnetization is refocused back to two positive peaks for the CH case. The decoupling of protons during carbon acquisition thus does not result in the cancelation of any peaks. The spectrum that is obtained contains decoupled peaks with enhanced intensity. Figure 5-22 compares the various experiments for chloroform. 

As a conclusion for this section, INS studies of KHCOs single-crystals provide the most detailed, and hopefully the most tutorial, view of proton dynamics ever obtained. The limitation of optical techniques to establishing an unambiguous representation of proton dynamics is emphasized. Effective oscillator masses of 1 amu are determined for each normal mode. Then arises a new fundamental question which mechanisms can account for the decoupling of proton dynamics from the lattice ... 

In pulsed or gated decoupling of protons (broadband decoupling only between FIDs), coupled C NMR spectra are obtained in which the CH multiplets are enhanced by the nuclear Overhauser effect (NOE, see Section 1.9). This method is used when CH coupling constants are required for structure analysis because it enhances the multiplets of carbon nuclei attached to protons the signals of quaternary carbons two bonds apart from a proton are also significantly enhanced. Figure 1.10 demonstrates this for the carbon nuclei in the 4,6-positions of 2,4,6-trichloropyrimidine. 

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. 

Tunnelling spectroscopy is unique to observing quantum nonlinear dynamics in crystals. Evidence for proton transfer along hydrogen bonds is another outstanding contribution of INS. It is another manifestation of the decoupling of proton dynamics from the crystal lattice. The quantum nature of proton transfer dynamics even at room temperature is quite unforeseen and contrasts with mechanisms based on semiclassical diffusion jumps. 

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