Decoupling Methods

In NMR-SIM broadband heteronuclear decoupling can be switched on by assigning a rf channel to the decoupled nucleus. As shown in Fig. 2.9 decoupling can be switched on during acquisition and/or prior to the detection pulse. In the following section the most common decoupling methods will be briefly discussed. 

In a homonuclear decoupling experiment a particular multiplet is irradiated suppressing the coupling interaction between the irradiated nucleus and its coupling partners. A comparison of the standard coupled ID spectrum and the selectively homonuclear decoupled spectrum reveals which nuclei are coupled. Whether a homonuclear decoupling experiment or a 2D homonuclear COSY experiment would be the best solution for multiplet analysis in a one-dimensional spectrum depends very much on the nature of the problem under investigation. If a large number of multiplets need to be irradiated then a two-dimension approach may be preferable. 

Technically homonuclear decoupling has to be implemented in a special way. In order to be selective the irradiation power (in Rz) is comparable to the sweepwidth so to prevent the detected signals being masked by the decoupler output the decoupler and the reeeiver are alternatively gated-on and off during the detection period. 

2-3 Hz, but with the ability to shim a sample so that its narrowest peaks have line widths of less than 0.5 Hz, the can be readily detected and quantified. In some instances, J s as large as 7 Hz are observed. 

W-coupling. A four-bond J-coupling occurring when two NMR-active spins are separated by four bonds held in a static, planar conformation that forms the letter W . 

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. 

Homonuclear single-frequency decoupling can also be used to simplify the multiplicities of other resonances. Flomonudear decoupling, however, can potentially overload the receiver, because irradiation may 

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For simplicity, we define T - and T (A iooTe/At). As explained by Luo and Tanner (1989), the decoupled method requires a suitable variable transfonna-tion in the governing equations (3.20) and (3.21). This is to ensure that the discrete momentum equations always contain the real viscous term required to recover the Newtonian velocity-pressure formulation when Ws approaches zero. This is achieved by decomposing the extra stress T as... 

The most common way to record heteronuclear 2D /-resolved spectra is the gated decoupler method, so called because the decoupler is gated, i.e., switched on during the preparation period (for nOe) during the first... 

Hamelberg, D. McCammon, J. A., Standard free energy of releasing a localized water molecule from the binding pockets of proteins Double-decoupling method, J. Am. Chem. Soc. 2004,126, 7683-7689. 

A.M. Dunker. The Direct Decoupled Method for Calculating Sensitivity Coefficients. J. Chem. Phys., 81 2385-2393,1984. 

The most economical and efficient method of sensitivity enhancement in 13C NMR of organic molecules is the pulse Fourier transform technique (PFT) in combination with decoupling methods such as proton broad band decoupling and polarization transfer. These methods will be described in the following sections. 

In the work described in the following sections, 13C assignments are made utilizing a variety of H decoupling methods which are conveniently summarized here. 

In solid state cross polarization the spin lock is obtained with a long, high power pulse, but for HOHAHA such a single, unmodulated pulse is not effective. Instead, the pulse cycle MLEV-16, as described in Section 9.6, or a variant with an additional pulse, MLEV-17, covers a sufficiently broad frequency range, much as in the broadband decoupling methods we discussed in Section 9.6. 

FIG. 16. A portion of the two-dimensional J-spectrum of pyridine corresponding to the C site, obtained by the "gated decoupler" method. Aliasing caused by the low sampling rate has drawn the two halves of the spectrum together by 72 Hz, From ref 153,... 

Decoupled methods as well as direct Newton methods have been used to accoimt for the nonhnearities in the constitutive equations. Those methods exhibit limited convergence, especially when flows with singularities are considered. Newton methods are efficient but need a high storage area and powerful computational flidlities. 

In this paragraph, a Lesaint-Raviart method is presented. A Newton algorithm allowing fixed values of the viscoelastic extra-stress components outside the finite elements is used. A fixed-point algorithm on those exter extra-stress components is also involved. Tffis quasi-Newton method needs a storage requirement of the same size as that related to a classical decoupled method, but allows improved convergence [39]. 

In the off-resonance decoupling method, the residual carbon-hydrogen coupling constant Jj-es of the methylene group of dioxane is measured. Two sequential, off (proton)-resonance experiments are performed, one with the decoupler situated at a higher frequency than the chemical shift of the methylene protons and a second with the decoupler located at... 

Using Lee-Goldburg based decoupling methods, a fwhmh of 150—170 Hz has been reported for the aliphatic H resonances in L-alanine78—this is demonstrated in part c of Figure 5, where, for comparison, the (a) static and (b) MAS (pr = 30 kHz) spectra are also shown. The spectrum in Figure 5c was recorded... 

Figure 15 presents the pulse sequence for a general separated local field (SLF) experiment.133-136 The basic principle of the SLF technique is that a spinning-sideband pattern, from which the heteronuclear dipolar coupling can be extracted, is obtained in the indirect dimension for each resolved resonance in the direct dimension, i.e., the dipolar interaction is separated from the chemical shift interaction. In the original SLF papers, a homonuclear decoupling method is applied in t, but recently McElheny et al. 

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