Selective spin decoupling

One-dimensional double-resonance or homonuclear spin-spin decoupling experiments can be used to furnish information about the spin network. However, we have to irradiate each proton signal sequentially and to record a larger number of ID H-NMR spectra if we wish to determine all the coupling interactions. Selective irradiation (saturation) of an individual proton signal is often difficult if there are protons with close chemical shifts. Such information, however, is readily obtainable through a single COSY experiment. 

A wide variety of ID and wD NMR techniques are available. In many applications of ID NMR spectroscopy, the modification of the spin Hamiltonian plays an essential role. Standard techniques are double resonance for spin decoupling, multipulse techniques, pulsed-field gradients, selective pulsing, sample spinning, etc. Manipulation of the Hamiltonian requires an external perturbation of the system, which may either be time-independent or time-dependent. Time-independent... 

Figure 3.59 displays the effect of selective spin decoupling on the neighboring protons of 3-butyne-l-ol... 

The overall information obtained from selective spin decoupling is connectivity —that is, progressing through the protons of the carbon chain. 

As described under spin decoupling (Section 3.15), it is possible to irradiate strongly a particular nucleus at its resonance frequency (v2) and detect loss of coupling (decoupling) in all the nuclei that were coupled to the irradiated nucleus. Detection of the decoupled nuclei is carried out with the usual probe the added decoupler delivers strong, selective pulsed radiation at the resonance frequency of irradiated nucleus. 

The presence of two nuclear spins means that there is considerable choice in the selection of basis functions the reader who wishes to practice virtuosity in irreducible tensor algebra is invited to calculate the matrix elements in the different coupled representations that are possible In fact the sensible choice, particularly when a strong magnetic field is to be applied, is the nuclear spin-decoupled basis set t], A N, S, J, Mj /N, MN /H, MH). Again note the possible source of confusion here MN is the space-fixed component of the nitrogen nuclear spin /N, not the space-fixed component of N. This nuclear spin-decoupled basis set was the one chosen by Wayne and Radford in their analysis of the NH spectrum. 

Prior to the advent of 2D methods, selective spin decoupling was used extensively in both proton NMR and in heteronuclear (especially 13C) NMR to ascertain which sets of nuclei contribute to observed spin coupling. Such information is critical to assignment of resonances and to the elucidation of the structure of an unknown molecule. 2D methods now largely supply this information much more efficiently, by correlations that depend on the existence of spin coupling. The homonuclear version of one such experiment is called COSY (correlation spectroscopy), and the heteronuclear version is known by several acronyms, most commonly HETCOR (lieferonuclear correlation). 

Inversion pulses are very important because of the wide variety of applications in experiments involving coherence selection and spin decoupling. One of the very first methods for spin inversion was the rapid adiabatic passage introduced by Bloch [18]. 

Nagayama, K., Bachmann, P., Ernst, R. R. and Wiithrich, K. (1979) Selective spin decoupling in the /-resolved two-dimensional H NMR spectra of proteins. Biochemical and Biophysical... 

Heteronuclear coupling constants (1,b7c,h) are most commonly measured from heteronuclear 2D experiments. The 3/c H couplings can be easily extracted from /-resolved spectra as well as from f or F2 proton coupled HSQC spectra. The undesired evolution of "/CH during q can be eliminated with use of an appropriate bilinear rotation decoupling (BIRD) pulse, such as BIRDd,x in. /-resolved spectroscopy35 and 111RD in Fi-coupled HSQC.36 Spin-state selective excitation techniques, S3E and S3CT37 38 (spin-state-selective coherence transfer), can also be used for the measurement of... 

In spin decoupling, a second radiofrequency signal is applied while the spectrum is scanned in the usual fashion. This radiofrequency can be set to irradiate a selected group of protons in the molecule being measured. Irradiation causes the selected group of protons to undergo rapid transitions... 

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