Spin decoupling methods

Lemieux and Stevens used the spin-decoupling method to extend the chemical-shift assignments for the n.m.r. spectra of hexoses and pentoses in aqueous solutions, and comments on the method have been made. ... 

In this section we shall outline some of the potential applications of N.M.D.R. to problems in carbohydrate chemistry. A brief discussion will be given of both the spin-decoupling (3, 23) and the spin-tickling (3, 21) methods together with an indication of their respective advantages. Since excellent reviews of the N.M.D.R. method have been published (8, 9) it is only necessary here to mention a few relevant items of nomenclature. 

The total correlation spectroscopy (TOCSY) techniques, which come in both 1- and 2-D versions, offer an alternative to 1-D spin decoupling and COSY methods for establishing through-bond connectivities. The important difference between the two is that TOCSY methods allow easy identification of isolated spin systems. For example, using our trusty morpholine compound once more, you can see that it is possible to identify the -CH2-CH2- spin system between the nitrogen and the oxygen atoms, these hetero-atoms, effectively isolating the protons from all others in the molecule. 

During the last few years the versatility of ENDOR spectroscopy has been improved by a number of new techniques which make use either of special types of pumping fields (CP-ENDOR, PM-ENDOR), of more than one rf field (DOUBLE ENDOR, multiple quantum transitions, nuclear spin decoupling) or a different display of the spectrum (EI-EPR). In addition to these techniques, alternative methods have been developed (electron spin echo and electron spin echo ENDOR) which are able to supplement or to replace the ENDOR experiment under certain conditions. The utility of all these various advanced techniques, particularly in studies of transition metal compounds, has recently been demonstrated. 

Subsequently, Uhrinova et al.29 reconsidered the problem using both proton-and carbon-detected experiments. For example, couplings of anomeric carbons were measured from the 13C satellites in proton NMR spectra. The critical factor in these methods is the suppression of signals from protons bound to, 2C atoms. In the pulse-sequence proposed, these protons were selectively inverted by a BIRD (Bilinear Rotation Decoupling) pulse,30 and the spin-echo method introduced by Bendall et al.31 was used. 

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). 

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. 

As an alternative to the above method for eliminating the NOE an instrumental technique is available. This depends upon the realization (247) that the time-dependent behaviour of the NOE and of spin decoupling are different. Thus the NOE takes a time comparable for Tj to build up or to decay after application or removal of a rf field, whereas spin decoupling effects appear or disappear almost instantaneously. Consequently if the proton decoupler is gated off immediately prior to... 

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]. 

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