Spin-decoupling

In the case of broad-band decoupling, the precise frequency of the irradiation is not important, but if it is far away - a few hundred Hz away from the resonances involved - then the decoupling is incomplete small couplings (over more than one bond) disappear, but the large ones are retained. This leads to a simplification of spectra in which there are many overlapping coupling patterns of different nuclei. But note that this technique, called off-resonance decoupling, has now been replaced almost completely by two-dimensional correlation methods, which we address in Section 4.13. 

Schematic pulse sequences for (a) a broad-band decoupling experiment, in which the X nucleus is irradiated during the time of recording the FID, and (b) a variant using a series of composite 180° pulses instead of broad-hand decoupling. 

In C NMR spectroscopy, three kinds of heteronuclear spin decoupling are used In proton broadband decoupling of C NMR spectra, decoupling is carried out unselectively across a frequency range which encompasses the whole range of the proton shifts. The speetrum then displays up to n singlet signals for the n non-equivalent C atoms of the moleeule. 

Quantitative analysis of mixtures is achieved by evaluating the integral steps of lH NMR spectra. This is demonstrated in Fig. 1.11a for 2,4-pentanedione (acetylacetone) which occurs as an equili-brium mixture of 87 % enol and 13 % diketone. 

Relaxation 2,3,6 refers to all processes which regenerate the Boltzmann distribution of nuclear spins on their precession states and the resulting equilibrium magnetisation along the static magnetic field. Relaxation also destroys the transverse magnetisation arising from phase coherence of nuclear spins built up upon NMR excitation. 

Spin-lattice relaxation is the steady (exponential) build-up or regeneration of the Boltzmann dis-tribution (equilibrium magnetisation) of nuclear spins in the static magnetic field. The lattice is the molecular environment of the nuclear spin with which energy is exchanged. 

The spin-lattice relaxation time, Tj, is the time constant for spin-lattice relaxation which is specific for every nuclear spin. In FT NMR spectroscopy the spin-lattice relaxation must keep pace with the exciting pulses. If the sequence of pulses is too rapid, e.g. faster than 3 r/max of the slowest C atom of a molecule in carbon-13 resonance, a decrease in signal intensity is observed for the slow C atom due to the spin-lattice relaxation getting out of step. For this reason, quaternary C atoms can be recognised in carbon-13 NMR spectra by their weak signals. 

FIGURE 4.52. The geminal Karplus correlation. /HH for CH2 groups as function of zlH—C—H. 

This unusually high long-range coupling constant is attributed to the W conformation of the four a bonds between HA and Hh.  

Irradiation of one proton in a spin-coupled system removes its coupling effect on the neighboring protons to which it had been coupled. Thus successive irradiation of the protons of 1-propanol for example, yields the following results  

we have a powerful tool for determining the connectivity of protons through bonds and assigning proton peaks. Furthermore, overlapping peaks can be simplified by removing one of the couplings. 

FIGURE 4.53. (a) Partial spectrum of methyl 2,3,4-tri-O-benzoyl-jS-L-lyxopyranoside at 100 MHz in CDC13. (b) H2 and H3 decoupled, (c) H4 decoupled. Note that there are two diastereotopic H5 protons. Bz = C6H5CO. Irradiation may cause a detectable change in chemical shift of nearby peaks. 

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From the evidence discussed above, the framework of methylkasugaminide is determined to be methyl 2,4-diamino-2,3,4,6-tetradeoxyhexo-pyranoside in which hydrogens at C-4 and C-5 are axial-axial and hydrogens at C-l and C-2 are not in axial-axial relation. The structure was definitely proved by the application of the spin decoupling technique and, moreover, the relative relations of all hydrogens were confirmed. 

It now appears that such difficulties can largely be overcome by the application of proton-proton spin-decoupling and especially by means of the very high resolution now available in spectrometers of the superconducting solenoid type (see below). 

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. 

Figure 2. Partial 100 MHz P.M.R. Spectrum of 3,4,6-tri-O-acetyl-v-glucal (1) measured for a chloroform -d solution (A normal spectrum of the Hi and H2 resonances respectively (B) frequency sweep spin-decoupled spectrum of the Hi and H2 resonances, with a strong decoupling field centred on the Hs resonance (C), as in (B) above, but with an additional weak radiofrequency field centred on the high field transition of the H2 resonance (D), as in (B) above, but with a weak radiofreauency field centred on the low field transition...

In contrast, 4-azidoindane (71) yields, in addition to the expected isomeric cyclopentazepines 72 and 73. a third cyclopentazepine which, by lanthanide shift and spin decoupling measurements, was shown to be the isomeric cyclopent[c]azepine 74. 

Before the advent of 2D NMR spectroscopy, the classical procedure for determining proton-proton connectivities was by homonuclear proton spin decoupling experiments. Such experiments can still serve to determine some H/ H connectivities in simple molecules. 

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. 

Gated decoupling The decoupler is gated during certain pulse NMR experiments, so spin decoupling occurs only when the decoupler is switched on and not when it is switched off used to eliminate either H- C spincoupling or nuclear Overhauser effect in a ID C spectrum, and employed as a standard technique in many other H-NMR experiments, such as APT and y-resolved. 

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... 

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. 

Whereas spin decoupling, COSY and TOCSY techniques are used to establish connectivities between protons through bonds, techniques that make use of the nuclear Overhauser effect (NOE), such as 1-D NOE and NOESY, 1- and 2-D GOESY, 1- and 2-D ROESY, can establish connectivities through space. Before looking at these techniques in detail, it s worth spending a little time considering the NOE phenomenon itself - in a nonmathematical manner, of course ... 

Decoupling The saturation of a particular signal or signals in order to remove spin coupling from those signals. Also referred to as spin decoupling. 

The advancement of >400 MHz NMR instruments with spin decoupling and Fourier transform software now allows identification of individual olefinic protons of nanogram carotenoids53. We have shown two examples (lycopene and capsantin) for which the chemical shifts have been employed in the assignment of relative configuration49. As for review of the 13C NMR of carotenoids, Englert in 198154 gave information especially on the position of the cis double bonds in a polyene chain. 

Techniques of Spectral Integration and Spin-Spin Decoupling... 

R.3 in (3.1) is the transpose of the third column of R, and represents a unit vector directed along the static field B0. More generally R j, i = 1,2,3, denotes the ith column of R. This notation proved to be convenient in the description of ENDOR spectra, especially in more complex cases, e.g. in CP ENDOR, PM-ENDOR, DOUBLE ENDOR as well as in spin decoupling experiments (Sect. 4). 

Nuclear spin decoupling 4.4 two rf fields 2.5 Assignment of ENDOR transitions... 

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