Spin double-selective

A homonuclear spin-system may be excited with radiofrequency (r.f.) pulses that are so Intense (in the order of p.s), compared to the frequency width of the spectrum, that all resonances are excited essentially uniformly. This is a nonselective excitation. A homonuclear spin-system may also be excited with a relatively weak, r.f. pulse (in the order of ms), in the sense that all components of a given multiplet are inverted at time zero, whereas the other resonances in the spectrum remain essentially unperturbed this is a selective excitation. The r.f. pulse may be single-selective, that is, there is an inversion of one multiplet in the spectrum, or double-selective, triple-selective, and so on, where two, three, or more separate multiplets in the spectrum are inverted simultaneously while the remaining resonances remain unperturbed. 

According to these equations, the effect of selectively perturbing the spin states of spins i and j is to isolate the cross-relaxation paths common to these two spins. Combining Eqs. 15 and 19, the individual cross-relaxation terms are readily determined from single-selective and double-selective relaxation-rate measurements, that is. 

Fig. 2.—A. Normal, H-N.m.r. Spectrum of Asperlin (1) in Benzene-

Selective, spin-lattice relaxation-rates are measured by the inversion-recovery technique. A rather weak, 180° pulse of very long duration (10-50 ms) inverts a multiplet (single-selective) or two multiplets (double-selective) in the spectrum of asperlin (1 see Fig. 2 ) and the recovery of the... 

Fig. 5 Symmetry-based dipolar recoupling illustrated in terms of pulse sequences for the CN (a) and RNvn (b) pulse sequences, a spin-space selection diagram for the Cl symmetry (c) (reproduced from [118] with permission). Application of POST-CVj [31] as an element in a H- H double-quantum vs 13C chemical shift correlation experiment (d) used as elements (B panel) in a study of water binding to polycrystalline proteins (reproduced from [119] with permission)...

Doubly selective ID-TOCSY experiments have been proposed to specifically transfer in-phase magnetization from two designated spins [57, 58]. This transfer will only take place if the two spins are connected by a scalar coupling. This method is achieved by using a double-selective spin-lock after the selective excitation of transverse magnetization of a desired spin. The doubly selective spin-lock can be obtained by using cosine-modulated... 

In the derivation of these spin-interaction selection rules the harmonic approximation was made. In taking nuclear vibration into account2,77 these selection rules are often broken. In addition to coupling with the internal vibrational modes of a molecule, coupling with the phonon modes in the solid state may be important.124 Some use of double point group symmetry will be found in Sections IX, XI, and XII. 

Another method of producing composite hollow fibers, described by Kusuki etal. at Ube [108] and Kopp et al. at Memtec [109], is to spin double-layered fibers with a double spinneret of the type shown in Figure 3.37. This system allows different spinning solutions to be used for the outer and inner surface of the fibers and gives more precise control of the final structure. Often, two different polymers are incorporated into the same fiber. The result is a hollow fiber composite membrane equivalent to the flat sheet membrane shown in Figure 3.26. A reason for the popularity of composite hollow fiber membranes is that different polymers can be used to form the mechanically strong support and the selective layer. This can reduce the amount of selective polymer required. The tailor-made polymers developed for gas separation applications can cost as much as... 

In modem NMR, in order to obtain data on through-bond, scalar connectivities or through-space, dipolar connectivities between individual spins, double or multiple irradiation experiments are used. These rely on selective irradiation of a particular resonance line with a radio frequency field and observation of the resulting effects in the rest of the spectrum. With 2D ESR techniques as well as with 2D NMR techniques, limitations of one-dimentional methods connected with overlapping resonance have been overcome (Fig. 1.7). 

The experiment described above is termed selective population transfer (SPT), or more precisely in this case with proton spin inversion, selective population inversion, (SPI). It is important to note, however, that the complete inversion of spin populations is not a requirement for the SPT effect to manifest itself. Any unequal perturbation of the lines within a multiplet will suffice, so, for example, saturation of one proton line would also have altered the intensities of the carbon resonance. In heteronuclear polarisation (population) transfer experiments, it is the heterospin-coupled satellites of the parent proton resonance that must be subject to the perturbation to induce SPT. The effect is not restricted to heteronuclear systems and can appear in proton spectra when homonuclear-coupled multiplets are subject to unsymmetrical saturation. Fig. 4.20 illustrates the effect of selectively but unevenly saturating a double doublet and shows the resulting intensity distortions in the multiplet structure of its coupled partner, which are most apparent in a difference spectrum. Despite these distortions, the integrated intensity of the proton multiplet is unaffected by the presence of the SPT because of the equal positive and negative contributions (see Fig. 4.19d). Distortions of this sort have particular relevance to the NOE difference experiment described in Chapter 8. 

OPPOSE Double pulsed field gradient spin-echo (selective excitation) 9.3.3... 

Spin-state selective experiments have been an integral part in both the history and development of NMR spectroscopy. Experiments vary from the early simple double-... 

A number of other, more sophisdcated, selective averaging tools (includli spin echo, double resonance and two-dimensional techniques) are available, both for spectral editing purposes and for obtaining quantitadve information about inter-... 

The transitions between energy levels in an AX spin system are shown in Fig. 1.44. There are four single-quantum transitions (these are the normal transitions A, A, Xi, and X2 in which changes in quantum number of 1 occur), one double-quantum transition 1% between the aa and j8 8 states involving a change in quantum number of 2, and a zero-quantum transition 1% between the a)3 and fia states in which no change in quantum number occurs. The double-quantum and zero-quantum transitions are not allowed as excitation processes under the quantum mechanical selection rules, but their involvement may be considered in relaxation processes. 

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. 

Generated Total number of spin and symmetry adapted single and double excitations Selected Number of spin and symmetry adapted configurations selected by second-order perturbation theory and treatedvariationally Property calculated with respect to the center of mass... 

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... 

Significant progress in signal enhancement methods for the central transition has been achieved by the implementation of double frequency sweeps (DFS) [62]. The basic idea of DFS, applicable for both static and MAS experiments, is to invert simultaneously the STs so that the populations of the outer spin levels are transferred to the CT energy levels before they are selectively excited (Fig. 4). 

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