Quadrupolar nuclei/coupling

The predominant H-bond in proteins is the bridge between the backbone amide proton of one amino acid and the backbone carbonyl oxygen atom of a second amino acid (see insert to Fig. 9.3). Although scalar couplings across H-bonds to the magnetic isotope 170 are conceivable, the fast relaxation of this quadrupolar nucleus would prevent such observations in... 

While the terminology scalar relaxation of the first kind concerns J modulation (J spin spin or scalar or indirect coupling constant) by exchange phenomena, the usual example of the second kind is a spin 1/2 nucleus (7), J coupled to a fast relaxing quadrupolar nucleus (S) with relaxation times Tj and Tf (and spin number /s)- The relevant interaction is of the form... 

Figure 4 Schematic representation of the populations of the nuclear spin energy levels of a quadrupolar nucleus with spin 5/2 (such as Mg) under a strong magnetic field and a perturbative quadrupole coupling showing (A) populations at thermal equilibrium, (B) populations after complete saturation of the satellite transitions, and (C) populations after complete inversion of the satellite transitions, following the order first, inversion of STl and ST4 and then inversion of ST2 and ST3. The numbers at left of each level (named pj in the text) are proportional to the population of that level, with —hVl/ 2k T= 0. ...

Another source of microdynamic information comes from the deuterium relaxation rates. The quadrupolar coupling constant (e2qQ/h) expresses the magnitude of the coupling between the electric quadrupole moment Q of the nucleus and the efg q at the nucleus (e is the electronic charge, and h is Planck s constant). It provides a relaxation mechanism for the quadrupolar nucleus of its Fourier component at the Larmor (resonance) frequency for this nucleus is non-negligible. It has such a Fourier component because tumbling of the molecule the quadrupolar nucleus is part of, in a... 

Relaxation rate R,s 2nJ and R,s R/. This situation occurs, for example, where I = 1/2 and is coupled with a moderately large value of J to a quadrupolar nucleus S that relaxes rapidly. (Because of the rapid relaxation of S, the expected splitting of the resonance lines of I does not occur, as we have seen in previous chapters.) Equations for this type of scalar relaxation have been derived ... 

Correct setting of the magic angle is essential in solid-state NMR spectroscopy. The most commonly used method relies on the Br resonance in solid KBr. Br is a quadrupolar nucleus (spin 3/2) ensuring a short relaxation time. The Br first-order quadmpole coupling on the satellite transition ( 3 /2 1 /2) produces a set of distinct spinning... 

Figure 23 Second order shifts in spin-1/2 spectra arising from coupling to a quadrupolar nucleus with S = 5/2, as a function of the parameter %D lu,...

In some cases, when a spin-1/2 nucleus such as P, or is coupled to a quadrupolar nucleus, the simulation by a computer program of the unusual lineshape of the spin-1/2 may provide information about the chemical shift anisotropy, the quadrupole coupling constant and the indirect scalar spin-spin coupling constant involving the metal atom. This method has been applied to the P spectra of phosphines bound to cobalt in heteronuclear clusters [17]. 

Trebosc et al. introduced a frequency-selective (FS) REDOR approach to a multi-spin system Sl y where S is a quadrupolar nucleus. FS-REDOR may be used for accurate trough-space distance measurements in spin pairs that involve quadrupolar nuclei [106]. The experiment reveals heteronuclear dipolar and scalar couplings, which can be reintroduced selectively, site after site. Importantly, FS-REDOR may also be used under high-resolution provided by MQMAS, STMAS or I-STMAS. 

The principle of sample spinning has been described in Section 3.3.3. As a result of sample spinning, the interaction Hamiltonian and thus the resonance frequency (cf. eqn (3.3.6)) becomes time dependent. For the simple case of a pair ij of dipolar coupled spins i, equivalent to a quadrupolar nucleus spin 1 like with, the time-dependent spin Hamiltonian is given by [Mehl, Schl]... 

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