Quadrupolar echo modulation

The development of the double resonance experiments described in Section 3.8.2 was targeted at spin- /i systems. TRAPDOR and Rotational-Echo Adiabatic Passage Double Resonance (REAPDOR) experiments are designed specifically for quadrupolar nuclei and do not work on systems containing only spin- /2 nuclei. These MAS experiments still rely on the modulation of the heteronuclear dipolar coupling (as in the REDOR experiment) to prevent an echo from refocusing, but the modulation is no longer by 180° pulses. 

Fig. 11 a-d Pulse sequences and coherence transfer pathways for the 2D STMAS experiment a two-pulse sequence as described by Gan b amplitude modulated z-filter acquisition scheme c phase-modulated shifted-echo experiment d split-ti shifted-echo experiment. The values of k, k and k" are chosen to refocus the second-order quadrupolar broadening at the end of the period... 

Another very important experiment for spectral editing is a heterocorrelation experiment combining MQMAS (in the indirect dimension) with CP. Introducing CP at the MQMAS echo position following a split-fj MQMAS scheme creates a spin-j detected spectra (in the dimension), which is modulated by the isotropic frequency of the quadrupolar nucleus in F. HETCOR has been demonstrated on Na- P pairs, on 2 A1- P pairs and lately on a- H pairs,where proton resolution during F was obtained with the wPMLGS multiple pulse decoupling technique. ... 

With the commercial availability of pulsed EPR instrumentation, other pulsed methods such as electron-spin echo envelope modulation (ESEEM) or hyperfine sublevel correlation spectroscopy (HYSCORE), which are quite useful to study specific hyperfine and quadrupolar couplings, have also been applied to flavoproteins [59, 61, 62]. These studies have been reviewed recently, e.g., in [46]. 

The reason here is that the l N nucleus has a small magnetic moment, which is hard to drive by r.f. fields, the difficulty being compounded in the case of frozen solution samples by line broadening due to the l N nuclear quadrupolar coupling. Echo envelope modulation effects are, on the other hand, quite easy to see for nuclei with small moments. For the simple case of an I = 1/2 nucleus weakly coupled to an S = 1/2 electron, it can be shown that the modulation depth is independent of the nuclear moment and depends only on the ratio between the Zeeman field and the local field at the nucleus due to the electron.15 Breadth of the shfs line is, moreover, not a serious obstacle to detection, provided that at least one modulation cycle can be seen in the echo envelope. 

In order to avoid signal modulation that could lead to spurious sidebands at fractions of the spinning speed, it is very important to fulfil rotor synchronisation. The two pulses forming the echo on the observed quadrupolar nucleus must be rotor-synchronised to avoid minor modulation due to the quadrupolar interaction. Even though the recoupling sequences on spin 1/2 are rotor-synchronised by design, the start of excitation and reconversion parts must be rotor-synchronised as well, at all time during ti evolution. 

Two Dimensional Multiple Quantum MAS. For non-integer spin quadrupolar nuclei a two dimensional (2D) Multiple Quantum Magic Angle Spiiming (2D MQMAS) technique emerged in 1995 that has had a profound effect on their study, and has meant that much higher resolution spectra can be produced from these nuclei in sol-gel materials. This experiment correlates the (m, -m) multiple quantum transition to the (1/2, -1/2) transition (Fig. 3-2(c)). The resolution enhancement stems from the fact that the quadrupole frequencies for both transitions are correlated. At specific times, the anisotropic parts of the quadrupole interaction are refocused and an echo forms. The amplitude of the echo is determined onlyhy isotropic interactions, so that the experiment is repeated at different echo times which forms the 2D data set where the echo amplitude is modulated only by isotropic effects. FT of the echo amplitude then produces an isotropic spectrum that... 

For quadrupolar nuclides in solid samples that have a large electric quadrupole interaction, C, the echoes exhibit splittings and modulations that depend on the magnitude of C, which may thus be measured. 

Electron-spin echo envelope modulation (ESEEM) spectroscopy is a pulsed technique that detects weak hyperfine and quadrupolar couplings in the solid state. It is complementary to ENDOR in that it gives... 

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