Rotor cycle

Figure Bl.12.15. Some double-resonance pulse sequences for providing distance infomiation in solids (a) SEDOR, (b) REDOR, (c) TEDOR and (d) TRAPDOR. In all sequences the narrow pulses are 90° and the wide pulses 180°. For sequences that employ MAS the luimber of rotor cycles N is shown along the bottom.

Fig. 1 (a) REDOR experiment and (b) typical REDOR dephasing curve (D designates the dipole-dipole coupling, tr the rotor period, and Nc the number of rotor cycles for the REDOR experiment) (reproduced from [78] with permission), (c, d) REDOR based determination of statherin binding to a hydroxyapatite surface (reproduced from [79] with permission)... 

If j Rf is exactly the magic angle and infinite spinning speed is assumed, the first-order anisotropic terms are zero for both single and DQ coherence (33). This does not hold true for finite spinning speed, but a complete averaging of the first-order effect occurs at the exact rotor cycles. Therefore, the x evolution time has to match exactly a multiple of the rotor period. The second-order anisotropy refocusing occurs for... 

Rotational-echo double resonance (REDOR), originally introduced by Gullion and Schaefer [102], is a method to recouple heteronuclear spin pairs. The sequence relies on a train of rotor-synchronized n pulses applied to the I spin to interrupt the spatial averaging of the heteronuclear dipolar coupling under MAS to give a nonvanishing dipolar Hamiltonian over a full rotor cycle (Fig. 11.8). Typically, REDOR data are collected by col-... 

In our context, especially C-Li distances are of interest. A first successful Li- C REDOR study was undertaken in order to establish the structure of the previously discussed TMEDA complex of fluorenyllithium °, prepared from Li-enriched w-butyllithium and fluorene with at natural abundance. The REDOR pulse sequence used is depicted in Figure 20. The number of rotor cycles is increased in a symmetric fashion about the central jr-pulse in order to increase the dephasing time. 

Figure 3 Pulse sequences for the different CT-REDOR versions for two rotor cycles (A) MAS spin-echo reference experiment (B) CT-VPP-REDOR with the pulse position fpp of the /-spin dephasing pulses stepped from 0 to Tp and (C) CT-VPD-REDOR with the pulse width fpp of the /-spin dephasing pulses stepped from 0 to In.

Fig. 22. Pulse sequence used to obtain orientationally weighted 2D MAS exchange spectra, with C = y or x and alternate addition and subtraction of signals. Unweighted spectra are obtained by setting A to a value close to zero and C = y. The arrows indicate rotor cycles. (Reproduced from Tycko et al 1% with permission.)...

Pulse Sequence. In the TOSS pulse sequence a series of usually four (tt) pulses are applied in precisely placed points with respect to the rotor cycle prior to acquisition. The TOSS experiments are designed for MAS rates below 8 kHz. The setup of this experiment is identical to the one for the CP apart from the need for an accurate determination of the S (7T)-pulse. This can be difficult as a standard with relatively short T relaxation time is required. The S ( r)-pulse can also be set using the TOSS pulse sequence indicated by the lowest intensity of spinning sidebands. 

C. "Pb NMR spectra of PbZi03 obtained under MAS conditions (upper) and from a two-rotor-cycle 2D PASS experiment (lower) showing the simpliheation obtained by this method giving essentially an isotropic spectrum with no sidebands. From Vogt et al. (2000), by permission of the copyright owner. 

Special classes of multiple-pulse sequences were designed by Levitt et al, using the symmetry principles of internal spin Hamiltonians [77,86-88] where an integer number of RF pulse units spans a certain number of rotor cycles. These sequences are classified as CN and RN sequences [77]. 

Fig. 16 a observed REDOR experiment shown with a dipolar evolution period of ten rotor cycles, b REDOR results for a dipolar evolution period lasting ten rotor cycles. 

In this expression Aphasing angle given by the product of the dipolar coupling frequency coj, and the dipolar evolution time N-T, (number of rotor cycles multiplied by the rotor period) ... 

This 2D experiment is well suited for imaging, because the indirectly detected spectroscopic dimension requires stepping of the spectroscopic evolution time t from an arbitrary but fixed rotor phase in only 16 steps through one rotor cycle tg. This dimension introduces the information about molecular order into the spectrum. Following t, magnetization is cross-polarized from H to C, and the response is acquired by frequency... 

These dipolar-transition frequencies are time-dependent and repeat the cycle in the spinning. In the REDOR pulse sequence, a 7r-pulse is applied in the centre of the rotor period. In this case, the averaged angular velocity over one rotor cycle for each resonance is given by... 

Fig. 10.26. Carbon-observe, fluorine-dephase REDOR NMR spectra of a heterogeneous blend of [carbonyl- CJpolycarbonate and poly(p-fluorostyrene-co-styrene) as a function of the number of rotor cycles of 5 kHz magic-angle spinning. The natural-abundance background has been subtracted from both the top and bottom sets of spectra. The REDOR difference (top) arises exclusively from the polycarbonate carbons at the interface. (Reprinted with permission from Ref. [153]. 1995 American Chemical Society, Washington, DC.)...

Here, we only briefly discuss the REAPDOR. REAPDOR experiment is specifically designed to measure the heteronuclear dipolar interaction involving, at least, one quadrupolar nucleus (irradiated spin). The pulse sequence is illustrated in Fig. 1.9 [101]. The initial transverse magnetization of J-spin (the observed spin) is usually generated by CP from proton. In the J-spin channel, two ti pulses are applied every rotor cycle except that the n pulse in the very middle of the pulse train is absent. An adiabatic pulse is... 

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