Spin-flip pattern

We associate a variational parameter x v) to the spin-flips pattern... 

Three-spin systems can be readily analyzed by inspection only in the first-order cases AX2 and AMX. The second-order AB2 spectrum can contain up to nine peaks—four from spin flips of the A proton alone, four from spin flips of the B protons alone, and one from simultaneous spin flips of both the A and the B protons. The ninth peak is called a combination line and is ordinarily forbidden and of low intensity. Although these patterns may be analyzed by inspection, recourse normally is made to computer programs. The other... 

Fig. 8. Calculated heteronuclear DQ MAS NMR spinning sideband patterns for rotating phenylene groups and different flip angles or as indicated (Nrcpl DIS/ coR= 1.67).

Figure 7. Pulse sequence and coherence transfer pathway diagram for a H DQ MAS experiment using the BAB A recoupling sequence for the excitation and reconversion of DQCs. The rectangular blocks represent pulses of flip angle 90°, with the choice of the phases being described in, e.g., ref 25. If the q increment is set equal to a rotor period, a rotor-synchronized two-dimensional spectrum is obtained, while reducing q, and hence increasing the DQ spectral width, leads to the observation of a DQ MAS spinning-sideband pattern.

Figure 16. Pulse sequence and coherence transfer pathway diagram for the REPT-HDOR heteronuclear ( H-13C) experiment, which is suitable for recording rotor-encoded spinning-sideband patterns. Dark- and light-shaded rectangular blocks represent rf pulses of flip angle 90° and 180°, respectively. For more details, see ref 125.

The fact that mutually different cross sectional patterns were observed in the SC-2D NMR spectrum where 7 was chosen to be 0 ms indicates that both inter- and intramolecular cross relaxation rates and spin flip-flop rates between interacting pairs of protons are relatively slow. This can be understood if one considers that dipolar interactions are partially averaged out by fast translational and rotational molecular motions in the liquid crystalline phase in contrast to the solid phase. 

In conventional NMR experiments the multiplet pattern (Fig. 10.3) has routinely been collapsed by decoupling on JH. The XH decoupling flips the spin H and concomitantly the sign of BDD(t). Thus, during 15N-evolution each 15N spin is perturbed during half of the evolution time by BCSA(t)+BDD(t) and during the other half of the evolution time by BCSA(t)-BDD(t), which leads to a less favorable relaxation when compared with the relaxation of the component S12 selected by TROSY. 

Spectra with the form of the two terms on the right-hand side of Eq. (18) can be obtained by two extra modulation periods of length t in the exchange pulse sequence, one before and one after rm. The sine.sine and cosine.cosine terms of Eq. (18) are selected by suitable choices of the phases of the flip back pulses, labelled a and b in Fig. 38. The spectrum of the form of Eq. (18) is produced by the pulse sequence in Fig. 38(a), for non-spinning samples, while the spectrum of the form 1/2 S is produced using the sequence of Fig. 38(b), which matches that in Fig. 38(a) in terms of pulses and delays and so should produce a matched intensity spectrum, so that when the relevant spectra from the pulse sequence in Fig. 38(a) are subtracted from it, the desired pure-exchange spectrum is obtained. The A periods in both sequences are simply Hahn echoes, implemented to achieve non-distorted powder patterns in both spectral dimensions.2... 

Figure 16 Spectral lineshapes for powdered solids. (A) Peak doublet produced by dipole coupling between two spin 1/2 nuclei. The doublet is composed of two parts (shown dotted). They correspond to the observed proton flip occurring when its neighbor is spin up (left) or spin down (right). The indicated turning points correspond to the angle between the internuclear vector and 5b. (B) Chemical shift anisotropy pattern with shielding tensor components (Til, < 22. and (T33. (C) Combined DD and CSA spectrum. Note that this is not simply (A) -1- (B). (Reproduced with permission from Power WP and Wasylishen RE (1991) In Webb GA (ed.) Annual Reports in NMR Spectroscopy, vol. 23, p. 17. London Academic Press.)...

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