Flip angle calculation

Numerical calculations have been carried out on the effect of the angle on the conversion efficiency. The work of Amoureux and Fernandez (1998) is summarised in Table 2.9. One of the key observations is that the efficiency of the flip angles shows no real dependence on r. However, the conversion is quite an inefficient process so that the overall sensitivity of the experiment is poor relative to one-pulse experiments and much effort has gone into improving the excitation and reconversion efficiency. As I increases, the optimum flip angle for maximum excitation decreases. 

Table 2.9. Numerically calculated optimum flip angles and the relative efficiency of the sequences after Amoureux and Fernandez (1998) for Vq/vi = 1.25.

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).

An alternative approach to tailor the cross peaks is the z-filtered COSY spectrum (z-COSY) [5.130]. In Check it 5.4.1.7 the "small-flip angle COSY" spectrum of 2,3-dibromopropionic acid, the basic sequence of the z-COSY spectrum, is simulated. As such a z-COSY spectrum can not be calculated because the randomly changing delay which is the major part of the z-filter can not be simulated in the current version of NMR-SIM. A comparison of the results of Check it 5.4.1.7 and the E.COSY spectrum of the same spin system calculated in Check it 5.4.1.6 shows that due to the small flip angles the diagonal peaks of the z-COSY spectrum are reduced in intensity while the cross peaks are very similar. 

The Tc values evaluated from NMR relaxation times are always shorter than the values estimated using the SED model. The NMR relaxation takes place for one motion of a single dipole-dipole flip and can describe the time interval for the motion but not the amplitude. As proposed in our previous pap>ers,i-4 we assume that the Tc process is the small-angle librational motion of the cation. Assuming the T2 value calculated from t] correspond to a 360 rotation, the angles for the librational motions were calculated and plotted versus temperature in Fig. 19. The flip angle for EMlm is particularly larger than those for the other anions. The molecular flip amplitude becomes smaller at lower temperature. 

The temperature dependent T data are shown in Fig. 9. 7j values decrease from 28 ms at 21°C with increasing temperature, and show a minimum of 6.4 ms at 80° C. These results indicate the presence of the motion with a Larmor frequency of 30 MHz at this temperature. This minimum was found to be attributed to the flipping motion of a phenyl ring from the result of our other experiments discussed in later section.13 The jump rates of the flipping motion were estimated with a two-site jump model that a C-2H bond jumps between two equivalent sites separated by 180°, and that the angle made by the C-2H bond and the rotational axis is 60°. The quadrupole coupling constant of 180 kHz and the asymmetry parameter approximated to zero were used in the calculation. The calculated values for fitting with the... 

---