Selective inversion recovery

The explicit expressions for Ci and C2 can be derived for many particular cases [3], including the selective and non-selective inversion recovery experiments of interest here. For the selective case... 

Selective inversion recovery experiments i.e. only select frequencies within the powder pattern are excited, have also been performed on 2H for the purposes of studying molecular motion. Initial experiments were performed on deuterated dimethylsulfone (DMS) to demonstrate the utility of the experiment.46 Selective inversion recovery curves were fitted to a suitable motional model, a two-site jump model in the case of DMS, to yield the motional rates as a function of temperature. A significant feature of this work is that the activation energy for the motion so obtained differs markedly from that obtained from earlier 13C chemical shift anisotropy lineshape studies. 

Fig. 38. Selective Inversion Recovery (SIR) H NMR spectra for A-methyl exchange in 70g in toluene-Jg solution at 360 K (a) stack plot of spectra (b) a plot of signal intensity as a function of the variable delay t (c) linear correlation of data (Eq. 27).51 Reproduced with permission from the American Chemical Society.

Aspects of the experimental implementation of the technique have been studied in detail [41] and it has been shown that double sideband modulation and pulse shaping can be combined to improve the performance of selective pulses in solid state 2H NMR. Applications of the selective inversion-recovery experiment using a DANTE sequence to study ultraslow motions have been demonstrated [42,43]. 

Figure B2.4.8. Relaxation of two of tlie exchanging methyl groups in the TEMPO derivative in figure B2.4.7. The dotted lines show the relaxation of the two methyl signals after a non-selective inversion pulse (a typical experunent). The heavy solid line shows the recovery after the selective inversion of one of the methyl signals. The inverted signal (circles) recovers more quickly, under the combined influence of relaxation and exchange with the non-inverted peak. The signal that was not inverted (squares) shows a characteristic transient. The lines represent a non-linear least-squares fit to the data.

Selective, spin-lattice relaxation-rates are measured by the inversion-recovery technique. A rather weak, 180° pulse of very long duration (10-50 ms) inverts a multiplet (single-selective) or two multiplets (double-selective) in the spectrum of asperlin (1 see Fig. 2 ) and the recovery of the... 

MHz, solvent CDC13 (degassed) T ] determination of the carbon atoms by inversion recovery experiment selected 13C spectra... 

IR, inversion recovery SR, saturation recovery PS, progressive saturation STEAM, progressive saturation with stimulated echo acquisition mode SE, Hahn spin-echo sel. SE, frequency selective spin-echo. 

Experimental techniques are available to measure magnetization recovery under different conditions. These are, for instance, the selective and non-selective variants of the inversion recovery experiment described in Section 1.7.4, and will be discussed in more detail in Chapter 9. We anticipate here, from Chapters 7 and 9, in a qualitative way, the kind of information contained in these experiments. 

If it is desired to emphasize Tt differences, one easy choice is to repeat the pulse sequence with a short repetition time TR, so that magnetizations are unable to relax completely back to equilibrium. Thus signals Corn water molecules with long Tj are reduced in intensity. Alternatively, an inversion recovery sequence (180°, T, 90°) can be applied instead of just a 90° pulse to excite the resonance. With careful selection of the value of r, as indicated in Fig. 14.4, T, differences can be emphasized. 

---