Bloch-decay

For direct polarization experiments (DPMAS), data were acquired from the Bloch decay following a single 13C pulse. Alternatively, FT spectra and spin-lattice relaxation times were measured in a high-resolution probe... 

Fig. 18. 75.4-MHz 13C Bloch decay MAS spectra showing the dynamics of the toluenium ion. The cation was synthesized by reacting bromomethane-13C with benzene-13Q on AlBr3 at 233 K. The spectrum at 213 K shows all the peaks for the toluenium ion at 32 (methyl), 50 (C-4), 178 (C-3), 139 (C-2), and 201 ppm (C-l). The peak at 129 ppm was the unreacted benzene-13C6. At 243 K, the peaks were much sharper, and the 138 and 50 ppm peaks were NMR invisible. At 273 K, the spectrum shows two extra peaks at 128 and 73 ppm. All these spectral features are rationalized by the chemical exchange between the para and ortho isomers.

Despite that 31P NMR spectra can be recorded in a simple one-pulse (Bloch decay) experiment, very often MAS technique is combined with CP. CP under MAS has been extensively studied in the past.17 18... 

A typical spectrum was acquired with Bloch decays excited by 4 ysec pulses separated by 10 second recovery delays, and the data should give a reasonably quantitative estimate of the Si content. Previous work on other types of zeolites has demonstrated the importance of checking for complete relaxation if the spectra are to be used for quantitative studies. 

Fig. 1 SSNMR spectra of ibuprofen, 75.6 MHz. (A) Bloch decay experiment (single pulse), no decoupling, static, 240-sec pulse delay, 100 scans, 400-min experiment time. (B) Same as (A) but with decoupling ( 60 kHz). (C) Same as (B) but with 5-kHz MAS. (D) Cross-polarization experiment, with H decoupling ( 60 kHz), 5-kHz MAS, 1.5-msec contact time, 3-sec pulse delay, 100 scans, 5-min experiment time. (E) Same as (D) with the TOSS pulse sequence applied to suppress spinning sidebands. Note Asterisk ( ) denotes spinning sidebands sharp ( ) denotes spectrometer background artifact.

The resonances observed under MAS and strong proton decoupling were separated and analyzed as a function of the delay between scans to infer the relaxation times, T, and to extrapolate the corresponding intensities to infinite delay. No new spectral features were detected under a Bloch decay experiment, as compared to those under a CP/MAS experiment. The extrapolated Bloch decay intensities were then compared with that of a standard. The results of these measurements are presented in Table 1, The number of surface platinum atoms was also calculated on the basis of the Pt dispersion and Pt loading. Subsequently, the number of chemisorbed carbon atoms per surface platinum atom (Table 1, last column) was calculated. The results indicated that there were 4.0 0.6 chemisorbed carbon atoms per one surface Pt atom and 10.2 1.0 carbon atoms in the highly... 

In the last step, the frequency dependence of the spin density and the exponential Bloch decay have been replaced by a normalized spectroscopic FID sit,r). This is advantageous when arbitrary spectra Sio),r) need to be considered. Thus, the impulse response measured in the presence of magnetic-field gradients, maps the Fourier transform of the spin density Mo(r), as long as the signal decay introduced by the FID can be neglected. 

Figure 8 Plots of C signal intensities versus cross-polarization contact time for methanol and dimethyl ether on zeolite HZSM-5. Intensities were normalized by division by the intensity obtained for each species in a direct 90° flip-observe experiment. Note that in this case the cross-polarization signals are less intense than the Bloch decay signals for all choices of the contact time (CT).

Figure 17 Selected C MAS NMR results supporting the assignment of the methoxy resonance at 58 ppm (a) Bloch decay spectrum (b) cross-polarization spectrum (c) spectrum with cross-polarization and interrupted decoupling (d) slow-spced spectrum obtained at 830 Hz. All spectra were acquired at 298 K.

Figure 7 MAS spectra of "y-alumina (out of the bottle) (a) H Al CP-MAS (512 transients) (b) H-decoupled Bloch decay (16 transients).

Figure 40 Static and MAS spectra of polycrystalline (Bu4N)iMo207 (a) H- Mo spectrum with 3477 scans, 5-s recycle delay, and Lorentzian line broadening of 500 Hz (b) H-decoupled Bloch decay spectrum with the same line broadening and number of transients as in (a) (c) 4.7 kHz MAS spectrum with H decoupling, 6970 scans, 5-s recycle delay, and 50 Hz of line broadening. (From Ref. 31.)...

Fig. 3. solid-state NMR spectra of LaCo(CN)s-5H20 measured by Bloch decay (bottom, full line) and quad-echo (top, broken line) at 7.1 T. (Reproduced with permission from ref 47. 1997 American Chemical Society.)... 

Fig. 5. static Bloch-decay spectrum of K3[Co(CN)6] measured at 9.4 T. The deadtime delay employed was 5 ps. The NMR parameters of the calculated spectrum (upper trace) were taken from Table 1. 

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