Bloch decay experiments

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... 

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... 

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... 

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).

High-resolution C spectra of solid polymers can principally be obtained by two ways from normal Bloch decays (SPE single-pulse excitation) of the carbon magnetisation, just as in 1-NMR, or from cross-polarisation. These techniques are complementary. Discriminating experiments may consist of comparing CP/MAS and SPE spectra (the latter obtained without cross-polarisation). Whereas the former depends on proton relaxation, the latter is affected only by carbon relaxation. Because of the great segmental mobility in elastomers, these systems have shorter spin-lattice relaxation times (in the order of seconds), which makes SPE feasible. 

In agreement with this conclusion, the Bloch picture applied here to derive Equation (30) does not predict a decay of the first moment, since the Bloch description omits spectral diffusion processes. Nevertheless, it is possible to understand the existence of a peak shift within the Bloch description, and this suggests a qualitative interpretation for its decay. As we have seen from photon echo experiments on spectroscopic probes... 

In the case of coherent laser light, the pulses are characterized by well-defined phase relationships and slowly varying amplitudes (Haken, 1970). Such quasi-classical light pulses have spectral and temporal distributions that are also strictly related by a Fourier transformation, and are hence usually refered to as Fourier-transform-limited. They are required in the typical experiments of coherent optical spectroscopy, such as optical nutation, free induction decay, or photon echoes (Brewer, 1977). Here, the theoretical treatments generally adopt a semiclassical procedure, using a density matrix or Bloch formalism to describe the molecular system subject to a pulsed or continuous classical optical field, which generates a macroscopic sample polarization. In principle, a fully quantal description is possible if one represents the state of the field by the coherent or quasi-classical state vectors (Glauber, 1965 Freed and Villaeys, 1978). For our purpose, however. 

So far, we have considered only experiments with continuous-wave lasers under steady state conditions. With time-resolved experiments, on the other hand, energy transfer rates and transition probabilities can be obtained. Such measurements were carried out by mechanically chopping the laser beam directed into an external absorption cell together with the microwave radiation. Later, Levy et at reported time-resolved infrared-microwave experiments with an N2O laser Q-switched with a rotating mirror to produce pulses less than 1 /tsec in duration. They observed a transient nutation of the inversion levels of the molecule following the infrared laser pulse. Based on the Bloch equations, the observed phenomena could be explained quantitatively. From the decay envelope of the oscillations a value for the transverse relaxation time T2 was determined. Similar effects were produced by rapidly switching a Stark field which brings the molecules into resonance with the cw microwave radiation. 

In an NMR experiment the resonance signal contains components in-phase and out-of-phase with the incident radio frequency radiation so that a complex susceptibility x can usefully be defined, such that x = x x"- The dispersion component x is in-phase, and the absorption component x is out-of-phase. By introducing phenomenologically the exponential decay constants Ti and T2 for the nuclear magnetization parallel and perpendicular to the applied field Ho, Bloch et al. (1946) derived expressions for the magnetization as a function of frequency. These may be related to expressions for x and x", the results being ... 

Spin relaxation phenomena are usually described by the semiclassical theory developed by Wangsness, Bloch and Redfield and known as the WBR theory or Redfield theory. The semiclassical nature of the theory implies that the spin system is treated quantum mechanically, while the remaining degrees of freedom (such as molecular rotations) are treated classically. Few years ago, Segnorile and Zamar studied the issue of quantum decoherence (loss of system phase memory) in proton NMR of nematic liquid crystals. The spin dynamics - and the decay of the free induction decay - was found to be governed by several different processes, partly of purely quantum nature. During the period under the present review, the same group reported a related work concerned with the Jeener-Broekaert experiment on liquid crystals. ... 

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