Stepped-echo technique

The zircon study was followed up with Zr SSNMR experiments in 1992 by Bastow et al. on seven Zr-based metals and oxides at room temperature [38]. Zr chemical and Knight shifts, EFG parameters, and some relaxation times were reported. The stepped spin-echo technique was introduced for Zr spectral acquisition in this work (see Section 2 for further details, vide supra). As a proof of concept, the Zr SSNMR powder pattern of hep Zr metal was acquired at 9.4 T via a traditional 90—180° Hahn-echo experiment as well as the stepped spin-echo technique (Fig. 7). The Hahn-echo experiment produced an imperfect, lopsided Zr powder pattern in ca. 27.5 h the authors credited this to a combination of instrumental factors, most likely excitation bandwidth limitations associated with hard pulses. In contrast, the stepped-echo technique produced a relatively weU-defmed, superior powder pattern in an experimental time of ca. 28 h, which clearly illustrated the benefits of the stepped-echo technique. 

Fast reactions (in the millisecond to second range) require special reactors with efficient mixing chambers. Faster reactions (down to the microsecond range and below) call for special techniques most of these are based on relaxation after an equilibrium state has been disturbed by an instantaneous pulse or step variation of conditions. With laser and photon-echo techniques the range has been extended down to femtoseconds. 

Fig. 6.1.4 Gradient paths for 3D reconstruction from projections. Only half a hemisphere is covered by the gradient paths, because signal for negative gradient values can be acquired by time inversion in echo techniques, (a) 3D space can be covered by a set of 2D projections, so that the 2D algorithm can be applied in two steps, (b) Optimization of the point density in 3D k space requires an integral approach to 3D reconstruction from projections. Adapted from [Lail] with permission from Institute of Physics.

With the use of a constant appHed magnetic field, field-swept techniques for acquisition ofbroad Zr powder patterns were no longer feasible, necessitating alternative techniques for Zr nuclei in lower-symmetry environments. Bastow introduced the stepped spin-echo technique for acquisition ofbroad Zr SSNMR powder patterns in 1992 [38]. This new technique permitted study of Zr in a variety of lower-symmetry environments, and Bastow et al. made extensive use of traditional one-pulse and Hahn-echo experiments as well as stepped spin-echoes to examine Zr in a variety of environments [38,43,48—52], including zirconia phases and zirconia-based materials with practical appfications. Flartmann and Scheler [35] also explored the feasibifity of Zr SSNMR through echo... 

The ultrafast infrared vibrational echo experiment and vibrational echo spectroscopy are powerful new techniques for the study of molecules and vibrational dynamics in condensed matter systems. In 1950, the advent of the NMR spin echo (1) was the first step on a road that has led to the incredibly diverse applications of NMR in many fields of science and medicine. Although vibrational spectroscopy has existed far longer than NMR, the experiments described here are the first ultrafast IR vibrational analogs of pulsed NMR methods. In the future, it is anticipated that the vibrational echo will be extended to an increasingly diverse range of problems and that the technique will be expanded to new pulse sequences, including multidimensional coherent vibrational spectroscopies such as the vibrational echo spectroscopy technique describe above. 

We have presented two types of nonlinear IR spectroscopic techniques sensitive to the structure and dynamics of peptides and proteins. While the 2D-IR spectra described in this section have been interpreted in terms of the static structure of the peptide, the first approach (i.e., the stimulated photon echo experiments of test molecules bound to enzymes) is less direct in that it measures the influence of the fluctuating surroundings (i.e., the peptide) on the vibrational frequency of a test molecule, rather than the fluctuations of the peptide backbone itself. Ultimately, one would like to combine both concepts and measure spectral diffusion processes of the amide I band directly. Since it is the geometry of the peptide groups with respect to each other that is responsible for the formation of the amide I excitation band, its spectral diffusion is directly related to structural fluctuations of the peptide backbone itself. A first step to measuring the structural dynamics of the peptide backbone is to measure stimulated photon echoes experiments on the amide I band (51). 

Despite the high cost of the equipment required and the time taken for sample preparation and spectra acquisition, MAS-HR NMR provides invaluable stmctural information about the species present in a reaction. Only a few milligrams of resin beads are required and they can be recovered as the technique is nondestructive. The complementarity of the technique with other analytical methods is clear MALDl-TOP cannot discriminate among compounds with the same MW and depends on the ionization properties of the resin-bound compound, while PTIR depends on the presence of selected functional groups in the molecule. MAS-HR NMR can be used independently from the nature of the performed reaction and the functional groups formed or lost during the SPS step. Additionally, two-dimensional MAS techniques such as 2D-COSY (correlated spectroscopy) and TOCSY (total correlated spectroscopy) (171) or 2D-SECSY (spin echo correlation spectroscopy) (181) can provide more detailed information that may be useful in specific cases. 

Hydrochloride salts have been popular materials to study, particularly in recent years, as evidenced by the reports of Bryce et al., Chapman and Bryce, and Hamaed et al. (see Figure 11 for an example). Data are summarized in Table 4. To the best of our knowledge, the first chlorine SSNMR report for a powdered hydrochloride salt appears to be that of Pines and co-workers, who studied cocaine hydrochloride in 1995. The study utilized multiple techniques to study the hydrochloride salt, including N NQR. The chlorine-35 SSNMR experiment was carried out at 7.0 T using a Hahn-echo pulse sequence, and a chlorine-35 Cq of 5.027 MHz was reported. To avoid the intensity distortions that result from a finite pulse applied to a broad line shape, a variable frequency offset approach, in which the frequency was stepped in 2 or 4 kHz increments over the entire spectral width, was used to acquire the spectrum. 

Uniform excitation of the entire spectral region is not always possible, since the spectra may span hundreds of kHz. As for the CPMG experiment discussed above (Section 3.3), this problem can be overcome by acquiring several subspectra with different frequency offsets. Recently, Schurko and co-workers demonstrated that undistorted Al NMR spectra of CTs which span up to 700 kHz, in a series of three- and five-coordinate aluminum compounds, may be acquired using a frequency-stepped acquisition method.These spectra were acquired using the Hahn-echo or quadrupolar CPMG (QCPMG vide infra) techniques. 

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