Hahn-echo experiments

RAPT signal-enhancement method (top) and a simple two-pulse Hahn-echo experiment (bottom). Both spectra were acquired at 11.75T, with the same experimental conditions. Reproduced from Cahill et al., ° Copyright 2009, with permission from Wiley. 

Figure A1.3.4 Semi-log plot of Spin Echo amplitude with changing Hahn Echo experiment interval, x, for an oilseed containing excess moisture (therefore some is free) and oil. Extrapolation of the fitted lines (dashed) to x = 0 gives amplitudes for excess (free) moisture (Aw and oil A0 modified from Schmidt, 1991).

The enhancement of the population difference across the CT leads to an enhancement of the spectral intensity of a static or MAS experiment of half-integer quadrupolar spins with either a single 90° pulse experiment or a Hahn-echo experiment preceded by a FAM block. " The schematic of the former is shown in Fig. 23b. 

Wrachtrup J, von Borczyskowski C, Bernard J, Brown R and Orrit M 1995 Hahn echo experiments on a single triplet electron spin Chem. Phys. Lett. 245 262-7... 

The 2-pulse or Hahn-echo experiment, originally developed in NMR is extensively used in pulsed ESR in solids for measurements of magnetic relaxation and of anisotropic hfc. 

Figure 1.5 The QCPMG pulse sequence is a modified Hahn echo experiment that includes additional delay (T3, T4) and acquisition periods. The portion inside the brackets is called a Meiboom-Gill (MG) loop and is repeated N times.

The point-by-point method involves the use of multiple Hahn-echo experiments to acquire broad SSNMR spectra, bypassing the limited... 

In 2003, Ganapathy et al. published a comprehensive study using Al, Si, and Ti SSNMR experiments to investigate the structure of titanium substituted ultra-stable zeolite Y (Ti-USY) [102]. Using a magnetic field of 11.7 T, static SSNMR Hahn-echo experiments were per-... 

In its simplest version, the method consists of two equal and rectangular gradient pulses of magnitude g and length (5, sandwiched on either side of the 180° RF-pulse in a simple Hahn echo experiment. For molecules undergoing free (Gaussian) diffusion characterized by a diffusion... 

The first viable solution devised for tackhng broad Zr SSNMR powder patterns was the acquisition of identical static Hahn-echo experiments at several different transmitter frequencies across the powder pattern. By plotting the normalized S/N, or intensity, of the signal obtained in each Hahn-echo experiment versus its transmitter frequency, a reasonable... 

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 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. 

Figure 7 Static SSNMR spectra of hexagonally close-packed Zr metal, acquired at a field of 9.4 T using (A) a set of stepped-echo experiments where signal intensity is plotted against transmitter frequency, and (B) a standard Hahn-echo experiment at a fixed transmitter frequency. Note the lack of broadband excitation in (B). The simulated Zr spectrum is shown in (C). Reprinted with permission from Ref. [38]. Copyright 1992 Elsevier.

Bastow et al. continued to explore applications of Zr SSNMR in 1998, detailing the use of multinuclear SSNMR spectroscopy to explore local structure within aluminides [43], a group of intermetaUic aluminum alloys. Several varieties of these AI3X (X = transition metal) materials were studied using Al, Sc, and Zr SSNMR Zr Hahn-echo experi-... 

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