Hartmann-Hahn match condition

The spin-locking and CP behavior of the most commonly used SQ coherence (CT) in quadrupolar nuclei under static and MAS conditions has been described in detail by Vega using the fictitious spin-1/2 approximation [223]. In a static sample, the Hartmann-Hahn matching condition requires that co = nut where co ut is one of the nutation frequencies associated with the SQ coherence of the quadrupolar S spin (see Sect. 2.3.4). In the simple case of on-resonance SQ-CP this translates to [224]... 

Figure 2.24. A. One of the pulse sequences for modulating the Hartmann-Hahn match condition by ramping the X-nucleus transmitter amplitude. B. Comparison of H —> C CP for conventional CP (SACP) matched on the centreband and a sideband with RAMP-CP on a sideband of N-t-Boc-alanine showing the very much improved quantitative data for the six equally populated carbon sites. Taken from Metz, Ziliox and Smith (1996) with permission of the copyright owners.

Cross-polarization between spin 1=112 and S=l/2 systems is a relatively simple and well-understood process. For a static sample, under on-resonance conditions, there is a unique Hartmann-Hahn matching condition [73] at which polarization is transferred between the two sets of nuclei, given by... 

The first of these arises when the long spin-lock pulse acts in an analogous fashion to the last 90" pulse of the COSY experiment so causing coherence transfer between J-coupled spins. The resulting peaks display the usual antiphase COSY peak stmcture and tend to be weak so are of least concern. A far greater problem arises from TOCSY transfers which arise because the spin-lock period in ROESY is similar to that used in the TOCSY experiment (Section 5.7). This may, therefore, also induce coherent transfers between J-coupled spins when these experience similar rf fields, that is, when the Hartmann-Hahn matching condition is satisfied. Since the ROESY spin-lock is not modulated (i.e. not a composite pulse sequence), this match is restricted to mutually coupled spins with similar chemical shift offsets or to those with equal but opposite... 

HHCP is a fundamental technique widely used in ssNMR spectroscopy. With this mechanism, the polarization of one nucleus can be transferred to another nucleus [26, 27]. This transfer requires the rotating frame Hartmann-Hahn matching condition to be satisfied ... 

A new approach to markedly reduce RF power for observed nuclei during H-homonuclear decoupled CP has been described Nishimura and Naito. The reduction of RF power to satisfy the Hartmann-Hahn matching condition for observed nuclei and homonuclear dipolar decoupling at H nuclei were achieved simultaneously based on time averaging of nutation frequency of H nuclei by frequency switched Lee-Goldburg decoupling with unequal duration times. The RF nutation frequency and RF field for observed nuclei were shown experimentally to be reduced by factor 3 and 10, respectively, at NMR signals. 

In this section, the acquisition of NMR spectra for spin-1/2 nuclei in diamagnetic compounds is discussed see Section 5 for a discussion of NMR spectroscopy on paramagnetic complexes. Obtaining NMR spectra for spin-1/2 isotopes in solid samples became practical with the development of two widely used techniques MAS and CP under the Hartmann-Hahn matching condition. These techniques have been discussed extensively. Below, the acquisition of NMR spectra for abundant-spin ligand nuclei ( H and F), as well as the direct observation of the NMR signals from spin-1/2 metal nuclei, are discussed. 

Hartmann-Hahn matching condition for triple-quantum CP is very dependent on the crystallite orientation. Wiench et al. have used — Al CP with multiple-quantum HETCOR to correlate the aluminum and phosphorus... 

H O CP experiments are not routine. One problematic issue is the orientation dependency of the quadrupolar interaction. As a result, only a fraction of the nuclear spins in a powder satisfies the Hartmann—Hahn matching condition, leading to distorted lineshapes. Ando and co-workers [14,16,17] recorded a series of H—CP static spectra of polypeptides. The resultant spectra exhibit clear lineshape distortions, as a result of the combination of the CP experiment and low magnetic field detections. Witterbort and co-workers [28,52,97] successfully apphed a modified H—CP sequence in single-crystal experiments of bio- and organic molecules to regain sensitivity. 

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