Hartman-Hahn condition

Cross-polarization is a double-resonance experiment in which the energy levels of the H and 15N spins are matched to the Hartman-Hahn condition in the rotating frame [47], Under this condition the energy between the two coupled spin systems may be exchanged and the 15N magnetization (S) develops as a function of time at the expense of the proton magnetization according to the relation ... 

An improved variant of the popular double cross-polarisation (DCP) experiment for heteronuclear dipolar re-coupling in solid state NMR spectroscopy under MAS was introduced by Bjerring and Nielsen. By simple phase and amplitude modulation of the RF irradiation at the Hartman-Hahn sideband conditions, the new pulse sequence, dubbed iDCP, was shown to enable broadband excitation with the high efficiency of y-encoded coherence transfer. The efficiency and robustness of iDCP towards isotropic chemical shift variations and chemical shift anisotropies in the case of uniformly C, N-labelled proteins has been demonstrated numerically and experimentally by N to C coherence transfer for N-labelled N-Ac-L-valyl-L-leucine and C, N-labelled-L-threonine. 

Carbon-13. Carbon-13 solid state NMR is particularly difficult on non-enriched samples because of the low density of the sites, in addition to the small natural abundance (1.1%) and weak sensitivity of carbon-13 (Tc/Yh = 0-25). The acquisition of NMR spectra is normally realized with sequences implying a polarization transfer from the protons to the carbons, cross polarization (CP, Scheme 2) under the Hartman-Hahn optimal conditions coh = YhBh = YcBc = c- This has the advantage to greatly enhance the intensity of the carbon signals. However, in the case of the characterization of surface species, it is still necessary to enrich samples with carbon-13 (10-100%). Yet, despite these problems, it has become a crucial technique for the identification of surface species. 

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