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Sensitivity improvement of 1 H‐ 15 N cross‐polarization at high MAS frequency applied to NMR structural characterization of organic solids
Author(s) -
Palmas Pascal,
Ilas Simon,
Girard Elzéar
Publication year - 2011
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.2834
Subject(s) - chemistry , adiabatic process , anisotropy , protonation , characterization (materials science) , sensitivity (control systems) , analytical chemistry (journal) , nuclear magnetic resonance spectroscopy , nitrogen , molecular physics , stereochemistry , optics , thermodynamics , chromatography , physics , organic chemistry , ion , electronic engineering , engineering
15 N CP/MAS solid state NMR should be a method of choice to obtain essential structural information on organic materials containing nitrogen atoms. However, the technique is generally not selected for the characterization of non‐labelled chemical compounds, which represents the most common situation encountered by chemists. Actually, due to the poor sensitivity of 15 N the method is time‐consuming and a very fine calibration is often a prerequisite to reach a sufficient signal/noise. The main drawback comes from the weakness of 15 N‐ 1 H dipolar couplings which leads to a splitting of the static Hartman Hahn condition into very narrow sideband conditions under MAS. Practically, it is more difficult to obtain a high enough CP transfer level on 15 N for the entire spectrum than on other more conventional nuclei like 13 C. An experimental investigation of the CP efficiency using the ramp and adiabatic CP transfer experiments is here proposed. Preliminary adjustments of experimental settings were first made on an 15 N‐labeled substituted heterocyclic model system, and then applied to several other organic compounds. Particular attention was paid to the detection of non‐protonated nitrogen atoms with a significant chemical shift anisotropy, which represented the least favourable case. It was experimentally demonstrated that, for these atoms, the adiabatic passage provided a much higher transfer level than the more conventional ramp sequence leading to an enhancement factor of up to 3.5 at a MAS frequency of 30 kHz. The resulting sensitivity rendered possible the detection of non‐protonated nitrogen atoms at natural abundance with 2.5‐mm rotors at 9.4 T. Copyright © 2011 John Wiley & Sons, Ltd.