Nitrogen-14 NMR by Double Cross-Polarization

Phantom IRM

Pulse sequence for double cross polarization (DCP) for the indirect detection of 14N signals with corresponding coherence transfer pathway.

Phantom IRM

Experimental signal amplitudes of protons in glycine (NH3+CH2COO-) observed after a double 1H - 14N - 1H transfer as a function of the length of the contact pulses, equal to an integer number of rotor periods. Signals due to one-bond transfers are indicated by black dots, whereas signals that arise from long-range transfers are indicated by empty circles.

Nitrogen-14 NMR spectra at fast magic-angle spinning rates can be acquired indirectly by means of two-dimensional techniques based on double cross polarization transfer. Either one-bond or long-range correlations can be favored by choosing the length of the cross polarization contact pulses. In contrast to earlier methods that exploit second-order quadrupolar/dipolar cross-terms, cross polarization operates like the method of Hartmann and Hahn, even for 14N nuclei with quadrupolar couplings as large as 4 MHz. The experiments are straightforward to set up. The efficiency of indirect detection of 14N nuclei by double cross polarization is shown to be similar to that of isotopically enriched 13C nuclei.

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