Solid-state NMR indirect detection of nuclei experiencing large anisotropic interactions using spinning sideband-selective pulses

• Published in: Solid state nuclear magnetic resonance 2015-11, Vol.72, p.104-117
• Main Authors: Shen, Ming, Trébosc, Julien, Lafon, Olivier, Gan, Zhehong, Pourpoint, Frédérique, Hu, Bingwen, Chen, Qun, Amoureux, Jean-Paul
• Format: Article
• Language: English
• Subjects: Anisotropy; Indirect detection; Magnetic Resonance Spectroscopy - methods; Nitrogen-14; Overtone; Selective excitation; Through-space HMQC; Time Factors; Very broad spectra
• ISSN: 0926-2040; 1527-3326
• DOI: 10.1016/j.ssnmr.2015.09.003

Under Magic-Angle Spinning (MAS), a long radio-frequency (rf) pulse applied on resonance achieves the selective excitation of the center-band of a wide NMR spectrum. We show herein that these rf pulses can be applied on the indirect channel of Hetero-nuclear Multiple-Quantum Correlation (HMQC) sequences, which facilitate the indirect detection via spin-1/2 isotopes of nuclei exhibiting wide spectra. Numerical simulations show that this indirect excitation method is applicable to spin-1/2 nuclei experiencing a large chemical shift anisotropy, as well as to spin-1 isotopes subject to a large quadrupole interaction, such as 14N. The performances of the long pulses are analyzed by the numerical simulations of scalar-mediated HMQC (J-HMQC) experiments indirectly detecting spin-1/2 or spin-1 nuclei, as well as by dipolar-mediated HMQC (D-HMQC) experiments achieving indirect detection of 14N nuclei via 1H in crystalline γ-glycine and N-acetyl-valine samples at a MAS frequency of 60kHz. We show on these solids that for the acquisition of D-HMQC spectra between 1H and 14N nuclei, the efficiency of selective moderate excitation with long-pulses at the 14N Larmor frequency, ν0(14N), is comparable to those with strong excitation pulses at ν0(14N) or 2ν0(14N) frequencies, given the rf field delivered by common solid-state NMR probes. Furthermore, the D-HMQC experiments also demonstrate that the use of long pulses does not produce significant spectral distortions along the 14N dimension. In summary, the use of center-band selective weak pulses is advantageous for HMQC experiments achieving the indirect detection of wide spectra since it (i) requires a moderate rf field, (ii) can be easily optimized, (iii) displays a high robustness to CSAs, offsets, rf-field inhomogeneities, and fluctuations in MAS frequency, and (iv) is little dependent on the quadrupolar coupling constant. 1H–{14N1Q} D-HMQC signal of glycine versus offset (B0=21.1T, νR=60kHz). 14N1Q excitation uses: (a) two pulses (ν1=40kHz, tp=33μs), or two D15 trains with (b) ν1=80kHz, tp=0.8μs), or (c) ν1=20kHz and tp=15μs [Display omitted] •Selective long pulses for indirect MAS detection of very broad spectra.•Very robust to all experimental parameters with weak rf field.•Very simple to optimize.•Comparison of 1Q and 2Q-overtone 1H–{14N} spectra.