Shen Ming, Trébosc Julien, Lafon Olivier, Gan Zhehong, Pourpoint Frédérique, Hu Bingwen, Chen Qun, Amoureux Jean-Paul
UCCS, CNRS, UMR 8181, University of Lille, Villeneuve d'Ascq 59652, France; Physics Department & Shanghai Key Laboratory of Magnetic Resonance, East China Normal University, Shanghai 200062, China.
UCCS, CNRS, UMR 8181, University of Lille, Villeneuve d'Ascq 59652, France.
Solid State Nucl Magn Reson. 2015 Nov;72:104-17. doi: 10.1016/j.ssnmr.2015.09.003. Epub 2015 Sep 9.
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 (14)N. 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 (14)N nuclei via (1)H 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 (1)H and (14)N nuclei, the efficiency of selective moderate excitation with long-pulses at the (14)N Larmor frequency, ν0((14)N), is comparable to those with strong excitation pulses at ν0((14)N) or 2ν0((14)N) 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 (14)N 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.
在魔角旋转（MAS）条件下，施加于共振态的长射频（rf）脉冲可实现对宽核磁共振（NMR）谱中心带的选择性激发。我们在此表明，这些rf脉冲可应用于异核多量子关联（HMQC）序列的间接通道，这有助于通过具有宽谱的原子核的自旋1/2同位素进行间接检测。数值模拟表明，这种间接激发方法适用于经历大化学位移各向异性的自旋1/2原子核，以及受到大四极相互作用的自旋1同位素，如（14）N。通过间接检测自旋1/2或自旋1原子核的标量介导HMQC（J - HMQC）实验的数值模拟，以及通过在60kHz的MAS频率下对结晶γ - 甘氨酸和N - 乙酰 - 缬氨酸样品中通过（1）H间接检测（14）N原子核的偶极介导HMQC（D - HMQC）实验，分析了长脉冲的性能。我们在这些固体上表明，对于获取（1）H和（14）N原子核之间的D - HMQC谱，在（14）N拉莫尔频率ν0（（14）N）下用长脉冲进行选择性适度激发的效率，与在ν0（（14）N）或2ν0（（14）N）频率下用强激发脉冲的效率相当，前提是由普通固态NMR探头提供rf场。此外，D - HMQC实验还表明，使用长脉冲不会在（14）N维度上产生明显的谱失真。总之，对于实现宽谱间接检测的HMQC实验，使用中心带选择性弱脉冲具有优势，因为它（i）需要适度的rf场，（ii）易于优化，（iii）对化学位移各向异性、偏移、rf场不均匀性和MAS频率波动具有高鲁棒性，并且（iv）几乎不依赖于四极耦合常数。
