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首页> 外文期刊>JOM >Compound Radiofrequency-Driven Recoupling Pulse Sequences for Efficient Magnetization Transfer by Homonuclear Dipolar Interaction under Magic-Angle Spinning Conditions
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Compound Radiofrequency-Driven Recoupling Pulse Sequences for Efficient Magnetization Transfer by Homonuclear Dipolar Interaction under Magic-Angle Spinning Conditions

机译：魔角自旋条件下通过单核偶极相互作用进行有效磁化转移的复合射频驱动耦合脉冲序列

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摘要

The maximum of the transferred magnetization in rotating powdered solids under the radiofrequency-driven recoupling (RFDR) pulse sequence is enhanced by reducing the orientation dependence of the effective recoupled homonuclear dipolar interaction. The compound RFDR (CRFDR) pulse sequence for this enhancement consists of RFDR pulse units (τ_(i)-π-τ_(R)-π-τ_(i)) with different τ_(i), where τ_(R) is the sample rotation period, τ_(i) and τ_(i) (=τ_(R)-τ_(i)) are delays, and π is a 180° pulse. The delay τ_(i) modifies the zero-quantum spin operators and the sample rotation-angle dependence of the recoupled dipolar Hamiltonian. The CRFDR pulse sequences were optimized for mixing by varying τ_(i). Numerical simulation for the two-spin system only with a dipolar interaction and isotropic chemical shifts indicates that the transfer efficiency of CRFDR averaged over the powder is about 70%, which is 30% higher than the efficiency of the RFDR pulse over a broad range of about 1/τ_(R) in resonance frequency difference. The CRFDR sequences need about 60% longer mixing times to maximize the transferred magnetizaion in comparison with the original RFDR sequence. Chemical shift anisotropy, the other dipolar interactions, and relaxation generally reduce the enhancement by CRFDR. Experiments for fully ~(13)C-labeled alanine, however, show that the maximum of the magnetization transferred with CRFDR from the carboxyl to α carbon is about 15% greater than that with RFDR.
机译：通过降低有效的再耦合同核偶极相互作用的方向依赖性，可以增强射频驱动的再耦合（RFDR）脉冲序列下旋转粉末状固体中转移磁化的最大值。用于此增强功能的复合RFDR（CRFDR）脉冲序列由具有不同τ_（i）的RFDR脉冲单位（τ_（i）-π-τ_（R）-π-τ_（i））组成，其中τ_（R）是样本旋转周期τ_（i）和τ_（i）（=τ_（R）-τ_（i））是延迟，而π是180°脉冲。延迟τ_（i）修改了零量子自旋算子和重新耦合的偶极哈密顿量的样本旋转角依赖性。通过改变τ_（i）来优化CRFDR脉冲序列以进行混合。仅具有偶极相互作用和各向同性化学位移的双自旋系统的数值模拟表明，CRFDR在粉末上的平均转移效率约为70％，比宽范围内RFDR脉冲的效率高30％。共振频率差约为1 /τ_（R）。与原始RFDR序列相比，CRFDR序列需要更长的混合时间约60％，以使转移的磁化作用最大化。化学位移各向异性，其他偶极相互作用和弛豫通常会降低CRFDR的增强作用。但是，完全〜（13）C标记的丙氨酸的实验表明，用CRFDR从羧基转移到α碳的最大磁化强度比用RFDR转移的最大磁化强度高约15％。

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