Shrot Yoav, Frydman Lucio
Department of Chemical Physics, Weizmann Institute of Science, 76100 Rehovot, Israel.
J Chem Phys. 2008 Apr 28;128(16):164513. doi: 10.1063/1.2890969.
The so-called "ultrafast" nuclear magnetic resonance (NMR) methods enable the collection of multidimensional spectra within a single scan. These experiments operate by replacing traditional t(1) time increments, with a series of combined radiofrequency-irradiation/magnetic-field-gradient manipulations that spatially encode the effects of the indirect-domain spin interactions. Barring the presence of sizable displacements, the spatial patterns thus imparted can be read out following a mixing period with the aid of oscillating acquisition gradients, leading to a train of t(2)-modulated echoes carrying in their positions and phases the indirect- and the direct-domain spin interactions. Both the initial spatial encoding as well as the subsequent spatial decoding procedures underlying ultrafast NMR were designed under the assumption that spins remain static within the sample during their execution. Most often this is not the case, and motion-related effects can be expected to affect the outcome of these experiments. The present paper focuses on analyzing the effects of diffusion in ultrafast two-dimensional (2D) NMR. Toward this end both analytical and numerical formalisms are derived, capable of dealing with the nonuniform spin manipulations, macroscopic sample sizes, and microscopic displacements involved in this kind of sequences. After experimentally validating the correctness of these formalisms these were used to analyze the effects of diffusion for a variety of cases, including ultrafast experiments on both rapidly and slowly diffusing molecules. A series of prototypical schemes were considered including discrete and continuous encoding modes, constant- and real-time manipulations, homo- and heteronuclear acquisitions, and single versus multiple quantum modalities. The effects of molecular diffusion were also compared against typical relaxation-driven losses as they happen in these various prototypical situations; from all these situations, general guidelines for choosing the optimal ultrafast 2D NMR scheme for a particular sample and condition could be deduced.
所谓的“超快”核磁共振（NMR）方法能够在单次扫描中采集多维光谱。这些实验通过用一系列组合的射频照射/磁场梯度操作取代传统的t(1)时间增量来进行，这些操作对间接域自旋相互作用的影响进行空间编码。除非存在相当大的位移，否则在混合期之后，可以借助振荡采集梯度读出由此赋予的空间模式，从而产生一系列t(2)调制回波，其位置和相位携带间接域和直接域自旋相互作用。超快NMR的初始空间编码以及随后的空间解码程序都是在自旋在执行过程中在样品内保持静止的假设下设计的。但大多数情况下并非如此，可以预期与运动相关的效应会影响这些实验的结果。本文着重分析超快二维（2D）NMR中扩散的影响。为此，推导了能够处理此类序列中涉及的非均匀自旋操作、宏观样品尺寸和微观位移的解析和数值形式。在通过实验验证了这些形式的正确性之后，它们被用于分析各种情况下扩散的影响，包括对快速和缓慢扩散分子的超快实验。考虑了一系列典型方案，包括离散和连续编码模式、恒定和实时操作、同核和异核采集以及单量子和多量子模态。还将分子扩散的影响与这些各种典型情况下发生的典型弛豫驱动损失进行了比较；从所有这些情况中，可以推导出为特定样品和条件选择最佳超快2D NMR方案的一般指导原则。
