不同耦合构型多自旋体系单重态制备效率研究
Preparation Efficiency of Singlet States in Multi-spin Systems with Different Coupling Configurations
Received date: 2023-04-13
Online published: 2023-05-16
核自旋单重态是一种特殊的量子态，其存在寿命能远长于纵向弛豫时间（T1），能够被用于研究分子之间的慢扩散和慢运动等动力学过程．单重态的制备是其能成功应用的关键．目前文献中报道了多种制备单重态的方法，这些方法主要适用于孤立的两自旋体系，当单重态所涉及的核自旋受到其它自旋的耦合作用时，单重态的制备效率往往会降低．本文以三自旋体系为例，研究了不同耦合构型下单重态的制备效率受非单重态自旋耦合的影响，模拟结果表明当单重态自旋由弱耦合逐渐变为强耦合时，单重态的制备效率会在单重态自旋与非单重态自旋的耦合呈现对称性时保持一定的稳定性，此特性能够为复杂体系中选择合适的自旋制备单重态提供参考．我们利用N-乙酰-L-天门冬氨酸（NAA）分子的三个质子组成的自旋体系实验验证了以上结论，通过调节NAA分子的酸碱度，所选的三自旋体系能从弱耦合向强耦合转变，实验结果表明：当三自旋中单重态自旋处于强耦合时，单重态的制备效率明显高于弱耦合时的效率．
王子文 , 辛家祥 , 魏达秀 , 姚叶锋 . 不同耦合构型多自旋体系单重态制备效率研究[J]. 波谱学杂志, 2024 , 41(1) : 67 -76 . DOI: 10.11938/cjmr20233063
Nuclear spin singlet is a special quantum state that lasts longer than T1 and can be used to study slow diffusion and motion between molecules. The preparation of singlet states is the key to their successful application. Currently, various methods for preparing singlet states have been reported in the literature, which are mainly applicable to isolated two-spin systems. When the nuclear spin involved in the singlet state is coupled to other spins, the preparation efficiency of the singlet state often decreases. In this paper, taking a three-spin system as an example, we studied the effect of non-singlet spin coupling on the preparation efficiency of singlet states under different coupling configurations. The simulation results show that as the singlet spin changes from weak coupling to strong coupling, the preparation efficiency of the singlet state will maintain a certain stability when the coupling between the singlet spin and the non-singlet spin is symmetrical. This characteristic can provide a reference for selecting appropriate spins to prepare singlet states in complex systems. We experimentally verified this conclusion using a three-spin system in the N-acetyl-L-aspartic acid (NAA) molecule. By adjusting the pH of the NAA molecule, the three-spin system can transfer from weak coupling to strong coupling. The experimental results show that the preparation efficiency of the singlet state is significantly higher when the three spins are in strong coupling than that when the spins are in weak coupling.
Key words： NMR; nuclear spin singlet state; preparation efficiency; NAA; pulse sequence; optimal control
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