Long-lived states in multi-spin systems
Long-lived states in multi-spin systems
复制标题
多自旋系统中的长寿命状态
DOI：
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发表时间：
2015
期刊：
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通讯作者：
G. Stevanato
中科院分区：
文献类型：
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作者：
G. Stevanato
长寿命状态是核自旋配置，在适当的情况下，衰变非常缓慢，达到热平衡。关于该主题的第一篇论文报道了2，3-二溴噻吩在约20 mT下约100 s的一对不等价质子核之间的长寿命顺序。寿命超过纵向磁化T1的弛豫时间超过一个数量级。目前，许多系统甚至可以在几特斯拉的磁场中生存T1。长寿命可能有利于用于研究例如核自旋扩散、化学反应性和代谢过程的不同方法。此外，超极化方法可以受益于长寿命的状态，以提高灵敏度和时间分辨率。虽然这一研究领域相对年轻，第一次出版是11岁，约150调查迄今已发表在同行评议的科学期刊上对这个问题。在这项工作中，主要的重点是扩展到多自旋系统的分析。论文的结构由理论部分和实验部分组成。我们提出了一个模型的基础上核自旋排列，使用离散群理论的形式主义。这种方法允许根据核自旋排列对称性对核波函数和内部哈密顿算符进行分类，以预测长寿命阶数及其解析表达式。该数学结构也可以应用于研究对称性存在下自旋转换的基本边界。理论模型是基于一组近似用于定义对称操作和相应的置换对称群。实验部分包括在不同的磁性，几何和动态条件下发生的长寿命订单的例子。这种多样性的制度一方面显示了长寿物种的普遍性。另一方面，一个共同的特点是确定在正式的表征，使用置换和旋转对称的概念。出于这个原因，排列对称性的特点是旁边的实验描述。局部几何在刚性自旋系统中的作用是突出的，通过比较两个异构体与不同的本地自旋核的安排，并显示如何长寿命的顺序预测和检测只有在一种情况下。非刚性分子也可以显示出长寿命的特性。这一点可以通过考虑？-中的甲基13 CH 3来证明。甲基吡啶有趣的是，由于质子核是磁等效的，长寿命的顺序可达性不能采用相干机制。最后，萘的衍生物被证明在溶液中和室温下具有非常长的寿命。非常长的寿命的可访问性打开了将（超）极化存储为单态顺序并在稍后时间检索它的可能性。一套初步的溶解动态核极化实验也提出了在这个意义上的第一次尝试。
Long-lived states are nuclear spin configurations that, in suitable circumstances, decay very slowly towards thermal equilibrium. The first paper on the subject reported a long-lived order in 2,3-dibromothiophene between a pair of inequivalent proton nuclei of about 100 s at about 20 mT. The lifetime exceeded the relaxation time of longitudinal magnetization T1 by more than one order of magnitude. Currently many systems cansurvive T1 even at magnetic fields of several Tesla. Long lifetimes may benefit different methodologies used to investigate for example nuclear spin diffusion, chemical reactivity and metabolic processes. In addition hyperpolarization methods may profit from long-lived states in order to enhance both sensitivity and temporal resolution. Although this research field is relatively young, the first publication being 11 years old, about 150 investigations so far have been published on peer-reviewed scientific journals on this subject. In this work the main focus is to extend the analysis to multiple spin systems. The structure of the thesis is composed of a theoretical and an experimental part. We propose a model based on nuclear spin permutations that uses the formalism of discrete group theory. This approach allows the classification of nuclear wave functions and internal Hamiltonian operators, according to nuclear spin permutation symmetry, in order to predict the number of long-lived orders and their analytical expression. The mathematical structure can also be applied to investigate fundamental bounds on spin conversion in the presence of symmetry. The theoretical model is grounded on a set of approximations used to define the symmetry operations and the corresponding permutation symmetry groups. The experimental section includes examples of long-lived orders occurring under different magnetic, geometric and dynamic conditions. This large variety of regimes shows on one hand the ubiquitous character of long-lived species. On the other hand a common trait is identified in the formal characterisation that uses permutation and rotational symmetry concepts. For this reason a permutation symmetry characterisation is presented alongside the experimental description. The role of local geometry in a rigid spin system is highlighted by comparing two isomers with a different local arrangement of spin nuclei, and showing how a long-lived order is predicted and detected only in one case. Non rigid molecules can also display long-lived character. This is demonstrated by considering the methyl group 13CH3 in ?-picoline. Interestingly, as proton nuclei are magnetically equivalent, the long-lived order accessibility cannot employ coherent mechanisms. Finally a derivative of naphthalene is shown to possess an exceptionally long lifetime in solution and at room temperature. The accessibility of a very long lifetime opens up the possibility to store (hyper)polarization into singlet order and retrieve it later in time. A set of preliminary dissolution dynamic nuclear polarization experiments are also presented as a first attempt in this sense.