Stevanato Gabriele, Roy Soumya Singha, Hill-Cousins Joe, Kuprov Ilya, Brown Lynda J, Brown Richard C D, Pileio Giuseppe, Levitt Malcolm H
School of Chemistry, University of Southampton, University Road, Southampton, UK.
Phys Chem Chem Phys. 2015 Feb 28;17(8):5913-22. doi: 10.1039/c4cp05704j.
Clusters of coupled nuclear spins may form long-lived nuclear spin states, which interact weakly with the environment, compared to ordinary nuclear magnetization. All experimental demonstrations of long-lived states have so far involved spin systems which are close to the condition of magnetic equivalence, in which the network of spin-spin couplings is conserved under all pair exchanges of symmetry-related nuclei. We show that the four-spin system of trans-[2,3-(13)C2]-but-2-enedioate exhibits a long-lived nuclear spin state, even though this spin system is very far from magnetic equivalence. The 4-spin long-lived state is accessed by slightly asymmetric chemical substitutions of the centrosymmetric molecular core. The long-lived state is a consequence of the locally centrosymmetric molecular geometry for the trans isomer, and is absent for the cis isomer. A general group theoretical description of long-lived states is presented. It is shown that the symmetries of coherent and incoherent interactions are both important for the existence of long-lived states.
耦合核自旋簇可能形成长寿命的核自旋态，与普通核磁化相比，其与环境的相互作用较弱。迄今为止，所有长寿命态的实验证明都涉及自旋系统，这些自旋系统接近磁等价条件，即在对称相关核的所有对交换下，自旋 - 自旋耦合网络得以保留。我们表明，反式 - [2,3 - (13)C2] - 丁 - 2 - 烯二酸酯的四自旋系统表现出长寿命的核自旋态，尽管该自旋系统与磁等价相差甚远。通过中心对称分子核心的略微不对称化学取代可获得四自旋长寿命态。长寿命态是反式异构体局部中心对称分子几何结构的结果，而顺式异构体则不存在这种情况。给出了长寿命态的一般群论描述。结果表明，相干和非相干相互作用的对称性对于长寿命态的存在都很重要。
