Laboratory of Biomolecular NMR, Saint Petersburg State University, Saint Petersburg, Russia.
Phys Chem Chem Phys. 2019 Jan 30;21(5):2595-2600. doi: 10.1039/c8cp06883f.
The examination and optimized preparation of nuclear spin singlet order has enabled the development of new types of applications that rely on potentially long-term polarization storage. Lifetimes several orders of magnitude longer than T1 have been observed. The efficient creation of such states relies on special pulse sequences. The extreme cases of very large and very small magnetic equivalence received primary attention, while relatively little effort has been directed towards studying singlet relaxation in the intermediate regime. The intermediate case is of interest as it is relevant for many spin systems, and would also apply to heteronuclear systems in very low magnetic fields. Experimental evidence for singlet-triplet leakage in the intermediate regime is sparse. Here we describe a pulse sequence for efficiently creating singlets in the intermediate regime in a broad-band fashion. Singlet lifetimes are studied with a specially synthesized molecule over a wide range of magnetic fields using a home-built sample-lift apparatus. The experimental results are supplemented with spin simulations using parameters obtained from ab initio calculations. This work indicates that the chemical shift anisotropy (CSA) mechanism is relatively weak compared to singlet-triplet leakage for the proton system observed over a large magnetic field range. These experiments provide a mechanism for expanding the scope of singlet NMR to a larger class of molecules, and provide new insights into singlet lifetime limiting factors.
核磁单重态的检测和优化制备使得依赖于潜在长期极化存储的新型应用得以发展。已经观察到比 T1 长几个数量级的寿命。这种状态的高效产生依赖于特殊的脉冲序列。非常大和非常小的磁等价的极端情况受到了主要关注，而对于中间状态的单重态弛豫的研究则相对较少。中间情况是感兴趣的，因为它与许多自旋系统有关，并且也适用于非常低磁场中的异核系统。在中间状态中单重态-三重态泄漏的实验证据很少。在这里，我们描述了一种在宽带范围内高效地在中间状态中产生单重态的脉冲序列。使用专门合成的分子，在自建的样品提升装置中，在很宽的磁场范围内研究了单重态寿命。实验结果辅以使用从头计算获得的参数进行的自旋模拟。这项工作表明，与在大磁场范围内观察到的质子系统的单重态-三重态泄漏相比，化学位移各向异性（CSA）机制相对较弱。这些实验为将单重态 NMR 的范围扩展到更大类别的分子提供了一种机制，并为单重态寿命限制因素提供了新的见解。
