Department of Chemistry, Washington University in St. Louis, St. Louis, Missouri 63130, USA.
J Chem Phys. 2019 Sep 21;151(11):114107. doi: 10.1063/1.5113598.
Simulations describing the spin physics underpinning nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopy play an important role in the design of new experiments. When experiments are performed in the solid state, samples are commonly composed of powders or glasses, with molecules oriented at a large number of angles with respect to the laboratory frame. These powder angles must be represented in simulations to account for anisotropic interactions. Numerical techniques are typically used to accurately compute such powder averages. A large number of Euler angles are usually required, leading to lengthy simulation times. This is particularly true in broad spectra, such as those observed in EPR. The combination of the traditionally separate techniques of EPR and magic angle spinning (MAS) NMR could play an important role in future electron detected experiments, combined with dynamic nuclear polarization, which will allow for exceptional detection sensitivity of NMR spin coherences. Here, we present a method of reducing the required number of Euler angles in magnetic resonance simulations by analytically performing the powder average over one of the Euler angles in the static and MAS cases for the TEMPO nitroxide radical in a 7 T field. In the static case, this leads to a 97.5% reduction in simulation time over the fully numerical case and reproduces the expected spinning sideband manifold when simulated with a MAS frequency of 150 kHz. This technique is applicable to more traditional NMR experiments as well, such as those involving quadrupolar nuclei or multiple dimensions.
描述核磁共振（NMR）和电子顺磁共振（EPR）光谱学中自旋物理的模拟在新实验的设计中起着重要作用。当在固态中进行实验时，样品通常由粉末或玻璃组成，分子相对于实验室框架以大量角度取向。这些粉末角度必须在模拟中表示，以考虑各向异性相互作用。通常使用数值技术来准确计算这种粉末平均值。通常需要大量的欧拉角，导致模拟时间长。在宽谱，如 EPR 中观察到的那些，情况尤其如此。EPR 和魔角旋转（MAS）NMR 的传统分离技术的结合，再结合动态核极化，将在未来的电子检测实验中发挥重要作用，这将允许 NMR 自旋相干的非凡检测灵敏度。在这里，我们提出了一种通过在静态和 MAS 情况下对 TEMPO 氮氧自由基在 7 T 场中的一个欧拉角进行粉末平均来减少磁共振模拟中所需欧拉角数量的方法。在静态情况下，与完全数值情况相比，模拟时间减少了 97.5%，并且当以 150 kHz 的 MAS 频率模拟时，它再现了预期的旋转边带谱。该技术也适用于更传统的 NMR 实验，例如涉及四极核或多个维度的实验。
