Department of Chemistry, Yale University, 350 Edwards Street, New Haven, CT 06511, United States.
Department of Chemistry and Biochemistry, University of Denver, Denver, CO 80208, United States.
J Magn Reson. 2021 Jan;322:106875. doi: 10.1016/j.jmr.2020.106875. Epub 2020 Nov 13.
The microwave magnetic field, B, in the non-resonant structures typically used for DNP-enhanced NMR is relatively small, so calibration via continuous wave (CW) power saturation requires a sample with longer spin lattice relaxation times than the samples used as CW standards in X-band cavities. HPHT diamonds have strong, easily observed EPR signals from P1 centers (nitrogen defects), and are indefinitely stable. This makes HPHT diamonds attractive as secondary standards for calibration of electron B field strength in a variety of experimental arrangements. The concentrations of P1 centers is also typically in the 30-200 ppm range, or equivalently 10-60 mM, and therefore the EPR relaxation observed is relevant to DNP enhanced NMR employing free radical polarizing agents at similar concentrations. Pulsed and CW saturation relaxation measurements T and T are compared at X-band. Under CW conditions the relevant TT product of time constants in our samples at room temperature is found to be dominated by electron-electron spin diffusion, and the product is large enough that saturation will be possible with the B of typical DNP systems. The similarity of T and T values obtained by pulse measurements at X-band and Q-band suggests that the X-band results can be extrapolated to the higher EPR frequencies used for DNP experiments. The electron spin dynamics observed here in HPHT diamond samples identify them as useful model systems to better delineate the interplay of electron spin relaxation, magic angle spinning, and inhomogeneous microwave irradiation as they affect DNP enhancement.
在非共振结构中，通常用于 DNP 增强 NMR 的微波磁场 B 相对较小，因此通过连续波 (CW) 功率饱和进行校准需要自旋晶格弛豫时间比 X 波段腔中用作 CW 标准的样品更长的样品。高温高压钻石具有来自 P1 中心（氮缺陷）的强且易于观察的 EPR 信号，并且具有无限稳定性。这使得高温高压钻石成为各种实验装置中电子 B 场强度校准的次级标准具有吸引力。P1 中心的浓度通常也在 30-200 ppm 范围内，或者等效地在 10-60 mM 范围内，因此观察到的 EPR 弛豫与在类似浓度下使用自由基极化剂的 DNP 增强 NMR 相关。在 X 波段比较了脉冲和 CW 饱和弛豫测量 T 和 T。在 CW 条件下，我们在室温下的样品中相关 TT 时间常数乘积由电子-电子自旋扩散主导，并且乘积足够大，以至于可以用典型的 DNP 系统的 B 进行饱和。在 X 波段和 Q 波段进行脉冲测量获得的 T 和 T 值的相似性表明，X 波段的结果可以外推到用于 DNP 实验的更高 EPR 频率。在高温高压钻石样品中观察到的电子自旋动力学表明，它们是有用的模型系统，可以更好地描绘电子自旋弛豫、魔角旋转和不均匀微波辐射之间的相互作用，因为它们会影响 DNP 增强。
