Grey C P, Eijkelenboom A P, Veeman W S
Department of Chemistry, State University of New York at Stony Brook 11794-3400, USA.
Solid State Nucl Magn Reson. 1995 Feb;4(2):113-20. doi: 10.1016/0926-2040(94)00041-a.
A two-dimensional (2D) experiment has been used to show that 14N irradiation and magic-angle spinning (MAS) results in population transfers between the 14N Zeeman levels. This experiment was applied to a sample of N-acetyl-D,L-valine, a material where asymmetric doublets resulting from 13C-14N dipolar coupling are clearly resolved in the 13C spectrum at a field of 7 T for Carbon atoms directly bonded to the nitrogen atom. The 13C transverse magnetization was allowed to evolve in the F1 and F2 dimensions, and the 14N spins were irradiated during the mixing period. Cross-peaks were observed in the 2D 13C spectrum between the two peaks of the CH asymmetric doublet. Since one peak of the doublet results primarily from coupling to the [formula: see text] state and the other peak from coupling to [formula: see text] states, population changes between the 14N Zeeman levels have occurred during the mixing period. These population transfers are a consequence of the time dependence of the 14N quadrupole splitting Q under MAS conditions and 14N irradiation. Level anti-crossings of the 14N Zeeman levels occur at the zero-crossings of Q, and a continuous and slow change in Q will result in the transfer of 14N populations between the different Zeeman levels. If these passages are adiabatic, then the system returns to its original state after two zero-crossings. This is consistent with the experimental observation that the intensities of the cross-peaks for 14N irradiation are greater for half a rotor period than a full rotor period.
二维（2D）实验已被用于表明，14N 辐照和魔角旋转（MAS）会导致 14N 塞曼能级之间的布居转移。该实验应用于 N - 乙酰 - D,L - 缬氨酸样品，在 7 T 磁场下，对于直接与氮原子键合的碳原子，由 13C - 14N 偶极耦合产生的不对称双峰在 13C 谱中清晰可辨。13C 横向磁化在 F1 和 F2 维度中演化，并且在混合期对 14N 自旋进行辐照。在 CH 不对称双峰的两个峰之间的二维 13C 谱中观察到了交叉峰。由于双峰的一个峰主要源于与[公式：见原文]态的耦合，另一个峰源于与[公式：见原文]态的耦合，因此在混合期 14N 塞曼能级之间发生了布居变化。这些布居转移是 MAS 条件下 14N 四极分裂 Q 和 14N 辐照的时间依赖性的结果。14N 塞曼能级的能级反交叉发生在 Q 的零交叉处，并且 Q 的连续缓慢变化将导致 14N 布居在不同塞曼能级之间转移。如果这些跃迁是绝热的，那么系统在两次零交叉后会回到其原始状态。这与实验观察结果一致，即 14N 辐照的交叉峰强度在半个转子周期比在一个完整转子周期时更大。
