使用PRESTO或偶极介导的重聚焦INEPT方法在固态NMR中从质子到四极核的磁化转移
Magnetization transfer from protons to quadrupolar nuclei in solid-state NMR using PRESTO or dipolar-mediated refocused INEPT methods
摘要：
我们在这里比较了两种实现这种转移的方法的性能：Presto（相移再耦合实现了顺序的平滑转移），这是目前实现从质子到四极核的磁化转移的首选方法，并且已经显示出对于四极核和D-RINEPT，（偶极介导的通过极化转移增强的重聚焦不敏感核），其使用基于自旋的SR 4 ${}_1{}^2$再耦合，其已经被用于以相反的方式将磁化从半整数四极自旋转移到质子。我们还使用270个${}_0$ 90 ${}_180$复合$π$ -脉冲作为反转元件，测试了具有R16 ${}_7{}^6$再耦合的PRESTO序列。这种再耦合方案迄今还没有被用来重新引入偶极耦合与质子。这些不同的技术转移的磁化强度从质子到四极核进行了分析，使用（i）平均哈密顿理论，（ii）自旋动力学的数值模拟，和（iii）实验${}^1$ H $\rightarrow {}^{27}$铝和${}^{1}$ H $\rightarrow {}^{17}$ O转移合成的AlPO 4 -14和${}^{17}$ O标记的气相二氧化硅，分别。实验和模拟是在两个磁场（9.4和18.8 T）和几个纺丝速度（15，18-24和60 kHz）。该分析表明，由于其$γ$编码的特性，PRESTO在低磁场和MAS频率下产生最高的转移效率，而由于其对射频场不均匀性和化学位移的更高鲁棒性，D-RINEPT在高场和MAS频率下更敏感，特别是对于表现出大偏移或CSA的质子，例如参与氢键的那些。
英文摘要：
We compare here the performances of two approaches to achieve such transfer: PRESTO (Phase-shifted Recoupling Effects a Smooth Transfer of Order), which is currently the method of choice to achieve the magnetization transfer from protons to quadrupolar nuclei and which has been shown to supersede Cross-Polarization under Magic-Angle Spinning (MAS) for quadrupolar nuclei and D-RINEPT (Dipolar-mediated Refocused Insensitive Nuclei Enhanced by Polarization Transfer) using symmetry-based SR4${}_1{}^2$ recoupling, which has already been employed to transfer the magnetization in the reverse way from half-integer quadrupolar spin to protons. We also test the PRESTO sequence with R16${}_7{}^6$ recoupling using 270${}_0$90${}_180$ composite $π$-pulses as inversion elements. This recoupling scheme has not so far been employed to reintroduce dipolar couplings with protons. These various techniques to transfer the magnetization from protons to quadrupolar nuclei are analyzed using (i) an average Hamiltonian theory, (ii) numerical simulations of spin dynamics, and (iii) experimental ${}^1$H $\rightarrow {}^{27}$Al and ${}^{1}$H $\rightarrow {}^{17}$O transfers in as-synthesized AlPO4-14 and ${}^{17}$O-labelled fumed silica, respectively. The experiments and simulations are done at two magnetic fields (9.4 and 18.8 T) and several spinning speeds (15, 18-24 and 60 kHz). This analysis indicates that owing to its $γ$-encoded character, PRESTO yields the highest transfer efficiency at low magnetic fields and MAS frequencies, whereas owing to its higher robustness to rf-field inhomogeneity and chemical shifts, D-RINEPT is more sensitive at high fields and MAS frequencies, notably for protons exhibiting large offset or CSA, such as those involved in hydrogen bonds.
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