Widdifield Cory M, Schurko Robert W
Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario, Canada N9B 3P4.
J Phys Chem A. 2005 Aug 11;109(31):6865-76. doi: 10.1021/jp058059l.
The quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) and double frequency sweep (DFS)/QCPMG pulse sequences are applied in order to acquire the first solid-state 39K NMR spectra of organometallic complexes, the polymeric main group metallocenes cyclopentadienyl potassium (CpK) and pentamethylcyclopentadienyl potassium (CpK). Piecewise QCPMG NMR techniques are used to acquire a high S/N 39K spectrum of the broad central transition of CpK, which is ca. 200 kHz in breadth. Analytical and numerical simulations indicate that there is a significant quadrupolar interaction present at both potassium nuclei (C(Q)(39K) = 2.55(6)/2.67(8) MHz and 4.69(8) MHz for CpK (static/MAS) and CpK, respectively). Experimental quadrupolar asymmetry parameters suggest that both structures are bent about the potassium atoms (eta(Q)(39K) = 0.28(3)/0.29(3) for CpK (static/MAS) and eta(Q)(39K) = 0.30(3) for CpK). Variable-temperature (VT) 39K NMR experiments on CpK elucidate temperature-dependent changes in quadrupolar parameters which can be rationalized in terms of alterations of bond distances and angles with temperature. 13C CP/MAS NMR experiments are conducted upon both samples to quantify the carbon chemical shielding anisotropy (CSA) at the Cp' ring carbon atoms. Ab initio carbon CSA and 39K electric-field gradient (EFG) and CSA calculations are conducted and discussed for the CpK complex, in order to correlate the experimental NMR parameters with molecular structure in CpK and Cp*K. 39K DFS/QCPMG and 13C CP/MAS experiments prove invaluable for probing molecular structure, temperature-dependent structural changes, and the presence of impurities in these systems.
应用四极 Carr-Purcell Meiboom-Gill（QCPMG）和双频扫描（DFS）/QCPMG 脉冲序列，以获取有机金属配合物、聚合主族茂金属环戊二烯基钾（CpK）和五甲基环戊二烯基钾（CpK）的首个固态 39K NMR 谱。采用分段 QCPMG NMR 技术获取了 CpK 宽中心跃迁的高信噪比 39K 谱，其宽度约为 200 kHz。分析和数值模拟表明，两个钾核处均存在显著的四极相互作用（对于 CpK（静态/魔角旋转）和 CpK，C(Q)(39K)分别为 2.55(6)/2.67(8) MHz 和 4.69(8) MHz）。实验四极不对称参数表明，两种结构在钾原子处均发生弯曲（对于 CpK（静态/魔角旋转），eta(Q)(39K) = 0.28(3)/0.29(3)；对于 CpK，eta(Q)(39K) = 0.30(3)）。对 CpK 进行的变温（VT）39K NMR 实验阐明了四极参数随温度的变化，这可以根据键长和键角随温度的变化来合理解释。对两个样品均进行了 13C CP/魔角旋转 NMR 实验，以量化 Cp'环碳原子处的碳化学屏蔽各向异性（CSA）。对 CpK 配合物进行了从头算碳 CSA 以及 39K 电场梯度（EFG）和 CSA 计算，并进行了讨论，以便将实验 NMR 参数与 CpK 和 Cp*K 中的分子结构相关联。39K DFS/QCPMG 和 13C CP/魔角旋转实验对于探测这些体系中的分子结构、温度依赖性结构变化以及杂质存在情况证明是非常有价值的。
