NMR spectroscopic and quantum chemical characterization of the (E)- and (Z)- isomers of the penta-1,3-dienyl-2-cation
NMR spectroscopic and quantum chemical characterization of the (E)- and (Z)- isomers of the penta-1,3-dienyl-2-cation
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DOI：
10.1002/poc.662
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发表时间：
2003-08-01
影响因子：
1.8
通讯作者：
Gauss, J
中科院分区：
文献类型：
--
作者：
Siehl, HU;Müller, T;Gauss, J
利用高真空共缩合技术，在-135 ℃下，在SO2 ClF/SO2 F2中，4-氯-1，2-戊二烯（2）与SbF 5反应，得到了顺-1，3-二烯基-2-阳离子（1）的（E）（-）和（Z）（-）异构体的稀溶液。在HF-SCF、MP2、CCSD和CCSD（T）水平上，采用MP2/tzp几何构型，将两种异构体混合物的实验NMR谱与量子化学C-13 NMR化学位移计算结果进行了比较。量子化学位移的计算进行了tzp基础（9 s5 p1 d/5s 3 p1 d）的碳和dz基础（4s/2s）的氢使用规范，包括原子轨道（GIAO）。HF-SCF计算偏离显着的带正电荷的碳原子的烯丙基型共振系统显示高达40 ppm太去屏蔽值相比，实验观察到的化学位移。因此，HF-SCF方法不足以令人满意地预测这种类型的碳正离子的屏蔽张量。然而，电子相关的列入，允许一个明确的分配的Z-和E-异构体的光谱。在CCSD（T）水平上，（Z）-和（E）-1的NMR化学位移的实验值和计算值之间的平均偏差分别为1.8和2.0 ppm。二烯基阳离子（E/Z）-1是在超强酸溶液中作为持久物种产生的最小的乙烯基阳离子，并通过C-13 NMR光谱观察到。这些碳正离子的结构完全由先进的从头算量子化学计算的结构和NMR化学位移。版权所有（C）2003约翰威利父子有限公司。
Dilute solutions of the (E)(-) and (Z)(-) isomers of pent-1,3-dienyl-2-cations (1) were obtained from reaction of 4-chloro-1,2-pentadiene (2) with SbF5 in SO2ClF/SO2F2 at -135 degreesC using high-vacuum co-condensation techniques. The experimental NMR spectra of the mixture of the two isomers were compared with quantum chemical C-13 NMR chemical shift calculations at HF-SCF, MP2, CCSD and CCSD(T) levels using MP2/tzp geometries. Quantum chemical shift calculations were performed with a tzp basis (9s5p1d/5s3p1d) for carbon and a dz basis (4s/2s) for hydrogen using gauge-including atomic orbitals (GIAOs). The HF-SCF calculations deviate significantly for the positively charged carbon atoms of the allyl-type resonance system showing up to 40 ppm too deshielded values compared with the experimentally observed chemical shifts. The HF-SCF approach is therefore not sufficient in predicting satisfactorily the shielding tensors in this type of carbocation. Inclusion of electron correlation, however, allows an unequivocal assignment of the spectra of the Z- and E-isomers. The mean deviation between experimental and calculated NMR chemical shifts at the CCSD(T) level is 1.8 and 2.0 ppm for (Z)- and (E)-1, respectively. The dienyl cations (E/Z)-l are the smallest vinyl cations ever generated as persistent species in superacidic solutions and observed by C-13 NMR spectroscopy. These carbocations were structurally fully characterized by advanced ab initio quantum chemical calculations of structure and NMR chemical shifts. Copyright (C) 2003 John Wiley Sons, Ltd.