Department of Chemistry and Biochemistry, University of North Carolina Wilmington, Wilmington, NC 28403-5932, USA.
J Mol Graph Model. 2012 Sep;38:26-30. doi: 10.1016/j.jmgm.2012.07.008. Epub 2012 Aug 8.
The GIAO-HF method in Gaussian 03 was used to calculate the isotropic shielding value of the proximal hydrogen of a diatomic hydrogen probe oriented perpendicular to the plane and moved in a square grid 2.5Å above the plane of conjugated five-membered ring heterocyclic compounds: pyrrole, furan, phosphole and thiophene and their phosphorus containing analogs. Subtraction of the calculated isotropic shielding value of diatomic hydrogen from each of these isotropic shielding values gave the shielding increment (Δσ) for each probe position. Plotting this value against Cartesian coordinates of the probe position allowed determination of the computed through-space shielding increment surfaces for these compounds. Substantial shielding was observed above the center of each ring, as expected for aromatic compounds. The magnitude of the shielding increment 2.5Å above the heterocyclic ring center correlated reasonably well with the other published methods of assessing aromaticity of these systems ASE (aromatic stabilization energy), Λ (exaltation of magnetic susceptibility) and NICS(1) (nucleus-independent chemical shift) values, both of which are magnetic criteria. The magnitude of the shielding increment also correlated with the number of phosphorus atoms in the ring. Greatest shielding (5.5ppm) was observed 2.5Å above 2,3,4,5-tetraphosphathiophene.
使用 Gaussian 03 中的 GIAO-HF 方法计算了垂直于平面且位于共轭五元杂环化合物吡咯、呋喃、磷杂茂和噻吩及其含磷类似物平面上方 2.5Å 的二原子氢探针的各向同性屏蔽值：吡咯、呋喃、磷杂茂和噻吩及其含磷类似物。从这些各向同性屏蔽值中减去二原子氢的计算各向同性屏蔽值，得到每个探针位置的屏蔽增量（Δσ）。将该值绘制为探针位置的笛卡尔坐标，允许确定这些化合物的计算贯穿空间屏蔽增量表面。在每个环的中心上方观察到了大量的屏蔽，这是预期的芳香化合物的特征。杂环环中心上方 2.5Å 的屏蔽增量的大小与其他评估这些体系芳香性的方法（芳香稳定能 ASE、磁各向异性 Λ 和核独立化学位移 NICS(1) 值）相当吻合，这两种方法都是磁标准。屏蔽增量的大小也与环中磷原子的数量有关。在 2,3,4,5-四磷杂噻吩上方 2.5Å 处观察到最大屏蔽（5.5ppm）。
