Strohmeier M, Orendt A M, Alderman D W, Grant D M
Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, Utah 84112-0850, USA.
J Am Chem Soc. 2001 Feb 28;123(8):1713-22. doi: 10.1021/ja003599b.
Two of the three conformational polymorphs of dimethyl-3,6-dichloro-2,5-dihydroxyterephthalate are studied by solid-state NMR techniques. The structural differences between the polymorphs have previously been studied by X-ray. In these two polymorphs named white and yellow due to their color, the major structural difference is the torsional angle between the ester group and the aromatic ring. The yellow form has a dihedral angle of 4 degrees between the plane of the aromatic ring and the plane of the ester group, while the white form has two different molecules per unit cell with dihedral angles of 70 degrees and 85 degrees. This change greatly affects the conjugation in the pi-electronic system. In addition, there are differences in the hydrogen-bonding patterns, with the white form having intermolecular hydrogen bonds and the yellow form having intramolecular hydrogen bonds. In this work, the carbon isotropic chemical shift values and the chlorine electric field gradient (EFG) tensor information are extracted from the (13)C MAS spectra, and the principal values of the chemical shift tensors of the carbons are obtained from 2D FIREMAT experiments. Quantum chemical calculations of the chemical shift tensor data as well as the EFG tensor are performed at the HF and DFT levels of theory on individual molecules and on stacks of three molecules to account for the important intermolecular interactions in the white form. The differences between the spectral data on the two polymorphs are discussed in terms of the known electronic and structural differences.
通过固态核磁共振技术研究了二甲基-3,6-二氯-2,5-二羟基对苯二甲酸三种构象多晶型物中的两种。此前已通过X射线研究了这些多晶型物之间的结构差异。在这两种因其颜色而分别命名为白色和黄色的多晶型物中，主要的结构差异是酯基与芳环之间的扭转角。黄色晶型中芳环平面与酯基平面之间的二面角为4度，而白色晶型的每个晶胞中有两种不同的分子，其二面角分别为70度和85度。这种变化极大地影响了π电子体系中的共轭。此外，氢键模式也存在差异，白色晶型具有分子间氢键，而黄色晶型具有分子内氢键。在这项工作中，从(13)C MAS谱中提取碳各向同性化学位移值和氯电场梯度(EFG)张量信息，并通过二维FIREMAT实验获得碳化学位移张量的主值。在HF和DFT理论水平上，对单个分子和三个分子堆叠进行化学位移张量数据以及EFG张量的量子化学计算，以考虑白色晶型中重要的分子间相互作用。根据已知的电子和结构差异讨论了两种多晶型物光谱数据之间的差异。
