CRYSTAL-STRUCTURES, PHASE-TRANSITIONS AND ENERGY CALCULATIONS OF POLY(P-PHENYLENE) OLIGOMERS
CRYSTAL-STRUCTURES, PHASE-TRANSITIONS AND ENERGY CALCULATIONS OF POLY(P-PHENYLENE) OLIGOMERS
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DOI：
10.1016/0032-3861(93)90313-y
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发表时间：
1993-01-01
期刊：
影响因子：
4.6
通讯作者：
FARMER, BL
中科院分区：
文献类型：
--
作者：
BAKER, KN;FRATINI, AV;FARMER, BL
报道了三种聚对苯低聚物的室温晶体结构、110 K时的晶胞尺寸和相变。对醌苯基（PQP）、C30H22、对六苯基（PSP）、C36H26和对九苯基（PSePtiP）、C42H30的结构与较短的低聚物相似，均属于P2(1)/c空间群。PQP的单元格尺寸为a=22.056埃，b=5.581埃，c=8.070埃和beta=97.91度。PSP的单元格尺寸为a=26.241埃，b=5.568埃，c=8.091埃和beta=98.17度，PSeptiP的单元格尺寸为a=30.577埃，b=5.547埃，c=8.034埃和beta=100.52度。a轴随分子长度增加而增加。分子在这三种结构中都是线性和平面的。PQP和PSeptiP的人字形填料是相似的，而PSP的倾斜程度要大得多。在110 K时，单元电池参数b和c大约增加了一倍，而a几乎与室温电池相同。当晶体冷却到110 K时，PQP、PSP和PSeptiP的固态转变随时间变化。在高温下，热测量表明低聚物是热致液晶。报道了PQP、PSP、PSeptiP和对辛基苯基（POP） C48H34的晶晶转变温度。本文还介绍了PSP的构象和堆积的分子力学研究结果。特别研究了分子内力（如邻氢斥力）和分子间晶体堆积力之间的竞争。分子力学计算预测了分离的多苯基的非平面构象，这意味着苯基环之间的共轭不足以克服邻位氢排斥。然而，在晶体环境中，分子间的作用力倾向于形成平面构象。对PSP分子阵列的计算表明，从共面角度改变苯基-苯基扭转角增加了结构的总能量。低聚物之间最有利的分子间相互作用是在苯环共面构象中实现的。
The room temperature crystal structures, unit cell dimensions at 110 K and phase transitions of three poly(p-phenylene) oligomers are reported. The structures of p-quinquephenyl (PQP), C30H22, p-sexiphenyl (PSP), C36H26, and p-septiphenyl (PSePtiP), C42H30, each belonging to space group P2(1)/c, are similar to those of shorter oligomers. The unit cell dimensions are a=22.056 angstrom, b=5.581 angstrom, c=8.070 angstrom and beta=97.91-degrees for PQP a=26.241 angstrom, b=5.568 angstrom, c=8.091 angstrom and beta=98.17-degrees for PSP, and a=30.577 angstrom, b=5.547 angstrom, c=8.034 angstrom and beta=100.52-degrees for PSeptiP. The a axis increases with molecular length. The molecules are linear and planar in all three structures. The herringbone nature of the packing is similar for PQP and PSeptiP, while a considerably greater tilt occurs in PSP. At 110 K, the unit cell parameters b and c are approximately doubled while a remains nearly the same as in the room temperature cell. A time-dependent solid state transition is observed for PQP, PSP and PSeptiP when crystals are cooled to 110 K. At elevated temperatures, thermal measurements indicate the oligomers to be thermotropic liquid crystals. The crystal-smectic transition temperatures are reported for PQP, PSP, PSeptiP and p-octiphenyl (POP), C48H34. The results of a molecular mechanics study on the conformation and packing of PSP are also presented. The competition between intramolecular forces (such as ortho hydrogen repulsions) and intermolecular crystal packing forces was examined in particular. Molecular mechanics calculations predict non-planar conformations in isolated polyphenyls, implying that conjugation between phenyl rings is insufficient to overcome ortho hydrogen repulsions. In a crystalline environment, however, intermolecular forces tend to force a planar conformation. Calculations on arrays of PSP molecules show that changing the phenyl-phenyl torsion angles from the coplanar value increases the total energy of the structure. The most favourable intermolecular interactions between oligomers are achieved for conformations having the phenyl rings coplanar.