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首页> 外文期刊>Acta Crystallographica, Section B. Structural Science >Model for the crystal packing and conformational changes of biphenyl in incommensurate phase transitions
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Model for the crystal packing and conformational changes of biphenyl in incommensurate phase transitions

机译：在不相称的相变中联苯的晶体堆积和构象变化模型

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Standard atom - atom potentials for hydrocarbons and a torsional potential to account for the pi-electron conjugation energy were used to model the crystal structures and phase transitions of biphenyl. The model describes the high-temperature phase ( I) with its planar molecule as a stationary point of the energy hypersurface. Phase I represents a low-energy barrier between the symmetry minima of the ground state ( phase III), in which the molecule is twisted with torsion angles of opposite sign. Global-energy minimization was carried out by considering both regular structures, with one or two independent molecules, and quasi-one-dimensional superstructures built of N cells (N up to 16) of the high-temperature structure. The various energy-minimized biphenyl structures demonstrate remarkable similarity in their crystal packing; in particular, there are characteristic rows of cooperatively twisted molecules parallel to the superstructure dimension b. The structures built of centrosymmetric rows (P (1) over bar 1, Z = 4 and 8) are almost as low in energy as the basic structure ( an N = 2 superstructure, Pa, Z = 4); moreover, one of them is isostructural with the low-temperature p-quaterphenyl structure. With N > 8, structures of lower energy than that of the basic structure ( N = 2) were found; their common feature is an M-fold modulation of the twist angle over the supercell period, with M smaller than N and generally not a simple fraction of it. The global minimum was found to conform to the ratio k = M/ N = 6/14, which is close to the experimentally observed k = 6/13 in the incommensurate phase III. Enthalpy minimization showed an overall decrease in the magnitude of the twist angle down to tau similar or equal to 0degrees, as well as the evolution of the modulated structures towards the high-temperature structure with increasing pressure, in agreement with evidence for the high-pressure limit of the incommensurate biphenyl phases. [References: 45]
机译：碳氢化合物的标准原子-原子电势和π电子共轭能的扭转电势被用来模拟联苯的晶体结构和相变。该模型将高温相（I）的平面分子描述为能量超表面的固定点。 I相代表基态对称最小值（III相）之间的低能势垒，其中分子以相反符号的扭转角扭曲。通过考虑具有一个或两个独立分子的规则结构和由高温结构的N个单元（最多16个N）构成的准一维超结构，来实现全局能量最小化。各种能量最小化的联苯结构在其晶体堆积方面显示出惊人的相似性。特别地，存在平行于上部结构尺寸b的协同扭曲分子的特征行。由中心对称行构成的结构（在条1，Z = 4和8上的P（1））的能量几乎与基本结构（N = 2上层结构，Pa，Z = 4）低。而且，其中之一是具有低温对-四-苯基结构的同构。当N> 8时，发现能量的结构比基本结构低（N = 2）。它们的共同特征是在超级单元周期内对扭曲角进行M倍调制，其中M小于N，通常不是其中的一个简单分数。发现全局最小值符合比率k = M / N = 6/14，该比率接近于在不相称的阶段III中实验观察到的k = 6/13。焓最小化显示，扭曲角的大小总体减小，直到大约等于0度的tau，并且随着压力的增加，调制结构向高温结构的演变，与高压的证据相符联苯相的极限。 [参考：45]

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