空间位阻引起的并五苯大负热膨胀
Large Negative Thermal Expansion in Pentacene due to Steric Hindrance
摘要：
并五苯晶体中沿$a$的单轴负热膨胀是低聚并苯系列中的一个特殊性，并且对于结晶固体是异常大的。从120 K到413 K的全X射线结构分析表明，占主导地位的热运动是刚性分子围绕其长轴的振动，修改了描述层内人字形包装的分子间角。该人字形角随温度增加（每100 K增加0.3 - 0.6 $^{\circ}$），并引起层内分子的各向异性重排，即在$b$方向上的膨胀和沿$a$的明显收缩。此外，较大的人字形角改善了相邻平行分子之间的共面重叠，从而增强了吸引范德华力。
英文摘要：
The uniaxial negative thermal expansion in pentacene crystals along $a$ is a particularity in the series of the oligoacenes, and exeptionally large for a crystalline solid. Full x-ray structure analysis from 120 K to 413 K reveals that the dominant thermal motion is a libration of the rigid molecules about their long axes, modifying the intermolecular angle which describes the herringbone packing within the layers. This herringbone angle increases with temperature (by 0.3 -- 0.6$^{\circ}$ per 100 K), and causes an anisotropic rearrangement of the molecules within the layers, i.e. an expansion in the $b$ direction, and a distinct contraction along $a$. Additionally, a larger herringbone angle improves the cofacial overlap between adjacent, parallel molecules, and thus enhances the attractive van der Waals forces.
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