Murugan N Arul
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560012, India.
J Phys Chem B. 2005 Dec 22;109(50):23955-62. doi: 10.1021/jp052535q.
Detailed molecular simulations are carried out to investigate the effect of temperature on orientational order in cubane molecular crystal. We report a transition from an orientationally ordered to an orientationally disordered plastic crystalline phase in the temperature range 425-450 K. This is similar to the experimentally reported transition at 395 K. The nature of this transition is first order and is associated with a 4.8% increase in unit cell volume that is comparable to the experimentally reported unit cell volume change of 5.4% (Phys. Rev. Lett. 1997, 78, 4938). An orientational order parameter, eta(T), has been defined in terms of average angle of libration of a molecular 3-fold axis and the orientational melting has been characterized by using eta(T). The orientational melting is associated with an anomaly in specific heat at constant pressure (C(P)) and compressibility (kappa). The enthalpy of transition and entropy of transition associated with this orientational melting are 20.8 J mol(-1) and 0.046 J mol(-1) K(-1), respectively. The structure of crystalline as well as plastic crystalline phases is characterized by using various radial distribution functions and orientational distribution functions. The coefficient of thermal expansion of the plastic crystalline phase is more than twice that of the crystalline phase.
进行了详细的分子模拟，以研究温度对立方烷分子晶体取向有序性的影响。我们报道了在425 - 450 K温度范围内从取向有序的塑性晶体相到取向无序的塑性晶体相的转变。这与实验报道的在395 K的转变相似。这种转变的性质是一级的，并且与晶胞体积增加4.8%相关，这与实验报道的5.4%的晶胞体积变化相当（《物理评论快报》，1997年，第78卷，第4938页）。根据分子三重轴的平均摆动角度定义了一个取向序参量η(T)，并使用η(T)对取向熔化进行了表征。取向熔化与恒压比热（C(P)）和压缩率（κ）的异常有关。与这种取向熔化相关的转变焓和转变熵分别为20.8 J mol⁻¹和0.046 J mol⁻¹ K⁻¹。通过使用各种径向分布函数和取向分布函数对晶体相以及塑性晶体相的结构进行了表征。塑性晶体相的热膨胀系数是晶体相的两倍多。
