A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Science, Vavilova Street 28, 119991 Moscow, Russia.
Highest Chemical College of RAS, D. Mendeleev University of Chemical Technology of Russia, Miusskaya Square 9, 125047 Moscow, Russia.
Int J Mol Sci. 2022 Aug 13;23(16):9073. doi: 10.3390/ijms23169073.
The structure, thermodynamic parameters, and the character of thermal motion in octamethylcyclotetrasiloxane (D4) were investigated using the combination of experimental (single-crystal X-ray diffraction, thermochemistry) and theoretical (density functional theory calculations, ab initio molecular dynamics and metadynamics) methods. Single crystals of D4 were grown in a glass capillary in situ and the structures of high- (238-270 K) and low-temperature (100-230 K) phases were studied in detail. In the temperature interval 230-238 K, a phase transition with rather low enthalpy (-1.04(7) kcal/mol) was detected. It was found that phase transition is accompanied by change of conformation of cyclosiloxane moiety from boat-saddle (cradle) to chair. According to PBE0/6-311G(d,p) calculation of isolated D4, such conformation changes are characterized by a low barrier (0.07 kcal/mol). The character of molecular thermal motion and the path of phase transition were established with combination of periodic DFT calculations, including molecular dynamics and metadynamics. The effect of crystal field led to an increase in the calculated phase transition barrier (4.27 kcal/mol from low- to high-temperature phase and 3.20 kcal/mol in opposite direction).
使用实验（单晶 X 射线衍射、热化学）和理论（密度泛函理论计算、从头算分子动力学和元动力学）方法相结合，研究了八甲基环四硅氧烷（D4）的结构、热力学参数和热运动特性。D4 的单晶在玻璃毛细管中就地生长，并详细研究了高温（238-270 K）和低温（100-230 K）相的结构。在 230-238 K 的温度范围内，检测到一个相当低的焓（-1.04(7) kcal/mol）相变。发现相变伴随着环硅氧烷部分构象从船鞍（摇篮）到椅式的变化。根据孤立 D4 的 PBE0/6-311G(d,p)计算，这种构象变化的特征是低能垒（0.07 kcal/mol）。通过周期性 DFT 计算（包括分子动力学和元动力学）的组合，确定了分子热运动的特征和相变的路径。晶体场的影响导致计算的相变能垒增加（从低温相向高温相增加 4.27 kcal/mol，相反方向增加 3.20 kcal/mol）。
