Bačić Zlatko, Vlček Vojtěch, Neuhauser Daniel, Felker Peter M
Department of Chemistry, New York University, New York, NY 10003, USA.
Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095-1569, USA.
Faraday Discuss. 2018 Dec 13;212(0):547-567. doi: 10.1039/c8fd00082d.
Splittings of the translation-rotation (TR) eigenstates of the solid light-molecule endofullerenes M@C60 (M = H2, H2O, HF) attributed to the symmetry breaking have been observed in the infrared (IR) and inelastic neutron scattering spectra of these species in the past couple of years. In a recent paper [Felker et al., Phys. Chem. Chem. Phys., 2017, 19, 31274], we established that the electrostatic, quadrupolar interaction between the guest molecule M and the twelve nearest-neighbor C60 cages of the solid is the main source of the symmetry breaking. The splittings of the three-fold degenerate ground states of the endohedral ortho-H2, ortho-H2O and the j = 1 level of HF calculated using this model were found to be in excellent agreement with the experimental results. Utilizing the same electrostatic model, this theoretical study investigates the effects of the symmetry breaking on the excited TR eigenstates of the three species, and how they manifest in their simulated low-temperature (5-6 K) near-IR (NIR) and far-IR (FIR) spectra. The TR eigenstates are calculated variationally for both the major P and minor H crystal orientations. For the H orientation, the calculated splittings of all of the TR levels of these species are less than 0.1 cm-1. For the dominant P orientation, the splittings vary strongly depending on the character of the excitations involved. In all of the species, the splittings of the higher rotationally excited levels are comparable in magnitude to those for the j = 1 levels. For the levels corresponding to purely translational excitations, the calculated splittings are about an order of magnitude smaller than those of the purely rotational eigenstates. Based on the computed TR eigenstates, the low-temperature NIR (for M = H2) and FIR (for M = HF and H2O) spectra are simulated for both the P and H orientations, and also combined as their weighted sum (0.15H + 0.85P). The weighted sum spectra computed for M = H2 and HF match quantitatively the corresponding measured spectra, while for M = H2O, the weighted sum FIR spectrum predicts features that can potentially be observed experimentally.
在过去几年中，在固体轻分子内嵌富勒烯M@C60（M = H2、H2O、HF）的红外（IR）和非弹性中子散射光谱中观察到了归因于对称性破缺的平移 - 旋转（TR）本征态的分裂。在最近的一篇论文[费尔克等人，《物理化学化学物理》，2017年，19卷，31274页]中，我们确定客体分子M与固体中十二个最近邻C60笼之间的静电四极相互作用是对称性破缺的主要来源。使用该模型计算的内嵌邻 - H2、邻 - H2O的三重简并基态以及HF的j = 1能级的分裂与实验结果非常吻合。利用相同的静电模型，本理论研究考察了对称性破缺对这三种物质的激发TR本征态的影响，以及它们如何在模拟的低温（5 - 6 K）近红外（NIR）和远红外（FIR）光谱中表现出来。对于主要的P晶体取向和次要的H晶体取向，都通过变分法计算了TR本征态。对于H取向，这些物质所有TR能级的计算分裂小于0.1 cm-1。对于占主导的P取向，分裂强烈依赖于所涉及激发的特性。在所有这些物质中，较高转动激发能级的分裂大小与j = 1能级的相当。对于对应于纯平移激发的能级，计算的分裂比纯转动本征态的分裂小约一个数量级。基于计算得到的TR本征态，针对P和H取向模拟了低温NIR（对于M = H2）和FIR（对于M = HF和H2O）光谱，并将它们作为加权和（0.15H + 0.85P）进行组合。对于M = H2和HF计算得到的加权和光谱与相应的测量光谱在数量上匹配，而对于M = H2O，加权和FIR光谱预测了可能在实验中观察到的特征。
