Physikalische Chemie III, Technische Universität Dortmund, Otto-Hahn-Straße 6, 44227 Dortmund, Germany.
J Chem Phys. 2012 Apr 7;136(13):134320. doi: 10.1063/1.3699071.
We study the influence of thermal excitation on the electric susceptibilities for Sn(6) and Sn(7) clusters by molecular beam electric deflection and Monte-Carlo simulations in conjunction with quantum-chemical calculations. At low temperatures (40 K), no field-induced broadening of the Sn(6) and Sn(7) cluster beams are observed, in agreement with vanishing permanent electric dipole moments due to their centro-symmetrical ground states. The electric polarizabilities of Sn(6) and Sn(7), as inferred from the field-induced molecular beam deflection, are in good agreement with the quantum-chemical predictions. At elevated temperatures of 50-100 K, increased polarizabilities of about 2-3 Å(3) are obtained. Also, we found indications of a field-induced beam broadening which points to the existence of permanent dipole moments of about 0.01-0.02 D per atom at higher temperatures. These results cannot be explained by thermal excitations within a harmonic oscillator model, which would yield a temperature-independent polarizability and fluxional, but not permanent, dipole moments. We analyze this behavior by Monte-Carlo simulations in order to compute average temperature-induced electric dipole moments. For that purpose, we developed a novel technique for predicting observables sampled on the quantum-chemical potential energy surface by an umbrella sampling correction of Monte-Carlo results obtained from simulations utilizing an empirical potential. The calculated, fluxional dipole moments are in tune with the observed beam broadenings. The cluster dynamics underlying the polarizability appear to be intermediate between rigid and floppy molecules which leads to the conclusion that the rotational, not the vibrational temperature seems to be the key parameter that determines the temperature dependence of the polarizability.
我们通过分子束电偏转和蒙特卡罗模拟结合量子化学计算研究了热激发对 Sn(6)和 Sn(7)团簇的电感应系数的影响。在低温（40 K）下，没有观察到 Sn(6)和 Sn(7)团簇束的场致展宽，这与由于它们的中心对称基态而导致的消失的永久电偶极矩一致。从分子束偏转引起的场中推断出的 Sn(6)和 Sn(7)的极化率与量子化学预测非常吻合。在 50-100 K 的较高温度下，获得了约 2-3 Å(3)的增加的极化率。此外，我们发现了场致束展宽的迹象，这表明在较高温度下每个原子存在约 0.01-0.02 D 的永久偶极矩。这些结果不能用简谐振荡器模型中的热激发来解释，因为简谐振荡器模型会产生与温度无关的极化率和动态的，但不是永久的，偶极矩。我们通过蒙特卡罗模拟分析了这种行为，以便计算平均温度诱导的电偶极矩。为此，我们开发了一种新的技术，用于通过对蒙特卡罗结果进行伞状采样校正来预测在量子化学势能面上采样的可观测值，该蒙特卡罗结果是从利用经验势进行模拟中获得的。计算出的、动态的偶极矩与观察到的束展宽相协调。极化率所基于的团簇动力学介于刚性和柔软分子之间，这得出结论，决定极化率的温度依赖性的关键参数似乎是旋转温度，而不是振动温度。
