Department of Chemistry, Brown University, Providence, Rhode Island 02912, USA.
J Chem Phys. 2012 Jul 14;137(2):024506. doi: 10.1063/1.4733392.
Given the limited intermolecular spaces available in dense liquids, the large amplitudes of highly excited, low frequency vibrational modes pose an interesting dilemma for large molecules in solution. We carry out molecular dynamics calculations of the lowest frequency ("warping") mode of perylene dissolved in liquid argon, and demonstrate that vibrational excitation of this mode should cause identifiable changes in local solvation shell structure. But while the same kinds of solvent structural rearrangements can cause the non-equilibrium relaxation dynamics of highly excited diatomic rotors in liquids to differ substantially from equilibrium dynamics, our simulations also indicate that the non-equilibrium vibrational energy relaxation of large-amplitude vibrational overtones in liquids should show no such deviations from linear response. This observation seems to be a generic feature of large-moment-arm vibrational degrees of freedom and is therefore probably not specific to our choice of model system: The lowest frequency (largest amplitude) cases probably dissipate energy too quickly and the higher frequency (more slowly relaxing) cases most likely have solvent displacements too small to generate significant nonlinearities in simple nonpolar solvents. Vibrational kinetic energy relaxation, in particular, seems to be especially and surprisingly linear.
由于在密集液体中分子间的空间有限，高激发、低频率振动模式的大振幅给溶液中的大分子带来了有趣的困境。我们对溶解在液态氩中的苝的最低频率（“翘曲”）模式进行了分子动力学计算，并证明这种模式的振动激发应该会导致局部溶剂化壳结构的可识别变化。但是，虽然相同类型的溶剂结构重排会导致液体中高激发双原子转子的非平衡弛豫动力学与平衡动力学有很大的不同，但我们的模拟也表明，大振幅泛音的非平衡振动能量弛豫在液体中不应表现出与线性响应的这种偏差。这一观察结果似乎是大转动臂振动自由度的一般特征，因此可能不是我们选择的模型系统所特有的：最低频率（最大振幅）的情况可能会过快地耗散能量，而更高频率（更慢弛豫）的情况则很可能会产生溶剂位移太小，无法在简单的非极性溶剂中产生显著的非线性。振动动能弛豫，特别是，似乎特别且令人惊讶地是线性的。
