Yagasaki Takuma, Saito Shinji
Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Myodaiji, Okazaki, Aichi, Japan.
J Chem Phys. 2008 Apr 21;128(15):154521. doi: 10.1063/1.2903470.
Physical and chemical properties of liquid water are dominated by hydrogen bond structure and dynamics. Recent studies on nonlinear vibrational spectroscopy of intramolecular motion provided new insight into ultrafast hydrogen bond dynamics. However, our understanding of intermolecular dynamics of water is still limited. We theoretically investigated the intermolecular dynamics of liquid water in terms of two-dimensional infrared (2D IR) spectroscopy. The 2D IR spectrum of intermolecular frequency region (<1000 cm(-1)) is calculated by using the equilibrium and nonequilibrium hybrid molecular dynamics method. We find the ultrafast loss of the correlation of the libration motion with the time scale of approximately 110 fs. It is also found that the energy relaxation from the libration motion to the low frequency motion takes place with the time scale of about 180 fs. We analyze the effect of the hindered translation motion on these ultrafast dynamics. It is shown that both the frequency modulation of libration motion and the energy relaxation from the libration to the low frequency motion significantly slow down in the absence of the hindered translation motion. The present result reveals that the anharmonic coupling between the hindered translation and libration motions is essential for the ultrafast relaxation dynamics in liquid water.
液态水的物理和化学性质由氢键结构和动力学主导。最近关于分子内运动的非线性振动光谱的研究为超快氢键动力学提供了新的见解。然而，我们对水的分子间动力学的理解仍然有限。我们从二维红外（2D IR）光谱的角度对液态水的分子间动力学进行了理论研究。通过使用平衡和非平衡混合分子动力学方法计算了分子间频率区域（<1000 cm(-1)）的二维红外光谱。我们发现摆动运动的相关性在约110 fs的时间尺度上超快丧失。还发现从摆动运动到低频运动的能量弛豫发生在约180 fs的时间尺度上。我们分析了受阻平移运动对这些超快动力学的影响。结果表明，在没有受阻平移运动的情况下，摆动运动的频率调制以及从摆动到低频运动的能量弛豫都显著减慢。目前的结果表明，受阻平移和摆动运动之间的非谐耦合对于液态水中的超快弛豫动力学至关重要。
