Bakker H J, Rezus Y L A, Timmer R L A
FOM Institute AMOLF, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands.
J Phys Chem A. 2008 Nov 20;112(46):11523-34. doi: 10.1021/jp8012943. Epub 2008 Oct 28.
The molecular reorientation of liquid water is key to the hydration and stabilization of molecules and ions in aqueous solution. A powerful technique to study this reorientation is to measure the time-dependent anisotropy of the excitation of the O-H/O-D stretch vibration of HDO dissolved in D2O/H2O using femtosecond midinfrared laser pulses. In this paper, we present and discuss experiments in which this technique is used to study the correlation between the molecular reorientation of the water molecules and the strength of the hydrogen-bond interactions. On short time scales (<200 fs), it was found that the anisotropy shows a partial decay due to librational motions of the water molecules that keep the hydrogen bond intact. On longer time scale (>200 fs), the anisotropy shows a complete decay with an average time constant of 2.5 ps. From the frequency dependence of the anisotropy dynamics, it follows that a subensemble of the water molecules shows a fast reorientation that is accompanied by a large change of the vibrational frequency. This finding agrees with the molecular jumping mechanism for the reorientation of liquid water that has recently been proposed by Laage and Hynes.
液态水的分子重排是水溶液中分子和离子水合作用及稳定化的关键。研究这种重排的一种强大技术是使用飞秒中红外激光脉冲测量溶解在D2O/H2O中的HDO的O-H/O-D伸缩振动激发的时间相关各向异性。在本文中，我们展示并讨论了使用该技术研究水分子的分子重排与氢键相互作用强度之间相关性的实验。在短时间尺度（<200飞秒）下，发现各向异性由于水分子的摆动运动而出现部分衰减，这些摆动运动保持氢键完整。在较长时间尺度（>200飞秒）下，各向异性以2.5皮秒的平均时间常数呈现完全衰减。从各向异性动力学的频率依赖性可知，一部分水分子表现出快速重排，同时伴随着振动频率的大幅变化。这一发现与Laage和Hynes最近提出的液态水重排的分子跳跃机制相符。
