Femtosecond Mid-Infrared Study of the Dynamics of Water Molecules in Water-Acetone and Water-Dimethyl Sulfoxide Mixtures
Femtosecond Mid-Infrared Study of the Dynamics of Water Molecules in Water-Acetone and Water-Dimethyl Sulfoxide Mixtures
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DOI：
10.1021/jp512703w
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发表时间：
2015-04-23
影响因子：
3.3
通讯作者：
Bakker, H. J.
中科院分区：
文献类型：
--
作者：
Lotze, S.;Groot, C. C. M.;Bakker, H. J.
用偏振分辨飞秒中红外光谱研究了水-二甲基亚砜(DMSO)和水-丙酮二元混合物中HDO分子的振动弛豫动力学和重取向动力学。对于低溶质浓度，我们观察到水合DMSO和丙酮的疏水甲基的部分水分子的重定向减慢。对于水-DMSO混合物，慢速水分子的比例随着溶质浓度的增加比水-丙酮混合物陡峭得多，这表明丙酮分子已经在低浓度下表现出明显的聚集。在高溶质浓度下，水-二甲基亚砜和水-丙酮混合物的振动和重取向动力学表明，水分子向其他水分子提供氢键的动力学与水向溶质的S=O/C=O基团提供氢键的动力学明显不同。对于水-DMSO混合物，两种类型的水分子都表现出非常缓慢的重定向。与S=O基团形成氢键的水分子发生了重新定向，其时间常数从X-DMSO=0.33时的46+/-14ps下降到X-DMSO=0.95时的13+/-2ps。与丙酮的C=O基团形成氢键的水分子表现出更快的重定向，其时间常数从X-ACET=0.3时的6.1+/-0.2ps下降到X-ACET=0.9时的2.96+/-0.05ps。DMSO和丙酮在溶质结合水中的重排向时间常数相差很大，这是因为水与DMSO的S=O基团之间的氢键远强于水与丙酮的C=O基团之间的氢键。我们将水在富含DMSO和富丙酮混合物中的强烈不同行为归因于它们分子形状的不同。
We study the vibrational relaxation dynamics and the reorientation dynamics of HDO molecules in binary water-dimethyl sulfoxide (DMSO) and water-acetone mixtures with polarization-resolved femtosecond mid-infrared spectroscopy. For low solute concentrations we observe a slowing down of the reorientation of part of the water molecules that hydrate the hydrophobic methyl groups of DMSO and acetone. For water-DMSO mixtures the fraction of slowed-down water molecules rises much steeper with solute concentration than for water-acetone mixtures, showing that acetone molecules show significant aggregation already at low concentrations. At high solute concentrations, the vibrational and reorientation dynamics of both water-DMSO and water-acetone mixtures show a clear distinction between the dynamics of water molecules donating hydrogen bonds to other water molecules and the dynamics of water donating a hydrogen bond to the S=O/C=O group of the solute. For water-DMSO mixtures both types of water molecules show a very slow reorientation. The water molecules forming hydrogen bonds to the S=O group reorient with a time constant that decreases from 46 +/- 14 ps at X-DMSO = 0.33 to 13 +/- 2 ps at X-DMSO = 0.95. The water molecules forming hydrogen bonds to the C=O group of acetone show a much faster reorientation with a time constant that decreases from 6.1 +/- 0.2 ps at X-acet = 0.3 to 2.96 +/- 0.05 ps at X-acet = 0.9. The large difference in reorientation time constant of the solute-bound water for DMSO and acetone can be explained from the fact that the hydrogen bond between water and the S=O group of DMSO is much stronger than the hydrogen bond between water and the C=O group of acetone. We attribute the strongly different behavior of water in DMSO-rich and acetone-rich mixtures to their difference in molecular shape.