Cooper Richard J, O'Brien Jeremy T, Chang Terrence M, Williams Evan R
Department of Chemistry , University of California , Berkeley , California 94720-1460 , USA . Email:
Chem Sci. 2017 Jul 1;8(7):5201-5213. doi: 10.1039/c7sc00481h. Epub 2017 May 19.
The effects of ion charge, polarity and size on the surface morphology of size-selected aqueous nanodrops containing a single ion and up to 550 water molecules are investigated with infrared photodissociation (IRPD) spectroscopy and theory. IRPD spectra of M(HO) where M = La, Ca, Na, Li, I, SO and supporting molecular dynamics simulations indicate that strong interactions between multiply charged ions and water molecules can disrupt optimal hydrogen bonding (H-bonding) at the nanodrop surface. The IRPD spectra also reveal that "free" OH stretching frequencies of surface-bound water molecules are highly sensitive to the ion's identity and the OH bond's local H-bond environment. The measured frequency shifts are qualitatively reproduced by a computationally inexpensive point-charge model that shows the frequency shifts are consistent with a Stark shift from the ion's electric field. For multiply charged cations, pronounced Stark shifting is observed for clusters containing ∼100 or fewer water molecules. This is attributed to ion-induced solvent patterning that extends to the nanodrop surface, and serves as a spectroscopic signature for a cation's ability to influence the H-bond network of water located remotely from the ion. The Stark shifts measured for the larger nanodrops are extrapolated to infinite dilution to obtain the free OH stretching frequency of a surface-bound water molecule at the bulk air-water interface (3696.5-3701.0 cm), well within the relatively wide range of values obtained from SFG measurements. These cluster measurements also indicate that surface curvature effects can influence the free OH stretching frequency, and that even nanodrops without an ion have a surface potential that depends on cluster size.
利用红外光解离（IRPD）光谱和理论，研究了离子电荷、极性和大小对含有单个离子及多达550个水分子的尺寸选择水性纳米液滴表面形态的影响。M(HO)（其中M = La、Ca、Na、Li、I、SO）的IRPD光谱以及支持的分子动力学模拟表明，多价离子与水分子之间的强相互作用会破坏纳米液滴表面的最佳氢键（H键）。IRPD光谱还表明，表面结合水分子的“自由”OH伸缩频率对离子的身份和OH键的局部H键环境高度敏感。通过计算成本较低的点电荷模型定性地再现了测量的频率偏移，该模型表明频率偏移与离子电场引起的斯塔克位移一致。对于多价阳离子，在含有约100个或更少水分子的团簇中观察到明显的斯塔克位移。这归因于离子诱导的溶剂图案化延伸到纳米液滴表面，并作为阳离子影响远离离子的水的H键网络能力的光谱特征。将较大纳米液滴测量的斯塔克位移外推到无限稀释，以获得在本体空气 - 水界面处表面结合水分子的自由OH伸缩频率（3696.5 - 3701.0 cm），该值在从和频振动光谱（SFG）测量获得的相对较宽的值范围内。这些团簇测量还表明，表面曲率效应会影响自由OH伸缩频率，并且即使没有离子的纳米液滴也具有取决于团簇大小的表面电位。
