Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India.
Homi Bhabha National Institute, Training School Complex, Anushaktinagar, Mumbai 400 094, India.
J Phys Chem B. 2023 Apr 27;127(16):3701-3710. doi: 10.1021/acs.jpcb.3c01193. Epub 2023 Apr 14.
DMSO-water mixtures provide an intriguing hydrogen-bonding environment which has been a subject of various theoretical and experimental investigations. The structural dynamics of aqueous DMSO solutions has been investigated, using nitrosyl stretch of sodium nitroprusside (SNP, Na[Fe(CN)NO]) as a local vibrational probe, with the help of infrared (IR) absorption spectroscopy, vibrational pump-probe spectroscopy, and two-dimensional IR spectroscopy (2D-IR). Fourier transform infrared spectra of the nitrosyl stretch of SNP reveals that both the peak position and spectral broadening are very sensitive to the composition of the DMSO-water mixture and the subsequent structural changes occurring due to the addition of DMSO to water. The vibrational lifetime of the nitrosyl stretch displays two different linear variation regimes as a function of mole fraction of DMSO which has been assigned presumably to two different predominant structures at these compositions. However, the rotational depolarization measurements show that the reorientational times follow a bell-shaped profile, imitating the changes in the composition-dependent physical properties (viscosity) of DMSO-water solvent mixtures. To get a holistic picture of the system, 2D-IR spectroscopy of the NO stretch of SNP has been employed to study time scales of hydrogen-bond reorganization dynamics existing at different compositions. The analysis of frequency-frequency correlation function (FFCF) decay times reveal that the dynamics gets slower in intermediate DMSO concentrations than that of pure DMSO or pure water. A careful analysis reveals two anomalous regions of hydrogen-bond dynamics: ∼0.2 and 0.4, which indicates that different hydrogen-bonded structures exist in these regions that can be effectively probed by SNP which has remained mostly elusive to previous vibrational probe-based investigations.
DMSO-水混合物提供了一种有趣的氢键环境，这一直是各种理论和实验研究的主题。本研究使用亚硝酰基伸展的硝普酸钠（SNP，Na[Fe(CN)NO]）作为局部振动探针，借助红外（IR）吸收光谱、振动泵浦探测光谱和二维红外光谱（2D-IR），研究了含水 DMSO 溶液的结构动力学。SNP 的亚硝酰基伸展的傅里叶变换红外光谱表明，峰位置和光谱展宽都对 DMSO-水混合物的组成非常敏感，并且由于 DMSO 加入到水中，会发生随后的结构变化。亚硝酰基伸展的振动寿命显示出两个不同的线性变化区域，作为 DMSO 摩尔分数的函数，这可能归因于这些组成下两种不同的主要结构。然而，旋转去偏振测量表明，重新取向时间遵循钟形轮廓，模仿了 DMSO-水溶剂混合物的组成依赖性物理性质（粘度）的变化。为了全面了解该系统，我们采用 SNP 的 NO 伸展的 2D-IR 光谱来研究不同组成下氢键重组动力学的时间尺度。对频率-频率相关函数（FFCF）衰减时间的分析表明，在中间 DMSO 浓度下，动力学比纯 DMSO 或纯水慢。仔细分析揭示了氢键动力学的两个异常区域：∼0.2 和 0.4，这表明在这些区域存在不同的氢键结构，这可以通过 SNP 有效地探测到，而 SNP 在以前基于振动探针的研究中一直难以探测到。
