Structural Dynamics of Dimethyl Sulfoxide Aqueous Solutions Investigated by Ultrafast Infrared Spectroscopy: Using Thiocyanate Anion as a Local Vibrational Probe.

The microscopic structure of dimethyl sulfoxide (DMSO) aqueous solutions was investigated by Fourier transform infrared (FTIR) spectroscopy and ultrafast IR spectroscopy. The structural dynamics of the binary mixtures were reflected by using thiocyanate anion (SCN) as a local vibrational probe. FTIR spectra of SCN anion showed that the hydrogen bond networks of water are affected by the presence of DMSO molecules, and the peak position and bandwidth of SCN anions are red shifted and narrowed accordingly because of the weak hydration in the binary mixture. The vibrational lifetime of the SCN anion showed almost linear enhancement with the increase of DMSO, which can be explained by the weak interaction between SCN and the hydrophobic groups in the DMSO molecule. However, the rotational dynamics of SCN are slowing down significantly and showed a maximum response at X (mole fraction) of 0.35, which is mainly caused by the confinement of SCN anions positioned in the vicinity of the complex structure formed between DMSO and water molecules. The concentration-dependent rotational dynamics of water molecules and SCN anions are having similar behavior, indicating that the complex structure can be formed between water and DMSO molecules because of the strong interaction. The result also demonstrates that the structural inhomogeneity in aqueous solution can be unraveled by monitoring the vibrational relaxation dynamics of SCN anion serving as the local vibrational probe.