Quantifying Hydrogen‐Bond Populations in Dimethyl Sulfoxide/Water Mixtures

Abstract

Dimethyl sulfoxide (DMSO) disrupts the hydrogen-bond networks in water. The widespread use of DMSO as a cosolvent, along with its unusual attributes, have inspired numerous studies. Herein, infrared absorption spectroscopy of the S=O stretching mode combined with molecular dynamics and quantum chemistry models were used to directly quantify DMSO/water hydrogen-bond populations in binary mixtures. Singly H-bonded species are dominant at 10 mol %, due to strong DMSO-water interactions. We found an unexpected increase in non-hydrogen-bonded DMSO near the eutectic point (ca. 35 mol %) which also correlates with several abnormalities in the bulk solution properties. We find evidence for three distinct regimes: 1) strong DMSO-water interactions (<30 mol %), 2) ideal-solution-like (30-90 mol %), and 3) self-interaction, or aggregation, regime (>90 mol %). We propose a "step in" mechanism, which involves hydrogen bonding between water and the DMSO aggregate species.

Proton Transfer Reaction Dynamics of Pyranine in DMSO/Water Mixtures

“…The overall population changes of hydrogen‐bonded species, noted in this MD simulation, clearly indicated that the strongest interaction between DMSO and water takes place at around X DMSO =0.4, which leads to efficient packing of the molecules at the eutectic point. These interactions disrupt the hydrogen‐bonding network in neat water . These unique solvent structures have been understood to be responsible for the slow solvation dynamics observed in these media.…”

“…These units of well‐defined DMSO/water structures form an extensive network structure, where 1 DMSO:1 H 2 O and 2 DMSO:1 H 2 O are connected through a number of bridged water molecules. In an insightful contribution, Oh et al. have further quantified the number of hydrogen‐bonded populations throughout the entire composition of the DMSO/water mixtures, where a significant increase in aggregated DMSO populations is observed at the expense of free DMSO and one hydrogen‐bonded DMSO population.…”

“…These units of well-defined DMSO/waters tructures form an extensive network structure, where 1DMSO:1 H 2 Oa nd 2DMSO:1 H 2 Oa re connected through an umber of bridged water molecules. In an insightful contribution, [32] Oh et al have further quantified the number of hydrogen-bonded populations throughout the entire composition of the DMSO/waterm ixtures,w here as ignificant increase in aggregated DMSO populations is observed at the expense of free DMSO and one hydrogen-bonded DMSO population. It has been postulated that water primarily makesh ydrogen bonds with the aggregates pecies rather than free species.T he overall population changes of hydrogenbonded species, noted in this MD simulation, clearly indicated that the strongest interaction between DMSO and water takes place at around X DMSO = 0.4, which leads to efficient packing of the molecules at the eutectic point.…”

“…Thus, DMSO/water binary mixtures at various compositions are fascinating hydrogen‐bonding systems and investigations of various aspects of these mixtures, including both structure and dynamics, are important to better understand their unique behavior. A large majority of previous investigations have implicated the crucial role of water structure in regard to the anomalous behavior of these aqueous DMSO systems . Thus, reactions that are highly sensitive towards the structure and dynamics of water molecules can serve as sensitive probes for monitoring the dynamical features of such systems.…”

“…A. Awasthi Jai Hind College Mumbai-400 020 (India) Thus, DMSO/water binary mixtures at variousc ompositions are fascinating hydrogen-bonding systems and investigations of variousa spectso ft hesem ixtures, includingb oths tructure and dynamics, are importantt ob etter understand their unique behavior.Alarge majority of previousi nvestigationsh ave implicated the crucial role of water structure in regard to the anomalous behavior of these aqueous DMSO systems. [26,31,32] Thus, reactions that are highly sensitivet owardst he structure and dynamics of water molecules can serve as sensitivep robes for monitoring the dynamical features of such systems. In this regard, excited-state intermolecular protont ransfer (ESIPT) reactions of organic photoacids are very suitable, as the dynamics of protont ransferr eactions are intimately associated with the suitable availability of protona cceptors, mobility of the transferred proton, and the ability of the solventt os tabilize the transient ion pair (solvation)d uring the process.T hus, any changes in the structure and dynamics of the water molecule are expected to significantly impact the ESPT reaction.…”