Vibrational probes such as carbonyls and nitriles have been extensively used by the ultrafast 2D IR community to access molecular-level dynamics in a variety of environments. Hydrogen-bond populations and lifetimes can be straightforwardly extracted from 2D spectra.  DMSO/water binary mixtures are an interesting example of heterogeneous solutions which have been shown to exhibit microscopic phase separation leading to anomalous bulk properties, such as a freezing point of -70 °C. DMSO behaves like an amphiphilic molecule and above 30 mol% DMSO, dimers or more extended DMSO structures become thermodynamically favored, however, despite the wealth of simulations, this nanometer-scale heterogeneity has been difficult to measure experimentally. In this talk, I will discuss our recent experiments using small amides as vibrational probes to characterize the dynamics of DMSO/water mixtures. We perform 2D IR spectroscopy on the C=O stretch and show that in the same mixtures formamide, a small polar molecule, exhibits different dynamics compared to dimethyl-formamide, a less polar molecule. Timescales extracted from MD simulations are in semi-quantitative agreement with experiments, and analysis of the simulations shows that formamide is preferentially solvated by the aqueous phase, whereas dimethyl-formamide tends to associate with DMSO molecules. Hydrogen-bond networks become severely disrupted due to the imbalance of hydrogen-bond donors and acceptors in the mixture. Finally, we discuss how a combination of multiple probes can be used to extract molecular information in complex mixtures, such as the crowded interior of a cell.<br/>

Probe-dependent vibrational dynamics in heterogeneous mixtures

2019 Spring National Meeting

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