Local Electric Fields in Aqueous Electrolytes
Drexler, CI (Drexler, Chad, I); Cracchiolo, OM (Cracchiolo, Olivia M.); Myers, RL (Myers, Ryan L.); Okur, HI (Okur, Halil, I); Serrano, AL (Serrano, Arnaldo L.); Corcelli, SA (Corcelli, Steven A.); Cremer, PS (Cremer, Paul S.)
Journal of Physical Chemistry B, 2021, Volume 125, pp. 8484-8493.
Vibrational Stark shifts were explored in aqueous solutions of organic molecules with carbonyl- and nitrile-containing constituents. In many cases, the vibrational resonances from these moieties shifted toward lower frequency as salt was introduced into solution. This is in contrast to the blue-shift that would be expected based upon Onsager's reaction field theory. Salts containing well-hydrated cations like Mg2+ or Li+ led to the most pronounced Stark shift for the carbonyl group, while poorly hydrated cations like Cs+ had the greatest impact on nitriles. Moreover, salts containing I- gave rise to larger Stark shifts than those containing Cl-. Molecular dynamics simulations indicated that cations and anions both accumulate around the probe in an ion- and probe-dependent manner. An electric field was generated by the ion pair, which pointed from the cation to the anion through the vibrational chromophore. This resulted from solvent-shared binding of the ions to the probes, consistent with their positions in the Hofmeister series. The "anti-Onsager" Stark shifts occur in both vibrational spectroscopy and fluorescence measurements. |
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