Identification of Ion-Pair Structures in Solution by Vibrational Stark Effects

Abstract

Ion pairing is a fundamental consideration in many areas of chemistry and has implications in a wide range of sciences and technologies that include batteries and organic photovoltaics. Ions in solution are known to inhabit multiple possible states, including free ions (FI), contact ion pairs (CIP), and solvent-separated ion pairs (SSIP). However, in solutions of organic radicals and nonmetal electrolytes, it is often difficult to distinguish between these states. In the first part of this work, we report evidence for the formation of SSIPs in low-polarity solvents and distinct measurements of CIP, SSIP, and FI, by using the ν(C≡N) infrared (IR) band of a nitrile-substituted fluorene radical anion. Use of time-resolved IR detection following pulse radiolysis allowed us to unambiguously assign the peak of the FI. In the presence of nonmetal electrolytes, two distinct red-shifted peaks were observed and assigned to the CIP and SSIP. The assignments are interpreted in the framework of the vibrational Stark effect (VSE) and are supported by (1) the solvent dependence of ion-pair populations, (2) the observation of a cryptand-separated sodium ion pair that mimics the formation of SSIPs, and (3) electronic structure calculations. In the second part of this work, we show that a blue-shift of the ν(C≡N) IR band due to the VSE can be induced in a nitrile-substituted fluorene radical anion by covalently tethering it to a metal-chelating ligand that forms an intramolecular ion pair upon reduction and complexation with sodium ion. This adds support to the conclusion that the shift in IR absorptions by ion pairing originates from the VSE. These results combined show that we can identify ion-pair structures by using the VSE, including the existence of SSIPs in a low-polarity solvent.

Probing Intermolecular Electron Delocalization in Dimer Radical Anions by Vibrational Spectroscopy

“…It is worthwhile noting that the nitrile vibration of the anion is also sensitive to the presence of counter-ions and therefore the IR shift can be used to quantify ion pairing as well. 26,49 The IR absorption linewidth of ν(C≡N) is larger in DMF than we previously measured in THF. 45 The differences are small for the neutrals, but in some anionic cases, they are close to two times larger in DMF (e.g.…”

Detection of electron tunneling across plasmonic nanoparticle–film junctions using nitrile vibrations

“…Assuming Stark tuning rate (Δ μ CN ) to be 0.61 cm −1 /(MV/cm) for aromatic nitrile group 63 and a linear Stark shift relation Δ ν = −Δ μ CN · Δ E , we can calculate the strength of local electric fields: −41.0 MV/cm and +41.0 MV/cm from the blue (Δν=+25cm −1 ) and red (Δν=-25 cm −1 ) shifted nitrile frequencies, respectively. The observed shifts are larger than those induced by solvent effects (<15 cm −1 red shift) 34 and ion-pairing environments (<16 cm −1 red shift) 37 , indicating larger local fields within these NPoM gaps. Compared with the ensemble measurement, the nitrile peaks in single isolated NPoMs are more asymmetric (Fig.…”

“…The VSE occurs when the local electric field perturbs the electronic environment of a chemical bond and results in a change in its vibrational energy, which can be directly measured with infrared or Raman spectroscopy. 33 Due to these advantages, the VSE has been used to quantify local electric fields in a diverse range of chemical environments including organic solvents 34, 35 , ionic liquids 36, 37 , catalysts 31, 38 , proteins 39, 40 and biomembranes 41 .…”

Unexpected Salt/Cocrystal Polymorphism of the Ketoprofen–Lysine System: Discovery of a New Ketoprofen–l-Lysine Salt Polymorph with Different Physicochemical and Pharmacokinetic Properties

“…Secondly, analysis at t30 and t60 shows that KET-LYS P1 underwent smaller variations in the first 60 min after preparation of the solution compared to KET-LYS P2, as, over time, KET-LYS P1 remained in the same quadrant, while KET-LYS P2 made a significant change in its position on the gustatory map in the first 30 min, moving from the upper part of the left quadrant to the lower right quadrant (Figure 6). This phenomenon could be due to the formation of intimate ion pairs in solution that maintain over time (from t0 to t60) the memory of the different stereochemistry of the original crystalline structures, and this results in a different solvation and shielding of the ion and the counterion of the KET-LYS P1 and KET-LYS P2 systems [40].…”