Influence of solute-solvent coordination on the orientational relaxation of ion assemblies in polar solvents

Abstract

We have investigated the rotational dynamics of lithium thiocyanate (LiNCS) dissolved in various polar solvents with time and polarization resolved vibrational spectroscopy. LiNCS forms multiple distinct ionic structures in solution that can be distinguished with the CN stretch vibrational frequency of the different ionic assemblies. By varying the solvent and the LiNCS concentration, the number and type of ionic structures present in solution can be controlled. Control of the ionic structure provides control over the volume, shape, and dipole moment of the solute, critical parameters for hydrodynamic and dielectric continuum models of friction. The use of solutes with sizes comparable to or smaller than the solvent molecules also helps amplify the sensitivity of the measurement to the short-ranged solute-solvent interaction. The measured orientational relaxation dynamics show many clear and distinct deviations from simple hydrodynamic behavior. All ionic structures in all solvents exhibit multi-exponential relaxation dynamics that do not scale with the solute volume. For Lewis base solvents such as benzonitrile, dimethyl carbonate, and ethyl acetate, the observed dynamics strongly show the effect of solute-solvent complex formation. For the weak Lewis base solvent nitromethane, we see no evidence for solute-solvent complex formation, but still see strong deviation from the predictions of simple hydrodynamic theory.

An ultrafast vibrational study of dynamical heterogeneity in the protic ionic liquid ethyl-ammonium nitrate. I. Room temperature dynamics

“…23 Early MD simulations note few linear hydrogen bonds with water 24 and many arrangements of dipolar character, while more recent MD simulations of SeCN −25 find an average of 3-4 hydrogen bonds, with one of them being an axial (linear) hydrogen bond. Ultrafast infrared spectroscopy can use the ν 3 mode of pseudo-halide anions to investigate ultrafast vibrational dynamics of water and other polar solvents, 22,[26][27][28][29][30][31] concentrated ion solutions, [32][33][34] ILs, [35][36][37][38][39] supported IL membranes, 40 deep eutectic solvents, 41 and colloid emulsions. 42 Furthermore, a combination of MD simulations and ultrafast infrared spectroscopy has developed a spectroscopic map of SeCN − in D 2 O that describes the frequency dependence of the nitrile stretch on the hydrogen bonding environment.…”

Contact Ion Pair Formation between Hard Acids and Soft Bases in Aqueous Solutions Observed with 2DIR Spectroscopy

“…Time-resolved IR spectroscopy of molecular anions provides an alternative approach to characterizing CIP equilibria in aqueous solution for molecular ions. The IR spectra of a variety of anions shift measurably when they form a CIP with alkali and alkaline earth metal cations in polar solvents. − Of these molecular anions, thiocyanate (SCN¯), isocyanate (OCN¯), and azide (N 3 ¯) provide the most tractable IR spectroscopy, particularly for time-resolved measurements. ,,,,− We have used the NCS – anion to study contact ion pairing in water because of the long excited-state lifetime of the CN-stretch vibration. , The sensitivity of the nitrile stretch frequency to variations in the nitrile chemical and electrostatic environment has been used in a variety of prior experiments. , Under certain circumstances, the CN-stretch frequency provides a probe of the local electrostatic environment. , Stronger intermolecular interactions, such as CIP formation or hydrogen-bond formation to the nitrile group, lead to shifts in the CN-stretch frequency that cannot be accounted for with the Stark effect. , For the specific case of CIP formation between Mg 2+ or Ca 2+ and NCS – , bonding of the M 2+ cation to the nitrogen atom in the NCS – anion induces an electronic polarization that shifts electron density from the CS bond to the CN bond, leading to an increase in the CN-stretch frequency . This electronic polarization induced by CIP stabilizes the CN triple-bond resonance structure of thiocyanate, an alternative means of explaining the increase in the CN-stretch frequency.…”

Role of Water in the Lyotropic Liquid Crystalline Mesophase of Lithium Salts and Non-ionic Surfactants

“…The inset of Figure b shows the ν-CN stretching region of the same spectra. Notice that the ν-CN region is observed around 2060 cm –1 , but at such a high salt concentration, the SCN – ions cannot remain as free ions; it instead forms ion pairs (Li-SCN) and dimers [Li 2 (SCN) 2 ], which appear at 2081 and 2045 cm –1 , respectively, in the spectrum. , Clearly, different ionic species (free ion, ion pair, and dimer) coexist in the mesophase even in the solution phase. With the evaporation of water, specifically in the third region, the intensity of the ion-pair band (at ∼2081 cm –1 ) gradually increases at the expense of that of the free ion (∼2060 cm –1 ) (see the inset of Figure b).…”