Rotational Diffusion of Coumarins in Electrolyte Solutions:  The Role of Ion Pairs

Abstract

In an attempt to explore how electrolyte ions influence the friction experienced by solutes with different functional groups, rotational diffusion of two structurally similar coumarins, coumarin 343 (C343) and coumarin 334 (C334), has been studied in dimethyl sulfoxide (DMSO) at several concentrations of LiNO 3 . The two coumarins are almost identical except for the different functional groups in the 3-position; C343 has -COOH whereas C334 has -COCH 3 . Because of the presence of the -COOH functional group, C343 exists as both anionic and neutral species in DMSO. Although both anionic and neutral forms of C343 exist in the ground state, C343 predominantly exists as neutral species in the excited state. The measured reorientation time of C343 in DMSO is slower by a factor of 2 compared to that of C334 due to the hydrogen bonding between the -COOH group of the probe and sulfoxide group of the solvent. However, the viscosity normalized reorientation times (τ r /η) of C343 in LiNO 3 /DMSO solutions increase by 60% to 70% compared to that in pure DMSO. Addition of electrolyte ions shifts the equilibrium toward the anionic form of C343, which in turn forms ion pairs with Li + , and the large increase in τ r /η values is due to the association of DMSO solvent molecules with these ion pairs. On the other hand, τ r /η values of the neutral solute, C334, remain invariant in the entire range of the electrolyte concentration. The C343 -Li + ion pairs have been characterized using ab initio molecular orbital methods. Two approaches have been employed to model the friction experienced by the C343 -Li + ion pairs. In the first approach, the complex formed between C343 -Li + and the solvent molecules is treated as a rigid entity and the increase in the viscosity normalized reorientation time has been accounted for solely as an enhancement in the mechanical friction. Alternatively, the solvent association with the C343 -Li + ion pairs has been modeled as dielectric friction using the extended charge distribution theory.

Molecular Rotation as a Tool for Exploring Specific Solute–Solvent Interactions

“…[10] The total interaction strengths in the cases of DMDPP-EtOH and DMDPP-TFE are 64.3 and 76.7 kJ mol…”

“…[10] These two solvents were selected because they are almost identical in size but possess different hydrogen-bond-donating abilities. TFE, due to the presence of three fluorine atoms on the second carbon atom, is a better hydrogen-bond donor than EtOH.…”

“…Following their analogy, we have used the extended charge-distribution theory of Alavi and Waldeck [30,31] to model the hydrogen-bonding interactions experienced by DPP in dipolar solvents. [8,9] A dipolar solute rotating in a dipolar solvent polarizes the surrounding solvent medium, and the response of the solvent polarization to the solute's rotation is not instantaneous. Thus, the rotating solute molecule experiences an additional friction known as the dielectric friction.…”

“…The data presented are from ref. [10]. actual experimental situations, such as attachment of more than two solvent molecules per solute and variation of the relative strengths of the intermolecular interactions involving only solvent molecules, might also have considerable influence on the rotational reorientation times.…”

Rotational Dynamics of Nonpolar and Dipolar Molecules in Polar and Binary Solvent Mixtures

“…In other words, molecules having no net dipole moment can also experience dielectric friction. AW theory has been successful compared to NZ and ZH theories in modeling the friction in nonassociative solvents (Dutt and Ghanty, 2003). The expression for the dielectric friction according to this model is given by (Alavi and Waldek, 1991a) …”