Comparison of Diffusivity Data Derived from Electrochemical and NMR Investigations of the SeCN¯/(SeCN)₂/(SeCN)₃¯ System in Ionic Liquids

Abstract

Electrochemical studies in room temperature ionic liquids are often hampered by their relatively high viscosity. However, in some circumstances, fast exchange between participating electroactive species has provided beneficial enhancement of charge transport. The iodide (I¯)/iodine (I(2))/triiodide (I(3)¯) redox system that introduces exchange via the I¯ + I(2) ⇌ I(3)¯ process is a well documented example because it is used as a redox mediator in dye-sensitized solar cells. To provide enhanced understanding of ion movement in RTIL media, a combined electrochemical and NMR study of diffusion in the {SeCN¯-(SeCN)(2)-(SeCN)(3)¯} system has been undertaken in a selection of commonly used RTILs. In this system, each of the Se, C and N nuclei is NMR active. The electrochemical behavior of the pure ionic liquid, [C(4)mim][SeCN], which is synthesized and characterized here for the first time, also has been investigated. Voltammetric studies, which yield readily interpreted diffusion-limited responses under steady-state conditions by means of a Random Assembly of Microdisks (RAM) microelectrode array, have been used to measure electrochemically based diffusion coefficients, while self-diffusion coefficients were measured by pulsed field gradient NMR methods. The diffusivity data, derived from concentration and field gradients respectively, are in good agreement. The NMR data reveal that exchange processes occur between selenocyanate species, but the voltammetric data show the rates of exchange are too slow to enhance charge transfer. Thus, a comparison of the iodide and selenocyanate systems is somewhat paradoxical in that while the latter give RTILs of low viscosity, sluggish exchange kinetics prevent any significant enhancement of charge transfer through direct electron exchange. In contrast, faster exchange between iodide and its oxidation products leads to substantial electron exchange but this effect does not compensate sufficiently for mass transport limitations imposed by the higher viscosity of iodide RTILs.

Structural Dynamics in Ionic Liquid Thin Films: The Effect of Cation Chain Length

“…ILs BmimNTf 2 , (99%), HmimNTf 2 (99%), and DmimNTf 2 (99%) were purchased from IoLiTec. 1-Butyl/hexyl/decyl-3-methylimidazolium selenocyanate (Bmim/Hmim/Dmim SeCN) was synthesized according to a previously described method, 73 and the contents of residual potassium ions (8.9 ppm in Bmim SeCN, 21 ppm in Hmim SeCN, and 15 ppm in Dmim SeCN) in the ILs were measured using ICP−MS. Ethanol (200-Proof, 99.5%), toluene (99.9%), and methanol (extra dry, 99.8%) were from Fisher Chemicals.…”

A Combined SNIFTIRS and XANES Study of Electrically Polarized Copper Electrodes in DMSO and DMF Solutions of Cyanate (NCO−), Thiocyanate (NCS−) and Selenocyanate (NCSe−) Ions

“…The observed ν(CN) stretching frequencies for both Se 2 (CN) 2 and Se(SeCN) 2 were reported as being at 2144 cm −1 . We therefore speculate from this previous literature evidence that the peak could be due to an (NCSe) 3 − species, because this species is thermodynamically favored when in equilibrium with its progenitor species (SeCN) 2 and NCSe − , according to Solangi et al 51 In the presence of K + ion (from the use of KSeCN), the (SeCN) 3 − ion may precipitate and become "trapped" in this form as a K(SeCN) 3 film on the electrode. This compound is known from earlier studies 9 involving the oxidation of selenocyanate on platinum electrodes in aqueous electrolytes containing 0.5 mol L −1 KSeCN and 0.1 mol L −1 NH 4 O 2 CMe at pH 7 and was attributed to an intense band at 2143 cm −1 that was observed when applied potentials >0.8 V (NHE) were used.…”

Regioselective C–H Phosphorothiolation of (Hetero)arenes Enabled by the Synergy of Electrooxidation and Ultrasonic Irradiation

“…In addition, the CV of NH 4 SCN shows two oxidation processes between 0.7 and 1.7 V and a reduction process at 0.25 V, which correspond to the pseudohalide system of {SCN−(SCN) 3 −(SCN) 2 } (Figure S4). 18 On the basis of the control experiments and CV studies, a plausible mechanism was proposed, as shown in Scheme 5.…”