Effect of charge on the rotation of prolate nitroxide spin probes in room-temperature ionic liquids.

[Display omitted] • Doubly charged peroxylamine disulfonate (Frémy's salt) resides in the polar nanodomains of room-temperature ionic liquids. • Neutral di- tert -butyl nitroxide (DTBN) resides in the nonpolar nanodomains of room-temperature ionic liquids. • Rotational anisotropy of Frémy's salt increases with increasing cation alkyl chain length. • The activation energy of the rotation of Frémy's salt and di- tert -butyl nitroxide is the same as that of the viscous flow. • Long-range collective motion governs the activation energy of Frémy's salt and di- tert -butyl nitroxide rotation. We have studied the rotational diffusion of two prolate nitroxide probes, the doubly negatively charged peroxylamine disulfonate (Frémy's salt − FS) and neutral di- tert -butyl nitroxide (DTBN), in a series of 1-alkyl-3-methylimidazolium tetrafluoroborate room-temperature ionic liquids (RTILs) having alkyl chain lengths from two to eight carbons using electron paramagnetic resonance (EPR) spectroscopy. Though the size and shape of the probes are reasonably similar, they behave differently due to the charge difference. The rotation of FS is anisotropic, and the rotational anisotropy increases with the alkyl chain length of the cation, while the rotation of DTBN is isotropic. The hyperfine coupling constant of DTBN decreases as a function of the alkyl chain length and is proportional to the relative permittivity of ionic liquids. On the other hand, the hyperfine coupling constant of FS increases with increasing chain length. These behaviors indicate the location of each probe in RTILs. FS is likely located in the polar region near the network of charged imidazolium ions. DTBN molecules are predominately distributed in the nonpolar domains. [ABSTRACT FROM AUTHOR]