A study of Heisenberg spin exchange in paramagnetic ionic liquids

The paramagnetic ionic liquids 1-ethyl-3-methylimidazolium tetrachloroferrate ([Emim][FeCl4]) and 1-butyl-3-methylimidazolium tetrachloroferrate ([Bmim][FeCl4]), exhibiting Heisenberg spin exchange, were studied using Electron Paramagnetic Resonance (EPR) from 150 to 300 K. In [Emim][FeCl4], an exchange-narrowed EPR Lorentzian line was observed in the liquid phase and in the solid phase between the solid-solid and melting/crystallization temperatures. In comparison, below the solid-solid transition, an additional EPR line appeared, resulting in two exchange-narrowed EPR Lorentzian lines. The two EPR lines likely originate from the two polymorphic forms of [Emim][FeCl4], whose crystal structures have been presented in the literature. We calculated the EPR linewidth for both lines resulting from the Heisenberg spin exchange using the observed EPR linewidths and the distances between the nearest-neighbor Fe3+ ions, following the Anderson-Weiss theory. The value of the exchange integral estimated from the additional broader EPR line, named the "low-temperature EPR line," is 0.06 K. In contrast, the exchange integral estimated from the other narrower EPR line, named the "high-temperature EPR line," was 0.235 K, implying that the crystal structure of [Emim][FeCl4] associated with this line has a stronger superexchange coupling than that associated with the low-temperature EPR line. The solid-solid transition in [Emim][FeCl4] and the melting/crystallization transition exhibited hysteretic behavior, which was also observed in differential scanning calorimetry measurements. The EPR spectrum of [Bmim][FeCl4] shows only one Lorentzian EPR line throughout the measured temperature region, regardless of temperature treatment. The width change of this EPR line indicates the glass transition in [Bmim][FeCl4] at 185 K.