Molecular Shells and Range of Interactions in Ionic Liquids as a Function of Temperature

Room-temperature ionic liquids (RTILs) represent a versatile
class
of chemical systems composed entirely of oppositely charged species
whose bulk properties can be fine-tuned by adjusting molecular structures
and, consequently, intermolecular interactions. Understanding the
intricate dynamics between ionic species can aid in the rational design
of RTILs for specific applications in a range of fields, including
catalysis and electrochemistry. Here, we investigate the temperature
dependence of intermolecular interactions through magnetization transfer
by means of 1H–19F heteronuclear Overhauser
effect spectroscopy (HOESY) for two ionic liquids, namely, [BMIM][BF4] and [BMIM][PF6]. We find that the cross-relaxation
rates vary significantly over a rather small temperature range, even
changing sign. Molecular dynamics (MD) simulations on neat RTIL systems
replicate this behavior well and further show that the dynamic properties
rather than coordination changes of RTILs account for the observed
temperature behavior. Furthermore, the investigation of different
coordination shells highlights the change of interaction range with
temperature even to the point where inner and outer coordination shells
could be in distinct motional regimes with cross-relaxation rates
of opposite sign. Since temperature changes lead primarily to dynamic
changes rather than structural ones, these findings underscore the
versatility and high thermal stability of ionic liquids.