Structure of Novel Phosphonium-Based Ionic Liquids with S and O Substitutions from Experiments and a Mixed Quantum-Classical Approach

This article presents experimental characterization information
and synchrotron X-ray scattering measurements on a set of novel O-
and S-substituted phosphonium-based ionic liquids (ILs) all coupled
with the bis(fluorosulfonyl)imide (FSI‑) anion.
The ILs include the ethoxyethyltriethylphosphonium (P222(2O2)+) and triethyl[2-(ethylthio)ethyl]phosphonium
(P222(2S2)+) cations,
and we contrast results on these with those for unsubstituted triethylpentylphosphonium
(P2225+). The article
also introduces a physics-based protocol that combines classical force
field studies on larger simulation boxes with classical and first-principles
studies on smaller boxes. The method produces significantly improved S(q) functions in the regime which in prior
publications we have associated with inter- and intraionic adjacency
correlations. By understanding which shorter-range structural changes
improve S(q) in the q-regime of interest, we are also able to pinpoint specific deficiencies
in the classical force field model. The approach we take should be
quite general and could help study other complex liquids on different
length scales.