Trace Water Effects on Crystalline 1-Ethyl-3-methylimidazolium Acetate

Spontaneous room-temperature crystallization of 1-ethyl-3-methylimidazoliumacetate ([C(2)mim]-[OAc]) was observed upon removal of tracewater. Sample purity was confirmed using analytical nuclear magneticresonance spectroscopy to ensure that trace water or other contaminantsdid not produce this observation. Raman spectroscopy and simultaneousquartz crystal microbalance/infrared spectroscopy measurements wereused to study molecular reorganization during crystallization anddecrystallization using trace water in the form of atmospheric moisture.These experimental results were supplemented with density functionaltheory calculations that indicate imidazolium cation ring stackingand side chain clustering with an exclusive arrangement of the acetateanion in the cation ring plane upon water removal. Crystal structureformation was confirmed using two-dimensional wide-angle X-ray scattering.This natural crystallization is attributed to the removal of tracewater over extended periods of time and calls attention to the molecular-levelrole of water in the structure of hygroscopic ionic liquid systems.