  Several calorimetric measurements have shown that 1-ethyl-3-methylimidazolium
dicyanamide, [C2C1im][N(CN)2], is a glass-forming liquid, even though it is a
low-viscous liquid at room temperature. Here we found slow crystallization
during cooling of [C2C1im][N(CN)2] along Raman spectroscopy measurements. The
low-frequency range of the Raman spectrum shows that the same crystalline phase
is obtained at 210 K either by cooling or by reheating the glass
(cold-crystallization). Another crystalline phase is formed at ca. 260 K just
prior the melting at 270 K. X-ray diffraction and calorimetric measurements
confirm that there are two crystalline phases of [C2C1im][N(CN)2]. The Raman
spectra indicate that polymorphism is related to [C2C1im]+ with the ethyl chain
on the plane of the imidazolium ring (the low-temperature crystal) or
non-planar (the high-temperature crystal). The structural reason for the
glass-forming ability of [C2C1im][N(CN)2], despite of the relatively simple
molecular structures of the ions, was pursued by quantum chemistry calculations
and molecular dynamics (MD) simulations. Density functional theory (DFT)
calculations were performed for ionic pairs in order to draw free energy
surfaces of the anion around the cation. The MD simulations using a polarizable
model provided maps of occurrence of anions around cations. Both the quantum
and classical calculations suggest that the delocalization of preferred
positions of the anion around the cation, which adopts different conformations
of the ethyl chain, is on the origin of the crystallization being hampered
during cooling and the resulting glass-forming ability of [C2C1im][N(CN)2].
