  The low melting point of room temperature ionic liquids is usually explained
in terms of the presence of bulky, low symmetry and flexible ions, with the
first two factors related to the lattice energy while an entropic effect is
attributed to the latter. By means of molecular dynamics simulations, the
melting points of 1-ethyl-3-methylimidazolium hexafluorophosphate and
1-decyl-3-methyl-imidazolium hexafluorophosphate were determined and the effect
of the molecular flexibility over the melting point was explicitly computed by
restraining the rotation of dihedral angles in both the solid and the liquid
phase. The rotational flexibility over the bond between the ring and the alkyl
chain affects the relative ordering of the anions around the cations and
results in substantial effects over both the enthalpy and the entropy of
melting. For the other dihedral angles of the alkyl group, the contributions
are predominantly entropic and an alternating behavior was found. The
flexibility of some dihedral angles has negligible effects on the melting
point, while others can lead to differences in the melting point as large as 20
K. This alternating behavior is rationalized by the different probabilities of
conformation defects in the crystal.
