Why does a reduction in hydrogen bonding lead to an increase in viscosity for the 1-butyl-2,3-dimethyl-imidazolium-based ionic liquids?

1-Butyl-3-methyl-imidazolium chloride ([C(4)C(1)im]Cl) is a prototypical ionic liquid. Substitution for a methyl group at the 2-position of the cation to form 1-butyl-2,3-dimethyl-imidazolium ([C(4)C(1)mim]+) eliminates the main hydrogen-bonding interaction between the Cl anion and the imidazolium cation. Loss of this hydrogen-bonding interaction could be expected to lead to a reduction in melting point and a decrease in viscosity; however the opposite is observed experimentally; melting points and viscosity increase. The gas-phase structure and electronic properties of ion pairs formed from [C(4)C(1)mim]+ and Cl- are investigated to offer insight into this counter-intuitive behavior. We hypothesize that the effects due to a loss in hydrogen bonding are outweighed by those due to a loss in entropy. The amount of disorder in the system is reduced in two ways: elimination of ion-pair conformers, which are stable for [C(4)C(1)im]Cl but not [C(4)C(1)mim]Cl, and an increase in the rotational barrier of the butyl chain, which limits free rotation and facilitates alkyl chain association. The reduction in entropy leads to greater ordering within the liquid raising the melting point and increasing viscosity. The relative stabilities of 15 conformers with respect to anion position and alkyl chain rotation are reported at the B3LYP/6-31++G(d,p) level for [C(4)C(1)mim]Cl. Hydrogen bonding between the cation and the anion is examined on the basis of structural criteria and the computed vibrational spectra (IR and Raman). Spectra for the substituted and unsubstituted cations and ion pairs are compared, and modes are identified for [C(4)C(1)mim]Cl that could be used to differentiate between rotational conformers. A natural bond orbital analysis has also been carried out, and the resultant charge distribution is compared with that of the unsubstituted analogue [C(4)C(1)im]Cl.