The intermolecular interactions and dynamics of novel ionic liquids with alkylsilyl and alkylsiloxy substitutions on the cations are studied by measuring the intermolecular vibrational spectra and reorientational dynamics using femtosecond Kerr effect methods. The new ionic liquids include 1-dimethylphenylsilylmethyl-3-methylimidazolium (PhSi-mim(+)), and 1-methyl-3-pentamethyldisiloxymethylimidazolium (SiOSi-mim(+)) cations paired with the bis(trifluoromethylsulfonyl)imide (NTf2-) anion. Measured ionic liquid viscosities are surprisingly low for such bulky cation substituents. DFT electronic structure calculations on the isolated ions provide additional information about the electrostatic interactions.