Physical properties of 4 room-temperature ionic liquids consisting of the 1-butyl-3-methylimidazolium cation with various perfluorinated anions and the bis(trifluoromethylsulfonyl)imide (Tf2N-) anion with 12 pyrrolidinium-, ammonium-, and hydroxyl-containing cations are reported. Electronic structure methods are used to calculate properties related to the size, shape, and dipole moment of individual ions. Experimental measurements of phase-transition temperatures, densities, refractive indices, surface tensions, solvatochromic polarities based on absorption of Nile Red, 19F chemical shifts of the Tf2N- anion, temperature-dependent viscosities, conductivities, and cation diffusion coefficients are reported. Correlations among the measured quantities as well as the use of surface tension and molar volume for estimating Hildebrand solubility parameters of ionic liquids are also discussed.

A systematic investigation has been made into the degree of silylium ion character in species of the type i-Pr 3 SiY where Yis a weakly coordinating hexahalo-carborane anion, X 6 -CB 11 H 6 -, with X ) Cl, Br, I. By X-ray crystal structural criteria such as the average C-Si-C angle (117.3(4)°, 117.0(13)°, and 115.6(9)°for X ) Cl, Br, I, respectively) and the downfield 29 Si chemical shift (115, 110, and 97 ppm for X ) Cl, Br, I, respectively) the hexachloro-carborane anion is the least coordinating. Thus, the "ion-like" species i-Pr 3 Si δ+ (Cl 6 -CB 11 H 6 ) δbecomes the closest structural approach to date to a condensed phase silylium ion. With the aid of ab initio molecular orbital theory and silicon chemical shift calculations we address the question of how much silylium ion character this species represents. From structural, energetic, and NMR criteria (where expectation values for the condensed phase 29 Si chemical shifts are revised downward) we conclude that i-Pr 3 Si δ+ (Cl 6 -CB 11 H 6 ) δmay have >50% silylium ion character.