Effect of Confined Hindrance in Polyphenylbenzenes

A comprehensive thermodynamic study of the whole <i>ortho</i>-polyphenylbenzenes series from biphenyl (<i>n</i> = 1) to hexaphenylbenzene (<i>n</i> = 6) is presented. Combustion calorimetry and phase equilibria measurements for 1,2,3,4-tetraphenylbenzene (<i>n</i> = 4) and pentaphenylbenzene (<i>n</i> = 5) together with literature data were used to understand and quantify the constraint effect of <i>ortho-</i>substitution on the molecular energetics and phase stability of polyaromatic compounds. All of the derived thermodynamic properties (enthalpy of sublimation, entropy of sublimation, and gas phase molecular energetics) show a marked trend shift at <i>n</i> = 4 to 5, which is related to the change of the degree of molecular flexibility after 1,2,3,4-tetraphenylbenzene (<i>n</i> = 4). The greater intramolecular constraint in the more crowded members of the series (<i>n</i> = 5 and 6) leads to a significant change in the molecular properties and cohesive energy. The trend shift in the molecular properties is related with the decrease in molecular flexibility, which leads to lower molecular entropy and destabilization of the intramolecular interaction potential due to the increased hindrance in a confined molecular space

Effect of the Methylation and N–H Acidic Group on the Physicochemical Properties of Imidazolium-Based Ionic Liquids

This work presents and highlights the differentiation of the physicochemical properties of the [C₁Him]­[NTf₂], [C₂Him]­[NTf₂], [¹C₁²C₁Him]­[NTf₂], and [¹C₄²C₁³C₁im]­[NTf₂] that is related with the strong bulk interaction potential, which highlights the differentiation on the physicochemical arising from the presence of the acidic group (N–H) as well as the methylation in position 2, C(2), of the imidazolium ring. Densities, viscosities, refractive indices, and surface tensions in a wide range of temperatures, as well as isobaric heat capacities at 298.15 K, for this IL series are presented and discussed. It was found that the volumetric properties are barely affected by the geometric and structural isomerization, following a quite regular trend. A linear correlation between the glass transition temperature, <i>T</i>_g, and the alkyl chain size was found; however, ILs with the acidic N–H group present a significant higher <i>T</i>_g than the [¹C_<i>N</i>‑1³C₁im]­[NTf₂] and [¹C_<i>N</i>³C_<i>N</i>im]­[NTf₂] series. It was found that the most viscous ILs, ([¹C₁Him]­[NTf₂], [¹C₂Him]­[NTf₂], and [¹C₁²C₁Him]­[NTf₂]) have an acidic N–H group in the imidazolium ring in agreement with the observed increase of energy barrier of flow. The methylation in position 2, C(2), as well as the N–H acidic group in the imidazolium ring contribute to a significant variation in the cation–anion interactions and their dynamics, which is reflected in their charge distribution and polarizability leading to a significant differentiation of the refractive indices, surface tension, and heat capacities. The observed differentiation of the physicochemical properties of the [¹C₁Him]­[NTf₂], [¹C₂Him]­[NTf₂], [¹C₁²C₁Him]­[NTf₂], and [¹C₄²C₁³C₁im]­[NTf₂] are an indication of the stronger bulk interaction potential, which highlights the effect that arises from the presence of the acidic group (N–H) as well as the methylation in position 2 of the imidazolium ring