Diffusion of Benzene and Alkylbenzenes in Nonpolar Solvents

Do Electrostatics Control the Diffusive Dynamics of Solitary Water? NMR and MD Studies of Water Translation and Rotation in Dipolar and Ionic Solvents

NMR-based measurements of the diffusion coefficients and rotation times of solitary water and benzene at 300 K are reported in a diverse collection of 13 conventional organic solvents and 10 imidazolium ionic liquids. Proton chemical shifts of water are found to be correlated to water OH-stretching frequencies, confirming the importance of electrostatic interactions in these shifts. However, the influence of magnetic interactions in aromatic solvents renders chemical shifts a less reliable indicator of electrostatics. Diffusion coefficients (DB) and rotational correlation times (τB) of benzene in the solvents examined are accurately described as functions of viscosity (η) by DB ∝ η-0.81 and τB ∝ η0.64. Literature values of DB and τB in alkane and normal alcohols, which were not included among the solvents studied here, are systematically faster than predicted by these correlations, indicating that factors beyond solvent viscosity play a role in determining the friction on benzene. In contrast to benzene, water diffusion and rotation are poorly described in terms of viscosity alone, even in the dipolar and ionic solvents measured here. The present data and the substantial literature data already available on dilute water diffusion show a systematic dependence of DW on solvent polarity among isoviscous solvents. The aspect of solvent polarity most relevant to water dynamics is the ability of a solvent to accept hydrogen bonds from water, as conveniently quantified by the frequency of water's OH stretching band, ΔνOH. The friction on translation, ζtr = kBT/DW, and rotation, ζrot = kBTτW, are both well correlated by functions of the form ζ(η, ΔνOH) = a1ηa2 exp (a3ΔνOH), where the ai are adjustable parameters. Molecular dynamics simulations reveal a strong coupling between electrostatic and nonelectrostatic water-solvent interactions, which makes it impossible to dissect the friction on water into additive dielectric and hydrodynamic components. Simulations also provide a tentative explanation for the unusual form of the correlating function ζ(η, ΔνOH), at least in the case of ζrot.

Diffusion of Polymethylene Chain Molecules in Nonpolar Solvents

The translational diffusion constants, D, for solutes with polymethylene chains are compared with the predictions of a hydrodynamic bead model based on Kirkwood-Riseman theory. The solute-solvent combinations include (a) n-alkanes in n-alkanes; (b) n-alkanes in benzene, toluene, tetralin, decalin, and CCl₄; (c) 1-alkenes in n-alkanes; (d) 1-phenylalkanes in n-alkanes; and (e) 1-phenylalkanes in isocetane (2,2,4,4,6,8,8-heptamethylnonane), pristane (2,6,10,14-tetramethylpentadecane), and squalane (2,6,10,15,19,23-hexamethyltetracosane). The bead model gives good overall agreement with an average difference of less than 3% between 207 experimental and calculated diffusion constants that include published data as well as new D values determined for 1-alkenes and 1-phenylalkanes using capillary flow techniques. The calculated values are obtained using chain element (bead) radii that decrease as the solvent viscosity increases. The bead model's results are comparable to those obtained using cylinder and lollipop diffusion for many of the same solute-solvent systems; the three models are compared and discussed. The results for the 1-alkenes and n-alkanes in the n-alkanes are the first from a Kirkwood-Riseman analysis in a homologous series of solvents.

Long-lived water clusters in hydrophobic solvents investigated by standard NMR techniques

Unusual physical characteristics of water can be easier explained and understood if properties of water clusters are revealed. Experimental investigation of water clusters has been reported by highly specialized equipment and/or harsh experimental conditions and has not determined the properties and the formation processes. In the current work, we used standard ¹H-NMR as a versatile and facile tool to quantitatively investigate water clusters in the liquid phase under ambient conditions. This approach allows collection of data regarding the formation, long lifetime, stability, and physical properties of water clusters, as a cubic octamer in the liquid phase.