N.M.R. study of the dynamic structure of isopropylbenzene

Abstract

The dynamic molecular behaviour of isopropylbenzene, IPB, in the liquid and supercooled state has been investigated by N.M.R. The proton, deuterium and ¹³C relaxation times were measured in selectively deuterated IPB, over the temperature range 125 to 200 K and the pressure range 1 bar to 5 kbar. The viscosity range extended from 1 cP to 10 P. The most significant result of the study is that although at ambient pressures the modified Debye-Stokes-Einstein relation adequately describes the microscopic dynamics of IPB in the slip limit, at high pressures the reorientational correlation times are no longer linear in η/T. The rotational-translational coupling parameter was found to be strongly density dependent, but insensitive to kinetic energy changes. An isotopic dilution study was employed to separate the intermolecular and intramolecular contributions to T _1ρ. It was shown that the viscosity anomaly at T _k is related to a change in the type of translational rather than rotational motions. The intramolecular interactions were interpreted in terms of a Cole-Davidson distribution of correlation times. Torrey's isotropic jump diffusion model was used in the analysis of the intermolecular interactions.

Two different relaxation mechanisms, indicated by minima in the temperature dependence of T _1ρ were detected in both the chain and the ring motions of IPB and were associated with the dielectric loss peaks, α and β.