Abstract
In this study, ultrasonic relaxation spectroscopy and molecular orbital calculations have been used to investigate the rotational-isomeric equilibria in isobutyl halide (CH3)2CH–CH2X (X = Cl, Br and I) liquids as a function of frequency and temperature under isobaric conditions. The sound absorption in the a/f2 representation appeared to deviate from straight line with frequency in the MHz region revealing a single Debye-type relaxation process, which is attributed to thermal relaxation of a rotational isomeric equilibrium for all isobutyl halide systems. From the temperature dependence of the relaxation parameters, the associated thermodynamic parameters and the respective volume changes were estimated for each liquid and discussed in view of the outcome of density functional theory electronic structure calculations. The activation enthalpy was found to be strongly dependent on the ionic radius of the halide. Furthermore, the lower energy state of all isobutyl halide systems is two-fold degenerate. The experimental results indicate that a shear viscosity relaxation is questionable in the frequency region covered in this work. The observed-to-classical absorption ratio α/αcl and the volume-to-shear viscosity ratio nV/ns have been evaluated and used as a measure to monitor the strength of the intermolecular bonding. The values and the strong temperature dependence of these ratios are typical of a relaxation process associated to rotational isomers equilibrium. The results have been discussed in the context of inter- and intra-molecular interactions.
Published Version
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