A. Kalampounias
Jul 15, 2020
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Influential Citations
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Journal
Journal of Molecular Structure
Abstract
Abstract We report a detailed investigation of the ultrasonic absorption spectra of n-propyl formate in the liquid state over a wide frequency and temperature range under constant pressure. The ultrasonic absorption measurements were performed by standard acoustic resonator and pulse techniques. A single relaxational phenomenon has been observed and is attributed to a unimolecular reaction due to a conformational change of the n-propyl formate molecule in the liquid state. The characteristic relaxation frequency increases with increasing temperature and vise versa. In an effort to confirm the assignment of the relaxation process and to determine the corresponding thermodynamic properties, relaxation parameters were further investigated as a function of temperature. Activation enthalpy and enthalpy difference between the two isomeric forms have been estimated equal to ΔH∗ = 4.21 ± 0.16 kcal/mol and ΔH0 = 6.21 ± 0.29 kcal/mol, respectively. The corresponding energy barrier was estimated equal to ΔEexperimental = 10.42 ± 0.33 kcal/mol. Molecular orbital calculations have been applied to obtain the standard enthalpy of formation for n-propyl formate at various dihedral angles around C–O bond. The results exhibited two clear minimum standard enthalpies of formation and the energy barrier has been calculated equal to ΔE = 11.62 kcal/mol, close to the experimental value considering that the calculation was performed in a vacuum environment. The results confirm the absence of shear viscosity relaxation in this frequency range. The ratio of the volume viscosity to the static shear viscosity ηV/ηS has been determined revealing a clear temperature dependent and ηV>ηS supporting the assignment of the observed relaxation process to the rotational equilibrium. The results are analyzed in view of the current phenomenological status of the field.