Conformational energy barriers in methyl acetate – Ethanol solutions: A temperature-dependent ultrasonic relaxation study and molecular orbital calculations

Abstract

We report on a temperature-dependent ultrasonic relaxation study of the binary methyl acetate – ethanol solutions in a wide concentration and frequency range using the pulse-echo method. This system exhibits a single relaxation process assigned to a specific conformational process. The unique relaxation effect and the chemical simplicity of methyl acetate facilitate more accurate quantum mechanical calculations. From the temperature dependence of the corresponding ultrasonic relaxation parameters, we estimated the activation enthalpy (ΔH⁎ = 4.97 ± 0.42 kcal/mol) and the energy gap (ΔH0 = 5.72 ± 0.44 kcal/mol) between the cis- and trans-isomers of pure methyl acetate.Molecular orbital calculations were performed in order to investigate the structural, spectroscopic and thermodynamic properties of both cis- and trans-conformers of methyl acetate in a vacuum environment. The trans-isomer is confirmed as the most thermodynamically stable. The Synchronous Transit-Guided Quasi-Newton (STQN) method has been used for locating the transition structures. We confirmed that a single transition structure is present and we calculated the enthalpy of this state. Subsequently, the activation enthalpy from the trans- to the cis-conformer was estimated equal to 12.42 kcal/mol. The calculated value is close to the experimentally estimated energy barrier value (ΔEexperimental = 10.69 ± 0.56 kcal/mol). In methyl acetate-ethanol solutions thermal relaxation plays a predominant role over structural relaxation process. The deviation between the experimental and calculated effective sound velocity indicates strong interactions between unlike molecules in methyl acetate-ethanol solutions analogous to that observed recently using vibrational spectroscopies.