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

doi:10.1016/j.molliq.2020.112519

Journal of Molecular Liquids

Volume 302, 15 March 2020, 112519

https://doi.org/10.1016/j.molliq.2020.112519 Get rights and content

Highlights

•
The system exhibits a single relaxation process assigned to a conformational process.
•
Activation enthalpy and enthalpy difference between cis-trans isomers were estimated.
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Calculations confirmed the relative stability of trans-isomer compared to cis-isomer.
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The calculated energy barrier values approximate the experimental value.
•
In solutions, thermal relaxation dominates over structural relaxation process.

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 (ΔE_experimental = 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.

Graphical abstract

Introduction

The significance of the conformation of a molecule has been widely accepted since decades [1,2]. The reactivity, the stability and the physical properties possessed by a molecule are strongly related to its conformation [1,2]. Due to this interrelation, the conformational analysis appears among the most important topics of organic stereo-chemistry, polymer-chemistry and bio-chemistry [3]. Ultrasonic relaxation spectroscopy, belonging to the category of stationary relaxation techniques, has been employed to determine the energies related to the conformational changes taking place in various organic liquids, polymer solutions and biochemical systems [[4], [5], [6], [7], [8]]. Frequency and amplitude measurements conducted in the MHz region revealed different kinds of relaxation processes in molecular isomerization and indicated that there are stronger relaxation phenomena in the frequency region below 50 MHz, where only the ultrasonic pulse-echo technique has the required accuracy and reliability [9].

Before proceeding with the actual measurements, it is useful to overview the intermolecular nature of the relaxation process. In molecular isomerization molecules can occupy two or more inner energy levels. By applying the ultrasonic field, the propagation of the sound wave disturbs the equilibrium distribution of the molecules between these levels. For relatively low frequencies of the sound wave, the time between two successive cycles is long enough for the energy transfer from the ultrasonic wave to the internal degrees of freedom. As the ultrasonic frequency increases, a time delay is observed between the variation of the temperature and pressure of the wave and the restoration of the internal equilibrium. This phenomenon is observed experimentally, when measuring the ultrasound absorption coefficient a as a function of frequency f, as a maximum point of the ultrasonic absorption per wavelength μ and as a saddle point of the α/f² ratio for a specific resonance frequency f_r. The equilibrium between the trans- and cis-isomer is presented as: $A_{trans} \underset{k_{b}}{\Leftarrow} \overset{k_{f}}{\Rightarrow} B_{cis}$ where Β represents the higher energy state. The ultrasonic absorption per wavelength or the α/f² ratio as a function of ultrasonic frequency provide information about the reaction constants moving forward and backwards, the energy barriers and the energy difference between two energy levels. By the temperature dependence of the relaxation frequency f_r, the energy barrier can be calculated, while the temperature dependence of the maximum value of attenuation per wavelength μ, that is μ_max, provides the difference of the free energy.

In this work, the ultrasonic relaxation spectroscopy has been employed to undertake a systematic study of temperature- and concentration-dependence of the relaxation frequency for pure methyl acetate in the liquid state and in solutions with ethanol as solvent. Attention has been paid in receiving reliable values for the activation enthalpy and for the energy gap between the cis- and trans-isomers of pure methyl acetate. Density functional theory (DFT) 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. Furthermore, the Synchronous Transit-Guided Quasi-Newton (STQN) method has been used for locating the transition structures.

Section snippets

Materials

Methyl acetate was purchased from Penta Chemical Industry with 99.8% purity and used without further purification. Ethanol was purchased from Merck with (99.8%) and also used as received. Solutions corresponding to a wide range of concentrations were prepared by weighing. Before sample preparation, attention has been paid to reduce to a minimum the vapour space in the vessels to avoid evaporation losses during manipulation in order to avoid possible errors. The maximum water content in methyl

Methyl acetate

Methyl acetate liquid exhibits an ultrasonic relaxation caused by the disturbance of the balance between the cis- and trans-isomers. The two conformations and the relevant energy levels are presented in Fig. 1.

Initially, the structures of both isomers of methyl acetate were fully optimized without symmetry constraints (C₁ symmetry) and analytical frequency calculations were performed in order to confirm the optimized structures to be either a minimum or a first-order saddle point. Fig. 1

Conclusions

In this work, a temperature-dependent ultrasonic relaxation study has been undertaken to study ultrasonic relaxation of the binary methyl acetate – ethanol solutions over a wide concentration and frequency range by means of pulse-echo technique. Ultrasonic absorption and the corresponding relaxation frequency were estimated as a function of temperature under isobaric conditions. Furthermore, molecular orbital calculations had been performed in a vacuum environment, in order to elucidate the

CRediT authorship contribution statement

Georgios Stogiannidis: data curation; formal analysis Stefanos Tsigoias: data curation; formal analysis Angelos G. Kalampounias: Conceptualization; Formal analysis; Investigation; Methodology; Project administration; Resources; Supervision; Writing-original draft; Writing-review & editing

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to express our thanks to Professors Dr. C. Kosmidis, Dr. S. Kaziannis and the personnel of the Central Laser Facility of Ioannina University for access on their facilities and their help. Dr. Zoi Salta is also acknowledged for helpful discussions.

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

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Methyl acetate

Conclusions

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgments

Ultrasonics

J. Phys. Chem. Solids

Ultrasonics

Chem. Phys. Lett.

J. Am. Chem. Soc.

ChemPhysChem

Top. Stereochem.

Experienta

Organic Chemistry

J. Korean Phys. Soc.

J. Korean Phys. Soc.

Acta Phys. Pol. A

Acoustics

Ultrasonic Spectroscopy: Applications in Condensed Matter Physics and Materials Science

J. Acoust. Soc. Am.

Ultrasonic World Congress, Proceedings, Berlin