3,4,5-Trimethoxyphenol: A combined experimental and theoretical thermochemical investigation of its antioxidant capacity

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Abstract

The standard (p = 0.1 MPa) molar enthalpies of combustion and sublimation of 3,4,5-trimethoxyphenol were measured, respectively, by static bomb combustion calorimetry in oxygen atmosphere and by Calvet microcalorimetry. From these measurements, the standard molar enthalpy of formation in both the crystalline and gaseous phase, at T = 298.15 K, were derived: −(643.4 ± 1.9) kJ · mol−1 and −(518.1 ± 3.6) kJ · mol−1, respectively. Density functional theory calculations for this compound and respective phenoxyl radical and phenoxide anion were also performed using the B3LYP functional and extended basis sets, which allowed the theoretical estimation of the gaseous phase standard molar enthalpy of formation through the use of isodesmic reactions and the calculation of the homolytic and heterolytic O–H bond dissociation energies. There is good agreement between the calculated and experimental enthalpy of formation. Substituent effects on the homolytic and heterolytic O–H bond dissociation energies have been analysed.

Introduction

This work is part of a wider investigation on the thermochemistry of substituted phenols. We have already studied tert-butyl and di-tert-butylphenols [1], [2], methoxy- and dimethoxyphenols [3], cyanophenols [4], and methoxynitrophenols [5]. The study of the energetics [6], [7] of phenolic compounds has a considerable practical interest since this class of chemical compounds includes a large number of synthetic and naturally occurring antioxidants. They inhibit the oxidation of materials of both commercial and biological importance. This antioxidant function is due to the ability of phenols to trap the peroxyl radicals via the hydrogen transfer reactionROO+ArO-HROO-H+ArO

So, it is clear that the energetics of the O–H bond in phenols will be an important factor in determining the efficacy of an antioxidant as well as is important the study of how this energetics is affected by the number, nature, and position of substituents in the aromatic ring. A high rate for reaction (1) is expected to go hand in hand with a low O–H bond dissociation energy.

In the present work, we report the standard molar enthalpy of formation of 3,4,5-trimethoxyphenol in the gaseous phase, obtained from measurements of the combustion energy using a static bomb calorimeter and from the enthalpy of sublimation measured by microcalorimetry Calvet. In addition, density functional theory calculations for this compound were carried out in order to estimate the gaseous phase enthalpy of formation. Similar calculations were performed for the respective phenoxyl radical and phenoxide anion and were used to obtain the homolytic O–H bond dissociation enthalpy and gaseous phase acidity. A theoretical interpretation of the influence of the methoxy substituent on these properties is presented.

Section snippets

Material and purity control

3,4,5-Trimethoxyphenol [642-71-7] was obtained commercially from Aldrich Chemical Co with the mass fraction of purity of >0.97 and was purified by repeated sublimation under reduced pressure until the combustion results were consistent and the carbon dioxide recovery ratios were satisfactory. The average ratio, together with the standard deviation of the mean, of the mass of carbon dioxide recovered to that calculated from the mass of sample was as follows: 3,4,5-trimethoxyphenol (3,4,5-(MeO)3

Results and discussion

The enthalpy of fusion for 3,4,5-trimethoxyphenol was measured by differential scanning calorimetry (DSC) as ΔcrlHm=(31.94±1.32)kJ·mol-1. The obtained value corresponds to the temperature of fusion Tfus=(420.17 ± 0.12) K measured at the onset of the DSC peak. The uncertainties quoted for ΔcrlHm and Tfus represents twice the standard deviation of the mean of five independent runs.

The results of the combustion experiments are given in table 1. The symbols in this table have been previously

Conclusions

In this work, the thermochemistry of 3,4,5-trimethoxyphenol has been experimentally determined using combustion calorimetry and microcalorimetry Calvet. In addition, density functional theoretical calculations of isodesmic reactions have been done, allowing estimates of the standard molar enthalpy of formation in the gaseous phase. There is very good agreement of the calculated and experimental enthalpy of formation.

The substituent effects on the homolytic and heterolytic O–H bond dissociation

Acknowledgement

Thanks are due to Fundação para a Ciência e Tecnologia for financial support to Centro de Investigação em Química.

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