Elsevier

Thermochimica Acta

Volume 673, March 2019, Pages 169-176
Thermochimica Acta

Calorimetric and computational study of (1H-Indol-n-yl)methanol and 2-(1H-Indol-n-yl)ethanol (n=2, 3)

https://doi.org/10.1016/j.tca.2019.01.021Get rights and content

Highlights

  • ΔcUmo of 2-(1H-indol-3-yl)ethanol has been measured by static bomb calorimetry.

  • ΔcrgHmo of 2-(1H-indol-3-yl)ethanol has been measured by Calvet microcalorimetry.

  • ΔfHmo(g) of 2-(1H-indol-3-yl)ethanol has been derived from experimental measurements.

  • ΔfHmo(g) of (1H-Indol-n-yl)methanol and 2-(1H-Indol-n-yl)ethanol (n = 2,3) have been derived from G3 calculations.

Abstract

In the present work, the gas-phase standard molar enthalpy of formation of 2-(1H-indol-3-yl)ethanol was derived, at T = 298.15 K, from the enthalpy of combustion for the crystalline compound, measured by static-bomb calorimetry, and its enthalpy of sublimation obtained from Calvet microcalorimetry measurements. The standard molar enthalpies of formation of this compound and for (1H-indol-2-yl)methanol, (1H-indol-3-yl)methanol and 2-(1H-indol-2-yl)ethanol were calculated using the composite G3 method. The experimental value of the gas-phase enthalpy of formation of 2-(1H-indol-3-yl)ethanol is ‒(48.5 ± 3.3) kJ mol−1, being in excellent agreement with the G3 value, thus giving confidence to the estimates. The results were analysed in terms of the enthalpic methylene increments and compared with other related systems.

Introduction

Indole derivatives constitute an important class of therapeutic agents in medicinal chemistry, presenting diverse biological properties [1,2] and ample industrial applications [3]. Due to their high potential, there has been the emphasis on the novel and improved process for the synthesis of indole derivatives to overcome problems faced by existing therapeutic agents. As so, the knowledge of their thermodynamic properties is highly valuable in clarifying the chemical behavior inherent to those species.

The present work is part of a systematic thermochemical study of indole derivatives aiming at investigating the energetic effect of different substituents on the indole ring as well as to get information on their relative stabilities [[4], [5], [6]]. In this work, we report the standard (po = 0.1 MPa) molar enthalpy of formation in the crystalline phase and the standard molar enthalpy of sublimation for 2-(1H-indol-3-yl)ethanol, at T = 298.15 K, determined by static-bomb combustion calorimetry and Calvet microcalorimetry techniques, respectively.

The obtained experimental values led to the enthalpies of formation of 2-(1H-indol-3-yl)ethanol in the gaseous phase. Additionally, high level ab initio molecular orbital calculations have been performed at the G3 level for (1H-indol-2-yl)methanol, (1H-indol-3-yl)methanol, 2-(1H-indol-2-yl)ethanol and 2-(1H-indol-3-yl)ethanol, in order to estimate the gas-phase standard molar enthalpies of formation of these systems and, complementarily, to evaluate the energetic effect of the −CH2OH and −CH2CH2OH groups on the indole structure.

Section snippets

Compound and purity control

2-(1H-indol-3-yl)ethanol was obtained commercially from TCI Europe, with certified mole fraction purity of 0.997, and was purified by sublimation under reduced pressure (1 Pa). The purity was checked by gas-liquid chromatography performed on an Agilent 4890D Gas Chromatography equipped with an HP-5 column, cross-linked, 5% diphenyl and 95% dimethylpolysiloxane (15 x 0.530 mm i.d x 1.5 μm film thickness), using nitrogen as the carrier gas, (≥ 0.9998 mass fraction). The purity of the compound was

Combustion calorimetry

The detailed results for the combustion experiments of 2-(1H-indol-3-yl)ethanol are given in Table 2, together with the mean value of the standard massic energy of combustion ⟨Δcu°⟩, and its standard deviation of the mean. Δm(H2O) is the deviation of the mass of water added to the calorimeter from 2900.0 g (the mass assigned for εcal), ΔUƩ is the correction to the standard state, and the remaining terms are as previously described [13,28]. For the static-bomb measurements, as samples were

Discussion

For 2-(1H-indol-3-yl)ethanol, the gas-phase standard molar enthalpy of formation determined experimentally and that obtained from computational calculations are in excellent agreement, which confers confidence to the estimates of this thermodynamic parameter for the compounds which have not been studied experimentally.

In the Fig. 2 we show a scheme which allows analysing the effect of the addition of a −CH2‒ group in the linear chain of (1H-indol-2-yl)methanol and (1H-indol-3-yl)methanol, to

Conclusions

The standard molar gas-phase enthalpy of formation, at T = 298.15 K, of 2-(1H-indol-3-yl)ethanol has been obtained both by experimental and computational techniques. The enthalpy of formation was obtained from static bomb combustion calorimetry and Calvet microcalorimetry experiments. These values have been also estimated by G3 calculations and by considering a pertinent set of working reactions. The computed value is in excellent agreement with the experimental data herewith reported,

Acknowledgments

Thanks are due to Fundação para a Ciência e Tecnologia (FCT), Lisbon, Portugal, for the financial support to Project UID/QUI/0081/2013 and to FEDER through Program NORTE2020 for the financial support to Project POCI‐01‐0145‐FEDER‐006,980 and to Project "Sustained Advanced Materials", ref. NORTE-01-0145-FEDER-000028 (FCUP). LMPFA thanks to Programa Ciência 2008.

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  • Cited by (0)

    1

    LEPABE, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr Roberto Frias, 4200-465 Porto, Portugal.

    2

    REQUIMTE, Department of Chemistry and Biochemistry, Faculty of Science, University of Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal.

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