Enthalpies of formation of dihydroxybenzenes revisited: Combining experimental and high-level ab initio data

doi:10.1016/j.jct.2013.10.032

The Journal of Chemical Thermodynamics

Volume 73, June 2014, Pages 90-96

Dedicated to the memory of the late Professor Manuel Ribeiro da Silva

https://doi.org/10.1016/j.jct.2013.10.032 Get rights and content

Highlights

•
Thermochemistry of hydroxyphenols probed by experimental and theoretical methods.
•
A new paradigm for obtaining enthalpies of formation of crystalline compounds.
•
High-level ab initio results for the thermochemistry of gas-phase hydroxyphenols.
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Sublimation enthalpies of hydroxyphenols determined by Calvet microcalorimetry.

Abstract

Accurate values of standard molar enthalpies of formation in condensed phases can be obtained by combining high-level quantum chemistry calculations of gas-phase enthalpies of formation with experimentally determined enthalpies of sublimation or vapourization. The procedure is illustrated for catechol, resorcinol, and hydroquinone. Using W1-F12, the gas-phase enthalpies of formation of these compounds at T = 298.15 K were computed as (−270.6, −269.4, and −261.0) kJ · mol⁻¹, respectively, with an uncertainty of ∼0.4 kJ · mol⁻¹. Using well characterised solid samples, the enthalpies of sublimation were determined with a Calvet microcalorimeter, leading to the following values at T = 298.15 K: (88.3 ± 0.3) kJ · mol⁻¹, (99.7 ± 0.4) kJ · mol⁻¹, and (102.0 ± 0.9) kJ · mol⁻¹, respectively. It is shown that these results are consistent with the crystalline structures of the compounds.

Graphical abstract

Introduction

The accuracy of experimentally derived gas-phase standard enthalpies of formation of organic compounds is often lower than ca. 4 kJ·mol⁻¹ [1] – the so-called chemical accuracy limit. As outlined in figure 1, most of these data rely on combustion calorimetry experiments, yielding condensed-phase enthalpies of formation [2], and on several other methodologies (including calorimetry and vapour pressure vs. temperature plots) [2], [3], which lead to enthalpies of sublimation or vaporisation.

The problems and rewards of combustion calorimetry have been thoroughly discussed [2]. Sample purity and (in)complete combustion are two of the most important factors that an experimentalist has to consider. For instance, a minor impurity with a high heat of combustion may cause sizeable errors. Moreover, the number of researchers who are proficient in combustion calorimetry is steadily decreasing, suggesting that we are now seeing the dusk of the practise of this technique.

It can be argued that the experimental determination of phase-change enthalpies, on the other hand, does not require the same level of expertise as combustion calorimetry. But if this statement were true, then, for example, most of the available enthalpy of sublimation data should be highly accurate. Unfortunately this is not observed for many organic compounds, as illustrated by our case study, involving catechol, resorcinol, and hydroquinone.

What can we do to take the thermochemical database to the chemical accuracy limit? Two decades ago, or even less, most experimentalists were sceptical about the reliability of thermochemical values obtained from quantum chemistry calculations. This suspicion, often justified by frequent disagreements between experimental and theoretical data, was first addressed by developing error-cancellation computational procedures, such as isodesmic and homodesmic reactions [4]. Although these procedures are still very useful, the application of higher-level quantum chemistry methods, together with much better computational resources, ensures that we can obtain chemically accurate values for the gas-phase enthalpies of formation of molecules with ten or even more heavy atoms [5]. In other words, stating that high-level quantum chemistry calculations are now as reliable as the best experimental procedures is no longer heresy [5], [6], [7], [8], [9], [10], [11]. Nevertheless, a limitation of theory still prevails: the previous statement only applies to molecules in the gas phase. The theoretical methodologies which address the energetics of intermolecular interactions are not yet able to predict chemically accurate enthalpy of sublimation data for organic molecules. Therefore, the experimental determinations of enthalpies of sublimation remain indispensable and our efforts should now be focused on improving the reliability and the accuracy of these data. This implies, in turn, that the solid sample under study must be properly characterised and that the occurrence of thermal events, such as solid–solid phase or glass transitions, must be investigated [12]. There is enough evidence that some discrepancies between experimental enthalpies of sublimation can be attributed to the use of solid samples with different crystalline structures (i.e. different polymorphs) or crystallinity [13].

In summary, as illustrated in figure 2, molecular energetics lives a new paradigm: some of the most accurate gas-phase data are now obtained with quantum chemistry methods, which, together with experimentally determined phase-change enthalpies, may afford accurate enthalpies of formation of substances in liquid and solid phases.

The present work applies the above ideas to the redetermination of the standard molar enthalpies of formation of solid catechol, resorcinol, and hydroquinone. It involved the determination of the standard molar enthalpies of sublimation of the dihydroxybenzenes by Calvet drop microcalorimetry, using well characterised samples in terms of chemical and phase purity. The gas-phase enthalpies of formation were determined using high-level quantum chemistry methods.

Section snippets

General

Diffuse reflectance infrared Fourier-transform (DRIFT) spectra were obtained in the (400 to 4000) cm⁻¹ range, using a Nicolet 6700 spectrometer. The resolution was 2 cm⁻¹ and the pellets were ∼5% (w/w) of sample in KBr. The ¹H-NMR and ¹³C-NMR spectra were obtained in DMSO-d₆, (Aldrich 99.9% atom D, containing 0.03% v/v TMS) at ambient temperature, with a Bruker Ultrashield 400 MHz spectrometer. The GC–MS experiments were performed with an Agilent 6890 gas chromatograph equipped with an Agilent

Computational details

The structures of catechol, resorcinol, and hydroquinone were optimised with the B3LYP-D3 dispersion corrected [21] hybrid density functional [22], [23] together with the cc-pVTZ basis set [24]. Zero-point and thermal enthalpy corrections at T = 298.15 K were obtained from frequencies calculated with the same method (scaled by 0.985, see reference [5]) under the rigid rotor and harmonic oscillator approximations. The enthalpies of all species under study were subsequently determined with the

Characterization of the solid phases

The conformations adopted by catechol, resorcinol, and hydroquinone molecules in the solid phase, as determined from single crystal X-ray diffraction data [16], [17], [18], [19], by using the Mercury 3.0 program [36], are shown in figure 3. The intramolecular hydrogen bond in catechol is apparent.

Figure 4, also obtained from single crystal X-ray diffraction data, exhibits the packing diagrams of catechol, resorcinol, and hydroquinone, in the corresponding unit cell. For all three isomers the

Discussion

Table 3 summarizes the values of $Δ_{sub} H_{m}^{o}$ , $Δ_{f} H_{m}^{o} (cr)$ , and $Δ_{f} H_{m}^{o} (g)$ determined in this work for catechol, resorcinol, and hydroquinone. It also includes selected experimental data reported by other researchers.

Analysis of table 3 reveals that the disparities in the literature values for the enthalpies of sublimation of the dihydroxybenzenes (9 kJ · mol⁻¹ for catechol, 7 kJ · mol⁻¹ for resorcinol, and 11 kJ · mol⁻¹ for hydroquinone) are ca. twice the size of the chemical accuracy limit. Incidentally, the

Conclusions

In the current work, we have reassessed the solid-, liquid-, and gas-phase enthalpies of formation of catechol, resorcinol, and hydroquinone (table 3). Large discrepancies were found for the enthalpy of sublimation data collected from the literature for hydroxyphenols. We suggested that these could stem from differences in the purity, crystallinity, and particle size of the samples. These features can significantly influence the thermochemical data, and must therefore be investigated through a

Acknowledgments

This work was supported by Fundação para a Ciência e a Tecnologia (FCT), Portugal (PEst-OE/QUI/UI0612/2013). T.S.A. and F.A. thank FCT for post-doctoral grants (SFRH/BPD/20836/2004 and SFRH/BPD/74195/2010, respectively). We also acknowledge Prof. José Manuel Nogueira and Carlos Almeida (CQB-FCUL) for performing the GC-MS experiments, Dr. Carlos Bernardes (Instituto Superior Técnico) for help in the X-ray diffraction experiments, and Prof. Manuel Eduardo Minas da Piedade (CQB-FCUL) and Prof. Rui

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Enthalpies of formation of dihydroxybenzenes revisited: Combining experimental and high-level ab initio data

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

General

Computational details

Characterization of the solid phases

Discussion

Conclusions

Acknowledgments

J. Chem. Thermodyn.

Comput. Phys. Commun.

Thermochim. Acta

Thermochim. Acta

Thermochim. Acta

J. Chem. Thermodyn.

J. Chem. Thermodyn.

Molecular Energetics

Enthalpies of Vapourization of Organic Compounds. A Critical Review and Data Compilation

J. Am. Chem. Soc.

J. Chem. Phys.

J. Chem. Phys.

J. Chem. Phys.

J. Chem. Phys.

J. Chem. Phys.

J. Chem. Phys.

J. Chem. Phys.

J. Phys. Chem. B

Cryst. Growth Des.

Acta Crystallogr. B

Acta Crystallogr. Sect. B-Struct. Sci.

Z. Kristall.