Energetic study of benzothiazole and two methylbenzothiazole derivatives: Calorimetric and computational approaches

Dedicated to the memory of Professor Manuel Ribeiro da Silva
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Highlights

Abstract

This work reports an experimental and computational thermochemical study for benzothiazole and two of its methyl benzothiazole derivatives, 2-methylbenzothiazole and 2,5-dimethylbenzothiazole.

Values of the standard (p° = 0.1 MPa) molar energy of combustion of the three compounds were measured by rotating bomb combustion calorimetry. The standard molar enthalpy of the corresponding transitions from condensed to gaseous phases, at T = 298.15 K, was obtained from high temperature Calvet microcalorimetry measurements. The experimental results enable the calculation of the standard molar enthalpy of formation in the gaseous state, at T = 298.15 K, for the afore-mentioned compounds, the results being discussed in terms of structural and energetic contributions.

The gas-phase enthalpies of formation were computationally estimated from high-level ab initio molecular orbital calculations at the G3//B3LYP level of theory. The computed values compare very well with the experimental results obtained in this work and show that, in terms of enthalpy, the methyl substituents lead to an increase on the stability of the compounds, in a similar way to that already described for the corresponded benzoxazole derivatives.

Furthermore, this composite approach was also used to obtain information about the gas-phase basicity, proton and electron affinities and adiabatic ionization enthalpies.

Introduction

Benzothiazoles constitute a class of heterocyclic compounds with a wide spectrum of biological activity, being used for a large variety of therapeutic uses [1], [2], [3]. For example, 2-methylbenzothiazole may serve as promising lead scaffolds for further elaboration as anti-tuberculosis drugs and as possible anti-malaria drugs [4]. On the other hand, structures with benzothiazole moiety have large molecular hyper-polarizability, being a good alternative for the non-linear optics (NLO) materials [5]. In addition, such compounds can also have application as polidentate ligands, in laser technology as laser dyes [6].

The knowledge of the thermodynamic properties of these heterocyclic compounds, particularly the correspondent enthalpies of formation, in condensed and gaseous states, enables a better understanding of their chemical behaviour, for future development of their applications. Following our interest in the study of heterocycles with a benzene-fused ring associated to a five-membered ring containing heteroatoms of oxygen or sulfur [7], [8], we report herein the thermochemical experimental and computational results on benzothiazole, 2-methylbenzothiazole and 2,5-dimethylbenzothiazole (figure 1) in order to evaluate the energetic effects associated to the methylation on the homo and heterocyclic rings.

The experimental work available in the literature concerning the thermochemistry of benzothiazoles is scarce. Steele et al. [9] carried out a thermochemical study of benzothiazole for which the enthalpies of combustion and vaporization have been determined. From a theoretical study, Gomes et al. [10] estimated the gas-phase enthalpy of formation of benzoxazole from G3(MP2)//B3LYP calculations and provided a value about 17 kJ·mol−1 lower than that from the experiments [9]. The gas-phase enthalpy of formation of the benzothiazole was also calculated at the Gaussian-4 (G4) level of theory using the atomization reaction procedure [11], with a difference 12.1 kJ · mol−1 between the experimental [9] and the calculated G4 values. The present computational analysis indicates the same deviation, and that justifies why in this work we proceeded to the experimental redetermination of this value.

Additionally, high level ab initio calculations have been used to estimate the gas-phase standard molar enthalpy of formation of these compounds, as well as the gas-phase basicity, the proton and electron affinities and adiabatic ionization enthalpies.

Section snippets

Materials

The liquid compounds, benzothiazole [95-16-9] and 2-methylbenzothiazole [120-75-2], were purified by repeated fractional distillation under reduced pressure. The solid compound 2,5-dimethylbenzothiazole [95-26-1] was purified by repeated sublimation under reduced pressure. The purity of each compound was checked by gas chromatography and the initial and final purities of the samples are presented in table 1.

The combustion calorimetric system was calibrated with benzoic acid, NIST Standard

Computational details

Standard ab initio molecular orbital calculations were performed with the Gaussian 03 series of programs [30]. The G3//B3LYP composite method was used throughout this work [31]. This is a variation of the G3 theory [32] which uses the B3LYP density functional method [33] for geometries and zero-point energies. The B3LYP functional uses a combination of the hybrid three-parameter Becke’s functional, first proposed by Becke [33], together with the Lee–Yang–Parr non-local correlation functional

Temperature and enthalpy of fusion

The mean values of the temperature of fusion and of the enthalpy of fusion obtained from the DSC study for 2,5-dimethylbenzothiazole were Tfus=(313.0±0.1)K and ΔcrlHm(Tfus)=(18.3±0.2)kJ·mol-1, respectively. The uncertainties assigned to the results are twice the standard deviation of the mean of five independent runs. No phase transitions were detected in the crystalline phase, between T = 298 K and the temperature of fusion of the compound.

Enthalpies of formation in the condensed phase

Results of a typical experiment for each compound are

Final remarks

The new calorimetric measurements performed to determine the enthalpies of formation and vaporization of liquid benzothiazole, at T = 298.15 K, lead to a value for the enthalpy of formation of this compound in the gaseous phase that gives us confidence, since it is coherent with the results obtained for related compounds and it is well supported by the results determined computationally. The value of the enthalpy of formation in gaseous phase, previously reported in literature [9], is (204.14±0.37)

Acknowledgments

Thanks are due to Fundação para a Ciência e Tecnologia (FCT), Lisbon, Portugal, and European Social Fund for financial support given to Centro de Investigação em Química – University of Porto (strategic project PEst-C/QUI/UI0081/2011). Ana L.R. Silva thanks FCT (Portugal) for the award of a Ph.D. Grant (SFRH/BD/69606/2010).

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    Present address: Laboratoire Analyse et Modélisation pour la Biologie et l’Environnement, LAMBE – UMR8587, Université d’Évry val d’Essonne, Évry, France.

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