Experimental and computational thermochemical study of two fluorobenzazoles: 5-fluoro-2-methylbenzoxazole and 5-fluoro-2-methylbenzothiazole
Graphical abstract
Introduction
The present work is part of an extensive study on the thermochemical properties of 1,3-benzazole compounds, containing the pentagonal heterocyclic ring fused to a benzenic ring, in order to evaluate the energetic effect of different substituents on the rings, namely the methyl, the amine, the mercaptan, the chlorine and the fluorine groups [1], [2], [3], [4], [5], [6]. Further, we intend to analyse the energetic effect associated to different heteroatoms (nitrogen, oxygen and sulphur) in position one of the pentagonal ring for this class of molecules. Hence, we report new experimental results for the energies of combustion and the enthalpies of vaporization of two benzazole fluorinated derivatives, the 5-fluoro-2-methylbenzoxazole, FMBO, and the 5-fluoro-2-methylbenzothiazole, FMBT, (Fig. 1), measured by rotating bomb combustion calorimetry, at T = 298.15 K, and high temperature Calvet microcalorimetry, respectively. From these results, the standard (p° = 0.1 MPa) molar enthalpies of formation of both benzazole derivatives, in condensed and gaseous states, were derived. It is important to highlight that the combustion of FMBT, a compound containing fluorine and sulphur atoms, origins a final bomb solution containing a mixture of sulphuric and hydrofluoric acids whose thermodynamic parameters (e.g. enthalpies of dissolution of the gases on the solution) needed for the calculations are unknown. To overcome this difficulty, we designed comparison experiments using standard samples of benzoic acid and thianthrene, jointly with an aqueous solution of hydrofluoric acid, in adequate proportions to guarantee identical qualitative and quantitative compositions of the final states of these comparison experiments and those of the combustion reactions of FMBT [7]. Considering this procedure, corrections to the standard state are required only to the initial states, except a minor correction to the final states since the amounts of gas in the final states were slightly different.
Additionally, the gas-phase standard molar enthalpies of formation of these compounds were estimated computationally at the G3(MP2)//B3LYP level of theory, as well as their gas-phase basicities and proton affinities.
The results obtained allow correlation of the energetic parameters with the structural characteristics of the molecules, which is important to contribute for the knowledge on the reactivity of the species.
Section snippets
Materials and purity analysis
5-fluoro-2-methylbenzoxazole (FMBO, CAS No. 701-16-6) and 5-fluoro-2-methylbenzothiazole (FMBT, CAS No. 399-75-7) were supplied by TCI Europe (+98%), being then purified by repeated fraction distillation under reduced pressure (T = 376 K and p = 4.1 kPa and T = 428 K and p = 10.0 kPa, respectively). Gas chromatography analysis indicated that the distilled samples of both compounds had a mass fraction purity >0.999. The analyses were carried out on an Agilent 4890D gas chromatography–flame
Computational details
The standard ab initio molecular orbital calculations for the two fluorinated compounds were performed with the Gaussian 03 series of programs [16] and the composite G3(MP2)//B3LYP approach was the methodology employed [17]. In this method, the geometries and zero-point energies are obtained from B3LYP density functional theory [B3LYP/6-31G(d)]. Then, single-point calculations are carried out at higher levels of electronic structure theory: quadratic configuration interaction, QCISD(T,FC), and
Enthalpies of formation in the liquid phase
The detailed results of one combustion experiment of each compound and the corresponding comparison experiment for FMBT are given in Table 2. The values of the energy associated to the isothermal bomb process, ΔU(IBP), were calculated from Eq. (1):
in which Δm(H2O) is the deviation of the mass of water added to the calorimeter from 5222.5 g; ΔTad is the calorimeter temperature change corrected for the heat exchange,
Acknowledgments
This work was supported by Fundação para a Ciência e Tecnologia (FCT) of Portugal, Project UID/QUI/UI0081/2013, and FEDER, Projects POCI-01-0145-FEDER-006980 and NORTE-01-0145-FEDER-000028. ALRS thanks FCT for the award of a Ph.D. Grant (SFRH/BD/69606/2010).
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