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

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°=0.1MPa) 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=298.15K, 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=298.15K, 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.