Experimental and computational thermochemical studies of benzoxazole and two chlorobenzoxadole derivatives

Abstract

The energetic study of benzoxazole, 5-chloro-2-methylbenzoxazole, and 2-chlorobenzoxazole, in condensed and gaseous states, has been developed using experimental techniques and computational approaches. The values of the standard (p°=0.1MPa) molar enthalpy of formation, at T=298.15K, of crystalline benzoxazole (36.0±2.0)kJ·mol−1 and 5-chloro-2-methylbenzoxazole (145.6±2.2)kJ·mol−1 and liquid 2-chlorobenzoxazole (52.5±3.0)kJ·mol−1 were determined from the corresponding experimental standard molar energy of combustion in oxygen, −(3432.1±1.7)kJ·mol−1, −(3883.0 2.0)kJ·mol−1, and −(3298.0±2.8)kJ·mol−1, respectively , measured by static or rotating-bomb combustion calorimetry. At T=298.15K, the standard (p°=0.1MPa) molar enthalpy of sublimation of benzoxazole and 5-chloro-2-methylbenzoxazole and of vaporization of 2-chlorobenzoxazole, (69.2±0.8) kJ·mol−1, (82.4±2.2) kJ·mol−1, and (56.3±1.6) kJ·mol−1 respectively, were determined by a direct method, using the vacuum drop microcalorimetric technique. From the latter values and from the enthalpies of formation of the condensed compounds, the standard (p°=0.1MPa) enthalpies of formation of the gaseous compounds have been calculated. Standard ab initio molecular orbital calculations were performed using the G3(MP2)//B3LYP composite procedure and several working reactions in order to derive the standard molar enthalpy of formation of the three compounds. There exists a good agreement between the experimental and the computational data.