Experimental and computational thermochemical studies of benzoxazole and two chlorobenzoxadole derivatives

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° = 0.1 MPa) molar enthalpy of formation, at T = 298.15 K, 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 = 298.15 K, the standard (p° = 0.1 MPa) 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° = 0.1 MPa) 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.

► Energetic study of three benzoxazoles using experimental and computational techniques.
► ΔfHm∘ (cr, 298.15 K) of benzoxazole, 5-chloro-2-methylbenzoxazole, and 2-chlorobenzoxazole.
► Δcr,lgHm∘ of benzoxazole, 5-chloro-2-methylbenzoxazole, and 2-chloro-benzoxazole were derived.
► MO calculations for benzoxazole derivatives performed with G3(MP2)//B3LYP composite.