Structural, energetic and reactivity properties of phenoxazine and phenothiazine

• Experimental and computational gas-phase enthalpies of formation of phenoxazine and phenothiazine.
• Reactivity behavior of chromophoric frameworks – electrostatic potential energy maps for phenoxazine and phenothiazine.
• Homolytic N–H bond dissociation enthalpies on phenoxazine and phenothiazine.
• Phenoxazine and phenothiazine: energetic characterization, proton affinity, gas-phase acidity and basicity.

A combined experimental and computational study was developed with the aim of evaluate and understand the structural, energetic and reactivity properties of phenoxazine and phenothiazine. Experimentally, differential scanning calorimetry, static and rotating bomb combustion calorimetries, Knudsen effusion and Calvet microcalorimetry were employed to determine, respectively, the standard (p° = 0.1 MPa) molar enthalpies of fusion, ΔcrlHmo, at the temperature of fusion, the standard molar enthalpies of formation, in the crystalline phase, ΔfHmo(cr), at T = 298.15 K, the temperature-vapor pressures dependences, and the standard molar enthalpies of sublimation, ΔcrgHmo, at T = 298.15 K. These data allowed the derivation the experimental standard molar enthalpies of formation, in the gaseous phase, ΔfHmo(g), of phenoxazine, (100.8 ± 4.3) kJ · mol−1, and of phenothiazine, (273.5 ± 4.7) kJ · mol−1. Computationally, the composite G3(MP2)//B3LYP approach was used to optimize the structures of these two compounds and to estimate their ΔfHmo(g) values, which are found to be in very good agreement with the experimental ones. Calculations were also performed for additional analyses of their natural bond orbitals (NBO) and to obtain other gas-phase thermodynamic properties, namely N–H bond dissociation enthalpies, gas-phase acidities and basicities and proton affinities.

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