The thermodynamic properties of phenothiazine and 135 polybrominated phenothiazine (PBPTH) in the ideal gas state at 298.15 K and 101.3 kPa have been calculated at the B3LYP/6-31G* level using Gaussian 03 program and their thermodynamic parameters were obtained. The isodesmic reactions were designed to calculate standard enthalpy of formation ( ) and standard free energy of formation ( ) of PBPTH congeners. The relationship of these thermodynamic parameters with the number and position of Br atom substitution (NPBS) was established, finding that there exists high correlation (r2≥0.998) of thermal correction to energy (Eth), heat capacity at constant volume ( ), entropy ( ), enthalpy ( ), free energy ( ), standard enthalpy of formation ( ), standard free energy of formation ( ) with NPBS. The number of Br atom substitution is the main factor, which has the greatest impact on the values of the thermodynamics properties. The stability of PBPTH congeners was theoretically proposed based on the relative magnitude of their .

Key words: phenothiazine, the number and position of Br atom substitution (NPBS), density functional theory (DFT), thermodynamic property, relative stability