Thermochemical study of three dibromophenol isomers

This work reports the standard (p° = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, ΔfHm∘(g), of 2,4-, 2,6-, and 3,5-dibromophenol, at T = 298.15 K, respectively, as (59.6 ± 2.6) kJ · mol−1, (49.1 ± 2.2) kJ · mol−1 and (39.5 ± 2.0) kJ · mol−1. These experimental values were derived from the measurements of the standard molar enthalpies of formation, in the crystalline phase, ΔfHm∘(2,4-dibromophenol,cr)=-(140.9±2.1)kJ·mol-1, ΔfHm∘(2,6-dibromophenol,cr)=-(132.5±1.6)kJ·mol-1 and ΔfHm∘(3,5-dibromophenol,cr)=-(134.5±1.7)kJ·mol-1, at the same reference temperature, achieved from the standard molar enthalpies of combustion, in oxygen, to yield CO2(g) and HBr·600H2O(l), measured by rotating-bomb combustion calorimetry, together with measurements of the standard molar enthalpies of sublimation, at T = 298.15 K, as ΔcrgHm∘(2,4-dibromophenol)=(81.3±1.5)kJ·mol-1, ΔcrgHm∘(2,6-dibromophenol)=(83.4±1.5)kJ·mol-1 and ΔcrgHm∘(3,5-dibromophenol)=(94.3±1.8)kJ·mol-1, obtained using the Calvet high temperature vacuum sublimation technique. The standard molar enthalpy of sublimation, at T = 298.15 K, for the 3,5-dibromophenol was also determined from the temperature–vapour pressure dependence, obtained by the Knudsen mass loss effusion method, ΔcrgHm∘(3,5-dibromophenol)=(95.0±1.1)kJ·mol-1. For this isomer it is also reported the standard (p° = 0.1 MPa) molar entropy and Gibbs energy of sublimation, at T = 298.15 K.The experimental values of the gas-phase enthalpies of formation of each compound were compared with estimates using the empirical scheme developed by Cox and with the calculated values based on density functional theory calculations using the B3LYP hybrid exchange-correlation energy functional at the 6-311++G(d,p) basis set. These two methodologies were also used to estimate the enthalpies of formation in the gas-phase of the 2,3-, 2,5-, and 3,4-dibromophenol.