On the Calculation of Henry's Law Constants of Chlorinated Benzenes in Water from Semiempirical Quantum Chemical Methods

The Henry's law constants for all 12 polychlorinated benzene congeners were calculated using semiempirical quantum chemical solvation models (SM2, SM2.1, and SM3), and their performances are discussed. The values obtained by the SM3 method are underestimated compared with the experimental values and those calculated by SM2 and SM2.1 methods. This underestimation is larger as the degree of chlorination of the benzenes increases. This bad performance of SM3 to calculate Henry's law constants is attributable to the PM3 Hamiltonian, due to its incapacity to give a good description of electron density in chlorinated benzenes. The best results, within the 95% confidence limit of error of the experimental mean, are obtained using SM2 method. The discussion includes analysis of the different contributions to the free energy of solvation, namely, polarization free energy, cavitation, dispersion, and structural rearrangement of the solvent effects as well as partial atomic charges.