Thermodynamic Properties of Polychlorinated Biphenyls in the Gas Phase

The molecular structures, vibrational frequencies, and internal rotational potentials of 209 polychlorinated biphenyls were computed at the B3LYP/6-31G(d,p) density functional theory level. Standard entropies, S°(T), heat capacities, and enthalpies, H°(T) − H°(0) (100 K ≤ T ≤ 1500 K), were calculated using the rigid-rotor harmonic-oscillator approximation with correction for internal rotation. Enthalpies of formation, were calculated at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p) level using isodesmic reactions and the recommended values for biphenyl, benzene, and polychlorinated benzenes. The uncertainties of the calculated values are estimated to be 5−10 J K-1 mol-1 for and and 5−35 kJ mol-1 for The calculated thermodynamic properties are compared with values determined earlier by the semiempirical and group additivity methods.