SEMIEMPIRICAL MOLECULAR-ORBITAL ESTIMATION OF THE RELATIVE STABILITY OF POLYCHLORINATED BIPHENYL ISOMERS PRODUCED BY O-DICHLOROBENZENE PYROLYSIS

The pyrolysis of pure o-dichlorobenzene at temperatures between 1100 a nd 1400 K produced a small set of tri- and tetrachlorobiphenyls whose relative yields appear to be related to steric factors associated with the presence of o-chlorine atoms. To evaluate the hypothesis that pro duct stability governs the distribution of PCB isomers produced in thi s system, biphenyl and all 209 polychlorinated biphenyl (PCB) congener s were modeled using the semiempirical molecular orbital method AM 1 ( Austin model 1). Relative enthalpies and entropies of PCB isomers were calculated, from which a small set of parameters that can be used for estimation by the method of group additivity was derived. These calcu lations suggest that PCBs produced by o-dichlorobenzene pyrolysis belo w 1400 K are thermodynamically distributed.