Temperature dependence of DCDD/F isomer distributions from chlorophenol precursors.

The temperature dependence of the gas-phase, rate-limited formation of dichlorodibenzo-p-dioxin (DCDD) and dichlorodibenzofuran (DCDF) isomers from 2,6-dichlorophenol and 3-chlorophenol, respectively, has been studied experimentally in an isothermal flow reactor over the range 300-900 degrees C under pyrolytic, oxidative and catalytic conditions and computationally using semi-empirical molecular orbital methods. At high temperatures, distributions of sets of DCDD/F condensation products are consistent with the calculated thermodynamic distributions, indicating that the relative rates of formation are governed by differences in symmetry and steric hindrance present in the isomer product structures. At low temperatures, however, this is not the case. In the case of 1,6- and 1,9-DCDD formed from 2, 6-dichlorophenol via Smiles rearrangement, the 1,6 isomer is favored at low temperatures more than thermodynamically predicted. This result appears to be consistent with kinetic effects of either the expansion of the five-membered ring Smiles intermediate or a lower activation energy six-membered ring intermediate pathway that produces only the 1,6 isomer. For formation of 1,7-, 3,7- and 1,9-DCDF from 3-chlorophenol, the 1,7 isomer fraction increases at low temperatures whereas thermodynamics predicts a decrease. This result can be attributed to steric effects in alternative "sandwich-type" approach geometries of phenoxy radicals to form the o,o'-dihydroxybiphenyl (DOHB) intermediate via its keto-tautomers. Higher level molecular theory (ab initio) is needed to provide a more quantitative description of these kinetics.

Nonlinear grating behavior of phthalocyanine Langmuir-Blodgett films.

Optical second-harmonic generation was observed from nickel tetracumylphenoxy phthalocyanine films deposited on quartz by the Langmuir-Blodgett (L-B) technique. The L-B films consist of microcrystallites, i.e., phthalocyanine aggregates, which have a preferred in-plane orientation with respect to the deposition direction. The L-B films also display characteristics of a nonlinear grating with the same periodicity and groove depth over the entire surface. This is to our knowledge the first time that such nonlinear grating behavior has been observed from an organometallic multilayer film.