Abstract
Reference infrared vapor-phase spectra of 15 polychlorinated dibenzodioxin-<i>p</i>-dioxin (PCDD) isomers were recorded at low microgram concentrations. Ether linkage (COC) bond angles for these isomers and for the 22 tetrachlorodibenzodioxin (TCDD) isomers were calculated from infrared data, with the use of mass approximations for the terminal atom in a nonlinear XY<sub>2</sub> model and by neglecting the valence force equation symmetric stretch bending term. Calculated bond angles show a good correlation with x-ray diffraction (XRD) and carbon-13 nuclear magnetic resonance (<sup>13</sup>C NMR) relaxation results. Molecular geometries in PCDD isomers, as defined by the COC bond angle and the COC stretching frequencies, were found to range from near planar, in laterally substituted isomers with high electron withdrawing capacity aromatic rings, to tetrahedral, for isomers with low electron withdrawing capacity rings. Nonbonded interactions were also found to influence molecular geometry. Molecular geometry was used to assign structures for the 1,2,3,6,7,8 and 1,2,3,7,8,9-hexachlorodibenzodioxin isomer mixture and was found to be an important factor in estimations of COC bond strength from empirical data. Correlations between infrared data and PCDD LD<sub>50</sub> values suggest that molecular geometry, polarizability, and steric PCDD/receptor interactions are associated with isomer toxicity.
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