Reference infrared vapor-phase spectra of 15 polychlorinated dibenzodioxin-p-dioxin (PCDD) isomers were recorded at low microgram concentrations. Ether linkage (COC) bound 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 XY2 model and by neglecting the valence force equation symmetric stretch bending term. Calculated bond angles show a good correlation with x-ray differentiation (XRD) and carbon-13 nuclear magnetic resonance (13C 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. Non-bonded 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 bound strength from empirical data. Correlations between infrared data and PCDD LD50 values suggest that molecular geometry, polarizability, and steric PCDD/receptor interactions are associated with isomer toxicity.
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