Reference infrared vapor-phase spectra of the 22 tetrachlorodibenzodioxin (TCDD) isomers were recorded at low microgram concentrations. These reference spectra of synthetic mixture components separated chromatographically or by spectral subtraction exhibit distinct infrared spectra for each isomer. The infrared frequencies are delineated in correlation tables and are interpreted in terms of substitution patterns which determine the strength of the ether linkage. Absorbance values in the 1330-1280 cm<sup>−1</sup> (C-O-C asymmetric stretch) region correlate with specific substitution patterns and molecular geometry. Relative electron-withdrawing capacities for chlorinated aromatic rings in TCDD isomers were estimated on the basis of relative capacities determined for model compounds. Qualitative correlations were established between electron-withdrawing capacities and the effects of resonance and field interactions on the ether linkage absorption frequencies of individual TCDD isomers. Gas chromatography Fourier transform infrared (GC/FT-IR) isomer assignments are generally consistent with those obtained by proton Fourier transform nuclear magnetic resonance (<sup>1</sup>H FT/NMR) and flame ionization gas chromatography (GC/FID). A chromatographically independent method of assigning TCDD isomer structures on the basis of ether linkage asymmetric stretching frequencies was established by utilization of valence-bond approximations. GC/FT-IR assignments for several TCDD isomers differ from isomer assignments in previously published results. A user-generated, vapor-phase reference library, containing individual TCDD spectra and spectra of isomer pairs that are incompletely resolved on chromatographic columns, correctly identified each isomer in variety of mixtures by means of a software algorithm.

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