Reference infrared vapor-phase spectra of the 14 pentachlorodibenzo-p-dioxin (PnCDD) isomers were recorded at low microgram concentrations. A unique infrared spectrum corresponding to each chromatographically separated or spectrally subtracted mixture component was observed. The structures for individual isomers in each isomer pair were assigned by qualitative valence-bond evaluations and empirically derived quantitative estimations of ether linkage asymmetric stretching frequencies left bracket upsilon //C//O//C//(//a//s//y//m//) right bracket . Correlations between calculated linkage (C-O-C) bond angles and upsilon //C//O//C//(//a//s//y//m//) indicat existence of butressed 1,6 and 1,9 nonbonded interactions not observed in tetrachlorodibenzo-p-dioxin (TCDD) isomers.
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