A simple rule for classification of polychlorinated dibenzo-p-dioxin congeners on the basis of IR frequency patterns

The IR spectra of 76 dioxin congeners with zero to eight chlorines have bee n calculated by the DFT (B3LYP) method. Close inspection of the calculated spectra provides a simple rule for LR spectral analysis in polychlorinated dibenzo-p-dioxins (PCDDs). All congeners with a common structural motif of chlorine substitution in either of two benzene rings share the same main IR frequency, and the chlorination of one ring has little influence on the vi brational frequencies of the other ring. As a result, the main ring vibrati onal frequencies of all dioxin congeners could be classified into 10 groups according to the chlorine substitution pattern. This simplifies the comple x IR spectral analysis of PCDDs to identifying two main peaks in essence, e ach of which corresponds to either of two chlorinated benzene rings. The sy stematic trend originates from the mass effect of the substituted chlorines . The C-H bending (in plane) character in the ring skeleton vibration mode is affected in quite different ways, depending on whether the chlorine is s ubstituted on longitudinal or lateral positions. In particular, when all th e lateral positions (2, 3, 7, 8) are chlorinated in PCDDs, the vibrational frequency of a bl, mode (or its analogues in other symmetry) gives a charac teristic IR peak around 1392 cm(-1). This peak is unique to all toxic conge ners and could be used as an indicator for them.