Unveiling photodegradation mechanisms of short-chain chlorinated paraffins via integrated experimental and...

Short-chain chlorinated paraffins (SCCPs) exist as complex mixtures, complicating precise identification of photodegradation sites and products, thereby obscuring their environmental degradation pathways. This study integrates experimental photodegradation with density functional theory (DFT) simulations to elucidate SCCPs photodegradation mechanisms. First, ultraviolet (UV) irradiation experiments were conducted on 1-chlorodecane (1-CD), with degradation products characterized by gas chromatography-mass spectrometry. Subsequent DFT simulations calculated molecular bond dissociation energies (BDEs) and energy barriers (EBs) for 1-CD. By incorporating photon energy, degradation sites and intermediates were predicted, clarifying product formation pathways consistent with mass spectrometry results. This simulation approach was extended to three representative polychlorinated SCCPs, revealing principal degradation reactions: dehydrochlorination, dechlorination, and C-C bond cleavage. For pristine SCCPs, the energy requirements for these reactions are < 3 eV, ∼4-6 eV, and > 6 eV, respectively, indicating they can be induced by visible light (>400 nm), UV (200-400 nm), and deep UV (<200 nm) irradiation. Given sufficient irradiation, dehydrochlorination and dechlorination proceed until complete SCCP dechlorination. Electron cloud density distribution analysis demonstrates that C-C bond cleavage BDEs in dechlorinated radical intermediates decrease to levels activatable by 320-400 nm irradiation, suggesting sunlight can facilitate SCCP fragmentation. These findings provide critical insights into environmental SCCP photodegradation pathways and mechanisms.