Aqueous-phase photooxygenation of enes, amines, sulfides and polycyclic aromatics by singlet (a1Δg) oxygen: prediction of rate constants using orbital energies, substituent factors and quantitative structure-property relationships

Environmental contextTo aid the transition to sustainable chemistry there is a need to improve the degradability of chemicals and limit the use of organic solvents. Singlet oxygen, O-1(2), is involved in organic synthesis and photochemical degradation; however, information on its aqueous-phase reactivity is limited. We developed cheminformatics models for photooxidation rate constants that will enable accurate assessment of aquatic photochemistry without experimentation. AbstractTo aid the transition to sustainable and green chemistry there is a general need to improve the degradability of chemicals and limit the use of organic solvents. In this study we developed quantitative structure-property relationships (QSPRs) for aqueous-phase photochemical reactions by singlet (a(1)Delta(g)) oxygen. The bimolecular singlet oxygen reaction rate constant can be reliably estimated (R-2=0.73 for naphtalenes and anthracenes, R-2=0.86 for enes and R-2=0.88 for aromatic amines) using the energy of the highest occupied molecular orbital (E-HOMO). Additional molecular descriptors were used to characterise electronic and steric factors influencing the rate constant for aromatic enes (R-2=0.74), sulfides and thiols (R-2=0.72) and aliphatic amines. Mechanistic principles (frontier molecular orbital, perturbation and transition state theories) were applied to interpret the QSPRs developed and to corroborate findings in the literature. Depending on resonance, the speciation state (through protonation and deprotonation) can heavily influence the oxidation rate constant, which was accurately predicted. The QSPRs can be applied in synthetic photochemistry and for estimating chemical fate from photolysis or advanced water treatment.