Examining the degradation of environmentally-daunting per- and poly-fluoroalkyl substances from a fundamental chemical perspective.

In this work, ground and excited-state properties were used as descriptors for probing mechanisms as well as to assess potential alternatives for tackling the elimination of perfluorobutane sulfonic acid (PFBS) - C4F9SO2OH, perfluorooctane sulfonic acid (PFOS) - C8F17SO2OH, and perfluorooctanoic acid (PFOA) - C7F15COOH. For this purpose, density functional theory (DFT) and its time-dependent formalism (TD-DFT) at both CAM-B3LYP/6-311+G(2d,2p) and M06-2X/6-311+G(2d,2p) levels of theory in water (IEF-PCM) were employed. To gauge the accuracy of the DFT approaches for the current systems, wave function methods (Møller-Plesset, MP2, coupled-cluster with single and double excitations, CCSD, CCSD with perturbative triples, CCSD(T), and equation of motion CCSD, EOM-CCSD) and aug-cc-pVXZ (X = D and T) basis sets were used. Regarding PFBS and PFOS, all the excited states probed were found to be energetically accessible only in the high-energy vacuum UV region (<200 nm ≥6.20 eV); SO2O- is released when the first low-lying excited singlet state (21A) of both compounds is accessed. On the other hand, two lowest-lying excited singlet states of PFOA were computed at considerably lower energy (5.84 eV and 5.97 eV for 21A and 31A, respectively, at the TD-DFT/CAM-B3LYP/6-311+G(2d,2p)). In addition, intramolecular OH radical formation is suggested for protonated PFOA when interacting with radiation at 7.98 eV ≈ 155 nm, as determined at the TD-DFT/CAM-B3LYP/6-311+G(2d,2p) level of theory. Such intramolecularly generated hydroxyl may contribute to a faster degradation of PFOA (or of other per- and poly-fluoroalkyl substances (PFAS) that are usually found together with PFOA).

Photoinduced degradation of indigo carmine: insights from a computational investigation.

In this work, we present a computational investigation on the photoexcitation of indigo carmine (IC). Physical insights regarding IC photoexcitation and photolysis were obtained from a fundamental perspective through quantum chemistry computations. Density functional theory (DFT) was used to investigate the ground state while its time-dependent formalism (TD-DFT) was used for probing excited state properties, such as vertical excitation energies, generalized oscillator strengths (GOS), and structures. All the computations were undertaken using two different approaches: M06-2X/6-311+G(d,p) and CAM-B3LYP/6-311+G(d,p), in water. Results determined using both methods are in systematic agreement. For instance, the first singlet excited state was found at 2.28 eV (with GOS = 0.4730) and 2.19 eV (GOS = 0.4695) at the TD-DFT/CAM-B3LYP/6-311+G(d,p) and TD-DFT/M06-2X/6-311+G(d,p) levels of theory, respectively. Excellent agreement was observed between the computed and the corresponding experimental UV-Vis spectra. Moreover, results suggest IC undergoes photodecomposition through excited state chemical reaction rather than via a direct photolysis path. To the best of our knowledge, this work is the first to tackle the photoexcitation, and its potential connections to photodegradation, of IC from a fundamental chemical perspective, being presented with expectations to motivate further studies.