Building Chemical Intuition about Physicochemical Properties of C8-Per-/Polyfluoroalkyl Carboxylic Acids through Computational Means.

We have predicted acid dissociation constants (pK a), octanol-water partition coefficients (K OW), and DMPC lipid membrane-water partition coefficients (K lipid-w) of 150 different eight-carbon-containing poly-/perfluoroalkyl carboxylic acids (C8-PFCAs) utilizing the COnductor-like Screening MOdel for Realistic Solvents (COSMO-RS) theory. Different trends associated with functionalization, degree of fluorination, degree of saturation, degree of chlorination, and branching are discussed on the basis of the predicted values for the partition coefficients. In general, functionalization closest to the carboxylic headgroup had the greatest impact on the value of the predicted physicochemical properties.

Geometric and Electronic Effects in the Binding Affinity of Imidazole-Based N-Heterocyclic Carbenes to Cu(100)- and Ag(100)-Based Pd and Pt Single-Atom Alloy Surfaces.

We have conducted nonlocal periodic density functional theory (DFT) calculations of N-heterocyclic carbenes (NHCs) adsorbed to Pd/Cu(100), Pt/Cu(100), Pd/Ag(100), and Pt/Ag(100) single atom alloys (SAAs) utilizing the nonlocal optPBE-vdW functional. NHCs with electron donating groups (EDGs) are predicted to bind more strongly to the SAA surface compared to NHCs functionalized with electron withdrawing groups (EWGs). Our calculations show that NHCs typically bind to SAA geometries containing a small space between the heteroatom sites for the SAAs considered. Generally, this pattern is predicted to persist for a single NHCs or for a pair of NHCs bound to the SAA surfaces. Approximate linear relationships between NMR-based parameters and NHC-SAA binding energies are uncovered. We predict that the binding of NHCs to SAA surfaces is composition-dependent and heteroatom geometry dependent.

Resolving unknown isomers of emerging per- and polyfluoroalkyl substances (PFASs) in environmental samples using COSMO-RS-derived retention factor and mass fragmentation patterns.

Chromatographic retention factors (k) and mass spectral fragmentation patterns of per- and polyfluoroalkyl substances (PFASs) were determined using the optimized parameters in liquid chromatography with tandem high-resolution mass spectrometry (LC-HRMS) analysis. Characteristic fragment ions obtained at various collision energies (MS2 fragmentation) were used to determine the structures of newly discovered (emerging) PFASs detected from industrial effluent and surface water samples. Moreover, COnductor-like Screening MOdel for Realistic Solvents (COSMO-RS) derived octanol-water partition coefficients (Kow), along with mean isotropic polarizabilities calculated from Density Functional Theory (DFT), of known PFASs were plotted against their experimental k values (kexp) to obtain a multivariable regression model that can be used to predict k values of unknown PFASs. The k values of different structural isomers of unknown PFASs were calculated and compared to kexp. The predicted k value for the isomer that matches the MS2 fragmentation observed was found to be within 4.2 % of kexp. This study demonstrates the applicability of an approach that combines the observed MS2 fragmentation patterns and k values, together with computationally-derived log Kow and polarizabilities, in assigning the structures of unknown PFASs at environmentally relevant conditions when no reference standards are available.

Electrochemical Atomic Force Microscopy and First-Principles Calculations of Ferriprotoporphyrin Adsorption and Polymerization.

The adsorption and subsequent electrooxidative polymerization of ferriprotoporphyrin IX chloride (hemin; FePPCl) was investigated on highly ordered pyrolytic graphite, glassy carbon, and polycrystalline Pt electrodes using electrochemical atomic force microscopy, first-principles calculations, and cyclic voltammetry. Hemin was shown to readily adsorb to all three surfaces; however, it was more continuous over the carbon surfaces compared to the Pt surface. This disparity in adsorption appears to be a major contributing factor to differences observed between the electrodes following hemin electropolymerization. Despite differences in roughness and morphology, hemin polymerized as a continuous layer over each electrode surface. Periodic density functional theory calculations were used to model FePP (without Cl) on both the Pt(111) and graphite surfaces using the vdW-DF-optPBE functional to account for the dispersion interactions. Our calculations suggest that the FePP molecule chemisorbs to the Pt surface while at the same time exhibiting intramolecular hydrogen bonding between the carboxylic acid groups, which are extended away from the surface. In contrast to FePP-Pt chemisorption, FePP was found to physisorb to graphite. The preferred spin state upon adsorption was found to be S = 2 on Pt(111), whereas on graphite, the high and intermediate spin states were nearly isoenergetic. Additionally, gas-phase calculations suggest that much of the surface roughness observed microscopically for the polymerized porphyrin layer may originate from the nonparallel stacking of porphyrin molecules, which interact with each other by forming four intermolecular hydrogen bonds and through dispersion interactions between the stacked porphyrin rings. Regardless of polymer thickness, the underlying electrode appears to be able to participate in at least some redox processes. This was observed for the hemin-polymerized Pt electrode using the 2H+/H2 redox couple and was suspected to be due to some Pt surface atoms not being specifically coordinated to the hemin molecules and therefore available to react with H+ that was small enough to diffuse through the polymer layer.