Catalytic Activity of Palladium(II) and Osmium(VIII) on the Oxidation of Chloramphenicol by Copper(III) Periodate Complex in Aqueous Alkaline Medium—A Comparative Kinetic and Mechanistic Approach

Kinetic UV-Vis Spectroscopic and DFT Mechanistic Study of the Redox Reaction of [OsVIIIO4(OH)n]n- (n = 1, 2) and Methanol in a Basic Aqueous Matrix

This combined experimental and computational study builds on our previous studies to elucidate the reaction mechanism of methanol oxidation by Os^VIII oxido/hydroxido species (in basic aqueous media) while accounting for the simultaneous formation of Os^VII species via a comproportionation reaction between Os^VIII and Os^VI. UV-Vis spectroscopy kinetic analyses with either CH₃OH or the deuterated analogue CD₃OH as a reducing agent revealed that transfer of α-carbon-hydrogen of methanol is the partial rate-limiting step. The resulting relatively large KIE value of approximately 11.82 is a combination of primary and secondary isotope effects. The Eyring plots for the oxidation of these isotopologues of methanol under the same reaction conditions are parallel to each other and hence have the same activation enthalpy [Δ^⧧H° = 14.4 ± 1.2 kcal mol^-1 (CH₃OH) and 14.5 ± 1.3 kcal mol^-1 (CD₃OH)] but lowered activation entropy (Δ^⧧S°) from -12.5 ± 4.1 cal mol^-1 K^-1 (CH₃OH) to -17.1 ± 4.4 cal mol^-1 K^-1 (CD₃OH). DFT computational studies at the PBE-D3 level with QZ4P (Os) and pVQZ (O and H) basis sets provide clear evidence to support the data and interpretations derived from the experimental kinetic work. Comparative DFT mechanistic investigations in a simulated aqueous phase (COSMO) indicate that methanol and Os^VIII first associate to form a noncovalent adduct bound together by intermolecular H-bonding interactions. This is followed by spin-forbidden α-carbon-hydrogen transfer (not O-H transfer) from methanol to Os^VIII by means of HAT, which is found to be the partial rate-limiting step. Without the organic and inorganic fragments dissociating from each other during the entire stepwise redox reaction (in order to avoid formation of highly energetically unfavorable monomer species), the HAT step is followed by PT and then ET before the final product monomers formaldehyde and Os^VI dissociate from each other. DFT-calculated Δ^⧧H° is within 5 kcal mol^-1 of the experimentally obtained value, while the DFT Δ^⧧S° is three times larger than that found from the experiment.

Reaction mechanism of chloramphenicol with hydroxyl radicals for advanced oxidation processes using DFT calculations

The structure properties of chloramphenicol (CAP), including bond information and the Fukui function for the atoms in the main chain, were investigated computationally by density functional theory (DFT). The result shows that the chiral carbons in CAP offer the most active positions for chemical reactions, which is in good agreement with the experiment. The detailed degradation mechanism for CAP with hydroxyl radicals in advanced oxidation processes is further studied at the SMD/M06-2X/6-311 + G(d,p) level of theory. The main reaction methods, including the addition-elimination reaction, hydrogen abstract reaction, hydroxyl radical addition, and bond-breaking processes, are calculated. The results show that the nitro-elimination reaction is the most likely reaction in the first step of the degradation of CAP, and the latter two processes are more likely to be hydrogen abstract reactions. The details for the transition states, intermediate radicals, and free energy surfaces for all proposed reactions are given, which makes up for a lack of experimental knowledge.

Factors and mechanisms that influence the reactivity of trivalent copper: A novel oxidant for selective degradation of antibiotics

Trivalent copper complexes are active intermediates in aquatic redox reactions that involve copper ions or structural copper, but their reactivity and selectivity toward pollutants remain unknown. We characterized copper(III) periodate, a representative trivalent copper compound, with phenol and several antibiotics as model contaminants. The results show that Cu(III) is highly reactive to phenol degradation; near-complete degradation was achieved after 10 min at a molar ratio of 3:1 (Cu[III]: phenol). Common alcohols, including methanol and 2-propanol, showed pH-dependent reactivity for Cu(III). In contrast to aquo trivalent copper ions that react rapidly with tert-butanol, Cu(III) showed limited reactivity toward tert-butanol. A mechanistic investigation showed that the degradation was caused by direct oxidation by Cu(III) and that no hydroxyl radicals were involved. Common water components such as chloride ions did not influence the reaction, which suggests that the use of Cu(III) may help mitigate the generation of chlorinated products. As a one-electron oxidant, Cu(III) showed high reactivity to degrade electron-rich compounds; the concentrations of sulfamethazine, sulfamethoxazole, and sulfadiazine (100 μg/L) were reduced to 1.8, 7.5, and 42.5 ng/L, respectively, after 2 min of reaction with 10 μM Cu(III). These results demonstrate a novel and highly efficient oxidant for selective removal of ubiquitous micropollutants from water bodies.

Optical and electronic configuration of a novel semiconductor-silver nitroprusside for enhanced electrocatalytic and photocatalytic performance

This study presents a novel n-type semiconductor material, silver nitroprusside, possessing a π-acceptor ligand bridged octahedral geometry with a poor spin state metal ligand charge transfer effect.

A DFT study to unravel the ligand exchange kinetics and thermodynamics of Os(VIII) oxo/hydroxido/aqua complexes in aqueous matrices

The Os(VIII) oxo/hydroxido complexes that are abundant in mild to relatively concentrated basic aqueous solutions are Os(VIII)O4, [Os(VIII)O4(OH)](-) and two cis-[Os(VIII)O4(OH)2](2-) species. Os(VIII) complexes that contain water ligands are thermodynamically unfavoured w.r.t. the abovementioned species. Os(VIII)O4 reacts with hydroxide in two, consecutive, elementary coordination sphere expansion steps to form the [Os(VIII)O4(OH)](-) complex and then the cis-[Os(VIII)O4(OH)2](2-) species. The Gibbs energy of activation for both reactions, in the forward and reverse direction, are in the range of 6-12 kcal mol(-1) and are relatively close to diffusion-controlled. The thermodynamic driving force of the first reaction is the bonding energy of the Os(VIII)-OH metal-hydroxido ligand, while of the second reaction it is the relatively large hydration energy of the doubly-charged cis-[Os(VIII)O4(OH)2](2-) product compared to the singly-charged reactants. The DFT-calculated (PBE-D3 functional) in the simulated aqueous phase (COSMO) is -2.4 kcal mol(-1) for the first reaction and -0.6 kcal mol(-1) for the second reaction and agree to within 1 kcal mol(-1) with reported experimental values, at -2.7 and 0.3 kcal mol(-1) respectively. From QTAIM and EDA analyses it is deduced that the Os(VIII)[double bond, length as m-dash]O bonding interactions are ionic (closed-shell) and that Os(VIII)-OH bonding interactions are polar covalent (dative). In contrast to QTAIM, NCI analysis allowed for the identification of relatively weak intramolecular hydrogen bonding interactions between neighbouring oxo and hydroxido ligands in both [Os(VIII)O4(OH)](-) and cis-[Os(VIII)O4(OH)2](2-) complexes.