New advances in conceptual-DFT: an alternative way to calculate the Fukui function and dual descriptor

Toxicity of persistent organic pollutants: a theoretical study

CONTEXT

Polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) are two families of persistent organic pollutants that are dangerous as they remain in the atmosphere for long periods and are toxic for humans and animals. They are found all over the world, including the penguins of Antarctica. One of the mechanisms that explains the toxicity of these compounds is related to oxidative stress. The main idea of this theoretical research is to use conceptual density functional theory as a theory of chemical reactivity to analyze the oxidative stress that PCBs and PBDEs can produce. The electron transfer properties as well as the interaction with DNA nitrogenous bases of nine PCBs and ten PBDEs found in Antarctic penguins are investigated. From this study, it can be concluded that compounds with more chlorine or bromine atoms are more oxidizing and produce more oxidative stress. These molecules also interact directly with the nitrogenous bases of DNA, forming hydrogen bonds, and this may be an explanation for the toxicity. Since quinone-type metabolites of PCBs and PBDEs can cause neurotoxicity, examples of quinones are also investigated. Condensed Fukui functions are included to analyze local reactivity. These results are important as the reactivity of these compounds helps to explain the toxicity of PCBs and PBDEs.

METHODS

All DFT computations were performed using Gaussian16 at M06-2x/6-311 + g(2d,p) level of theory without symmetry constraints. Electro-donating (ω-) and electro-accepting (ω +) powers were used as global response functions and condensed Fukui functions as local parameters of reactivity.

Prediction of •OH-Initiated and •NO3-Initiated Transformation Products of Polycyclic Aromatic Hydrocarbons by Electronic Structure Approaches

The abiotic reaction products of polycyclic aromatic hydrocarbons (PAHs) with hydroxyl radicals (^•OH) and nitrate radicals (^•NO₃) are nitro-, oxygen-, and hydroxyl-containing PAHs (NPAHs, OPAHs, and OHPAHs). Four methods of the highest occupied molecular orbital (HOMO), Fukui function (FF), dual descriptor (DD), and population of π electrons (PP-π) are selected to predict the chemical reactivity of PAHs attacked by ^•OH and ^•NO₃ in this study. The predicted ^•OH-initiated and ^•NO₃-initiated transformation products are compared with the main PAH transformation products (PAH-TPs) observed in the laboratory. The results indicate that PP-π and DD approaches fail to predict the transformation products of fused PAHs containing five-membered rings. By predicting the PAH-TPs of 13-14 out of the 15 parent PAHs accurately, HOMO and FF methods were shown to be suitable for predicting the transformation products formed from the abiotic reactions of fused PAHs with ^•OH and ^•NO₃.

From the Electron Density Gradient to the Quantitative Reactivity Indicators: Local Softness and the Fukui Function

Important reactivity measures such as the local softness, the Fukui function, and the global hardness have been calculated directly from first principles with the use of the electron density function, beyond the finite difference approximation. Our recently derived density gradient theorem and the principle of nearsightedness of the electronic matter have been instrumental in obtaining the original, albeit approximate, result on the local softness of an atom. By integration of the local softness s(r), we obtain the global softness S and the Fukui function f(r) = s(r)/S. Local and global softness values have also been calculated analytically for the basic hydrogenic orbitals; the general relation to the atomic number S = σZ ^-2 has been demonstrated, with constants σ characteristic for each orbital type. Global hardness η = 1/S calculated for atoms and ions has been favorably tested against its conventional measure given by the finite difference approximation: (I - A). Calculated test results for atoms and ions in rows 1-4 of the periodic table have been presented.

A DFT study of the double (3 + 2) cycloaddition of nitrile oxides and allenoates for the formation of spirobiisoxazolines

The molecular mechanism of the double (3 + 2) cycloaddition (32CA) reaction between nitrile oxides and allenoates has been studied using density functional theory at the M06-2X/6-311G (d,p) level of theory. In the first 32CA, the nitrile oxide adds chemo- and regio-selectively to the C-C double bond of the allenoate closest to the carboxylate group followed by a subsequent regioselective addition to the olefinic bond of the isoxazoline intermediate. The rate constant for the preferred pathway (formation of 4-methylene-2-isoxazoline intermediate) in the reaction of ethyl substituted allenoate and mesitonitrile oxide is 5.3 × 10² s^-1 in THF which is about 13 times faster than the closest competing step (formation of its regioisomer 5-methylene-2-isoxazoline intermediate) which has a rate constant of 4.4 × 10¹ s^-1. Strong electron-donating groups (EDGs) and electron-withdrawing groups (EWGs) decrease activation barriers and hence increase the reaction rate. Also, the dimerization of nitrile oxide to form furaxon is found to be kinetically unfavored.

Structural, electronic, and energetic investigations of acrolein adsorption on B36 borophene nanosheet: a dispersion-corrected DFT insight

The adsorption of acrolein (AC) onto the surface of B₃₆ borophene nanosheet was studied using dispersion-corrected density functional theory (DFT). The structural and electronic properties were scrutinized by several quantum chemical parameters such as HOMO-LUMO gap, condensed Fukui function, molecular electrostatic potential (ESP), and the density of states (DOS). The non-covalent interactions (NCI) were explored by combined reduced density gradient (RDG-NCI) and energy decomposition analysis (EDA) techniques. It was found that the adsorption of acrolein on both convex and concave surfaces of borophene is mainly governed by van der Waals interactions. Our calculations showed that the adsorption energy is strengthened and favored when multiple acrolein molecules adsorb on the edge sides of borophene through their terminal carbonyl oxygen atom. Furthermore, the calculated HOMO-LUMO energy gaps were significantly reduced upon adsorption affecting, therefore, the electrical conductance of borophene. These results should be useful in designing acrolein sensors.

Electrochemical antioxidant screening and evaluation based on guanine and chitosan immobilized MoS2 nanosheet modified glassy carbon electrode (guanine/CS/MoS2/GCE)

In this study, an electrochemical biosensor based on guanine and chitosan immobilized MoS2 nanosheet modified glassy carbon electrode (guanine/CS/MoS2/GCE) was developed and employed for antioxidant screening and antioxidant capacity evaluation. The oxidation peak current of guanine was improved and nearly tripled after modifications of chitosan and MoS2 nanosheet. The immobilized guanine could be damaged by hydroxyl radicals generated in Fenton solution. However, in the presence of antioxidants, the guanine was protected and the oxidation peak current of guanine increased. This process mimics the mechanism of antioxidant protection in human body. The factors affecting preparation of sensor and detection of antioxidant capacity were optimized. At the optimum conditions, the guanine/CS/MoS2/GCE showed wide linear range, low detection limit, satisfactory reproducibility and stability for detection. Ascorbic acid was used as a model antioxidant to evaluate the antioxidant capacity. A good linearity was observed with a correlation coefficient of 0.9959 in the concentrations between 0.5 and 4.0 mg L-1. The antioxidant capacities of three flavonoids were also tested and the rank of antioxidant capacities was ascorbic acid (51.84%), quercetin (45.82%), fisetin (34.39%) and catechin (16.99%). Due to the rapid measurement and low cost, this sensor could provide an available sensing platform for antioxidant screening and evaluation.