Interaction mechanism of water-soluble inorganic arsenic onto pristine nanoplastics.

Nanoplastics (NPLs) persist in aquatic habitats, leading to incremental research on their interaction mechanisms with metalloids in the environment. In this regard, it is known that plastic debris can reduce the number of water-soluble arsenicals in contaminated environments. Here, the arsenic interaction mechanism with pure NPLs, such as polyethylene terephthalate (PET), aliphatic polyamide (PA), polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), and polystyrene (PS) is evaluated using computational chemistry tools. Our results show that arsenic forms stable monolayers on NPLs through surface adsorption, with adsorption energies of 9-24 kcal/mol comparable to those on minerals and composite materials. NPLs exhibit varying affinity towards arsenic based on their composition, with As(V) adsorption showing higher stability than As(III). The adsorption mechanism results from a balance between electrostatics and dispersion forces (physisorption), with an average combined contribution of 87%. PA, PET, PVC, and PS maximize the electrostatic effects over dispersion forces, while PE and PP maximize the dispersion forces over electrostatic effects. The electrostatic contribution is attributed to hydrogen bonding and the activation of terminal O-C, C-H, and C-Cl groups of NPLs, resulting in several pairwise interactions with arsenic. Moreover, NPLs polarity enables high mobility in aqueous environments and fast mass transfer. Upon adsorption, As(III) keeps the NPLs polarity, while As(V) limits subsequent uptake but ensures high mobility in water. The solvation process is destabilizing, and the higher the NPL polarity, the higher the solvation energy penalty. Finally, the mechanistic understanding explains how temperature, pressure, pH, salinity, and aging affect arsenic adsorption. This study provides reliable quantitative data for sorption and kinetic experiments on plastic pollution and enhances our understanding of interactions between water contaminants.

Computational Insights on the Chemical Reactivity of Functionalized and Crosslinked Polyketones to Cu2+ Ion for Wastewater Treatment.

Today, the high concentrations of copper found in water resources result in an urgent problem to solve since human health and aquatic ecosystems have been affected. Functionalized crosslinked polyketone resins (XLPK) have demonstrated high performance for the uptake of heavy metals in water solutions. In addition, its green chemical synthesis makes these resins very attractive as sorbents for metal ions contained in wastewater. XLPK are not soluble in aqueous media and do not require any catalyst, solvent, or harsh conditions to carry out the uptake process. In this paper, a series of functionalized XLPK with pending amino-derivatives namely; butylamine (BA), amino 2-propanol (A2P), 4-(aminomethyl) benzoic acid (HAMC), 6-aminohexanoic acid (PAMBA), and 1,2 diamino propane (DAP) directly attached to the pyrrole backbone of the polymers and crosslinked by di-amine derivatives was investigated using Density Functional Theory (DFT) calculations. Our computational analysis revealed that dipole-dipole interactions played a crucial role in enhancing the adsorption of Cu2+ ions onto XLPKs. The negatively charged ketone moieties and functional groups within XLPKs were identified as key adsorption sites for the selective binding of Cu2+ ions. Additionally, we found that XLPKs exhibited strong electrostatic interactions primarily through the -NH2 and -C=O groups. Evaluation of the adsorption energies in XLPK-Cu(II) complexes showed that the DAP-Cu(II) complex exhibited the highest stability, attributed to strong Cu(II)-N binding facilitated by the amino moiety (-NH2). The remaining XLPKs displayed binding modes involving oxygen atoms (Cu(II)-O) within the ketone moieties in the polymer backbone. Furthermore, the complexation and thermochemical analysis emphasized the role of the coordinator atom (N or O) and the coordinating environment, in which higher entropic effects involved in the adsorption of Cu2+ ions onto XLPKs describes a lower spontaneity of the adsorption process. The adsorption reactions were favored at lower temperatures and higher pressures. These findings provide valuable insights into the reactivity and adsorption mechanisms of functionalized and crosslinked polyketones for Cu2+ uptake, facilitating the design of high-performance polymeric resins for water treatment applications.

Interaction mechanism of triclosan on pristine microplastics.

Urban wastewaters comprise different hydrophobic pollutants such as microplastics (MPs), pharmaceuticals, and personal care products. Among these pollutants, triclosan (TCS) shows a worrying interaction ability with MPs; recent studies show MPs serve as a vector between TCS and aquatic environments, whose interaction is still being studied to understand their combined toxicity and transport ability. Using computational chemistry tools, this work evaluates the TCS-MPs interaction mechanism, including pristine polymers, i.e., aliphatic polyamides (PA), polyethylene (PE), polystyrene (PS), polyvinyl chloride (PVC), and polyethylene terephthalate (PET). Our results show that TCS adsorption on MPs solely occurs via physisorption, where PA reaches the higher adsorption ability. Remarkably, MPs reach higher or comparable adsorption stability than carbon-based materials, boron nitrides, and minerals, indicating their worrying transport properties. Also, the adsorption capacity is strongly influenced by entropy changes rather than thermal effects, which determine the different sorption capacities among polymers and agree well with reported sorption capacities from adsorption kinetic experiments in the literature. MPs show a polar and highly susceptible surface to establish electrostatics and dispersion effects on TCS. Accordingly, the TCS-MPs interaction mechanism arises from the interplay between electrostatics and dispersion forces, with a combined contribution of 81-93 %. Specifically, PA and PET maximize the electrostatic effects, while PE, PP, PVC, and PS maximize the dispersion effects. From the chemical viewpoint, TCS-MPs complexes interact by a series of pairwise interactions such as Van der Waals, hydrogen bonding, C-H⋯π, C-H⋯C-H, C-Cl⋯C-H, and C-Cl⋯Cl-C. Finally, the mechanistic information explains the effects of temperature, pressure, aging, pH, and salinity on TCS adsorption. This study quantitatively elucidates the interaction mechanism of TCS-MP systems, which were hard to quantify to date, and explains the TCS-MPs sorption performance for sorption/kinetic studies.

Mechanistic insights into the adsorption of endocrine disruptors onto polystyrene microplastics in water.

Microplastics and endocrine disruptors (EDs) are contaminants of emerging concerns and ubiquitously present in aquatic ecosystems, establishing interactions that still are the subject of investigation due to their implications in the cotransport of pollutants. Then, we conducted mechanistic studies based on state-of-art computational chemistry methods to quantitatively understand the interaction mechanisms whereby polystyrene micro or nanoplastics (PS-MPs) interact with representative classes of EDs in water (Ethynylestradiol, Estradiol, and Bisphenol A). The results showed that PS-MPs increase their charge distribution when forming microparticles in water, giving a permanent dipole that explains their increasing solubility in aqueous conditions. In agreement with experimental assessments, the PS-MPs favorably adsorb EDs with adsorption energies larger than 15 kcal/mol, even with comparable stability to nanostructured materials for adsorption, removal, and/or analysis of pollutants. The adsorption occurs via physisorption without covalent binding, bond breaking, or structural preparation energies, where the molecular structure of EDs can favor inner or outer surface adsorption depending on the molecular structure of the adsorbates. A balanced contribution of dispersion and electrostatic stabilizing effects determines the interaction mechanisms, accounting for a whole contribution of 88-90%. The electrostatic contribution emerges from the favorable alignment of the PS-MPs and EDs dipoles upon interaction due to the mild charge transfer between them in solution. In contrast, the dispersion contribution emerges from electron-electron interactions due to the permanent dipoles in adsorbates and adsorbents. Furthermore, thermochemical analyses clarify the role of temperature and pressure effects on the relative adsorption stability among EDs in aquatic environments. Therefore, modeling the adsorption process contributes to new knowledge on the sorption properties of PS-MPs, providing a mechanistic basis to understand the cotransport of pollutants in water environments and their impacts on environmental pollution.

The interaction mechanism of polystyrene microplastics with pharmaceuticals and personal care products.

Microplastics (MPs) have been detected in the hydrosphere, with hazardous implications in transporting coexisting water pollutants. Our knowledge about the interaction mechanisms that MPs establish with organic pollutants are still growing, which is essential to understand the adsorption properties of MPs and their relative stability with adsorbates. Here, we used classical (force field methods) and ab-initio (density functional theory) computational chemistry tools to characterize the interaction mechanisms between Polystyrene-MPs (PS-MPs) and pharmaceuticals/personal care products (PPCPs). Adsorption conformations and energies, thermochemistry, binding, and energy decomposition analyses were performed to obtain the quantitative mechanistic information. Our results show that PS-MPs have permanent dipoles, increasing the interaction with neutral PPCPs while repelling the charged pollutants; in all cases, a stable physisorption takes place. Moreover, PS-MPs increase their solubility upon pollutant adsorption due to an increase in the dipole moment, increasing their co-transport ability in aqueous environments. The stability of the PS-MPs/PPCPs complexes is further confirmed by thermochemical and molecular dynamics trajectory analysis as a function of temperature and pressure. The interaction mechanism of high pKa pollutants (pKa > 5) is due to a balanced contribution of electrostatic and dispersion forces, while the adsorption of low pKa pollutants (pKa < 5) maximizes the electrostatic forces, and steric repulsion effects explain their relative lower adsorption stability. In this regard, several pairwise intermolecular interactions are recognized as a source of stabilization in the PS-MPs/PPCPs binding: hydrogen bonding, π-π, OH⋯π, and CH⋯π, CCl⋯CH and CH⋯CH interactions. The ionic strength in solution slightly affects the adsorption stability of neutral PPCPs, while the sorption of charged pollutants is enhanced. This mechanistic information provides quantitative data for a better understanding of the interactions between organic pollutants and MPs, serving as valuable information for sorption/kinetic studies.

Atmospheric microplastics and nanoplastics as vectors of primary air pollutants - A theoretical study on the polyethylene terephthalate (PET) case.

Polyethylene terephthalate (PET) microplastics and nanoplastics are ubiquitously present in the atmosphere as atmospheric and airborne forms (PET-aMPs). Using first-principles calculations, we analyze the uptake of primary air pollutants onto PET-aMPs, focusing on their stabilities, adsorption mechanisms, and thermochemistry. The results show that PET-aMPs are selective for the spontaneous adsorption of CO, CO2, NO, N2O, NO2, NH3, and SO2, reaching stable adsorption energies of 6-20 kcal/mol per molecule, with comparable uptake ability than carbon-based materials, metals/metalloids, and metal oxide surfaces. Then, PET-aMPs become a vector for coexisting air pollutants in the atmosphere, which adsorb by inner or outer adsorption depending on the molecular polarity (dipole moment) and atomic constitution (electronegativity) of gaseous molecules. Also, atmospheric H2O and O2 are not competitive molecules, and ozone could enhance adsorption due to surface oxidation and structure breakdown. The interplay of electrostatic (46-61%) and dispersion forces (21-58%) drives the adsorption mechanism, where low-polar pollutants display almost a balanced electrostatic vs. dispersion contribution, while high polar molecules display a higher electrostatic stabilization. The outer adsorption is reached by strong dispersion, hydrogen bonding, and dipole-dipole-induced pairs, while lone-pair-π interactions appear in the inner adsorption regime. These results expand the understanding of the hazards and risks of atmospheric and airborne microplastics/nanoplastics, their impacts, co-transport ability, and interaction with the environment.

Theoretical Insight into the Effect of Fluorine-Functionalized Metal-Organic Framework Supported Palladium Single-Site Catalyst in the Ethylene Dimerization Reaction.

Ethylene dimerization reaction is one of the most common mechanisms for the production of 1-butene. Recently, metal-organic frameworks (MOFs) have received extensive attention in this area since they combine all the advantages of homogeneous and heterogeneous catalysts in a single compound. Here a computational mechanistic study of MOF-supported palladium single-site catalyst for ethylene dimerization reaction is reported. Catalytic systems with both biphenyl-type backbone as organic ligand and its fluorine-functionalization have been investigated to reveal the origin of ligand effects on the catalytic activity and selectivity. The calculations revealed that the nonfluorinated palladium MOF catalyst undergoes dimerization over isomerization reaction. Then the influence of the fluorine-functionalized organic ligand was compared in the dimerization catalytic cycle, which was strongly favored in terms of activity and selectivity. Catalyst-substrate interactions were analyzed by energy decomposition analysis revealing the critical role of ligand backbone functionalization on the activity. This theoretical analysis identified three chemically meaningful dominant effects on these catalysts; steric, electrostatic and charge transfer effects. The steric effects promote nonfluorinated MOF catalyst, whereas the electrostatic effects are the dominant factor that promotes its fluorinated counterpart. This theoretical study provides feedback with future experimental studies about the role of fluorine ligand functionalization in palladium MOF catalysts for ethylene dimerization reaction.

Description of the Reaction Intermediate Stabilization for the Zimmerman Di-π-methane Rearrangement on the Basis of a Parametric Diabatic Analysis.

The mechanism of the Zimmerman di-π-methane rearrangement has been studied using a parametric diabatic analysis (PDA) on which the diagonal elements on the effective Hamiltonian defining the energies of the diabatic electronic states have been parametrized and modeled upon the use of the vertex form of a parabolic function. The PDA requires two inputs: the energy local minimum of an optimized structure along the intrinsic reaction coordinate and the maximum gradients associated with the barriers for the transition states. In the present work, the PDA was used to gain novel insights into the mechanism of the triplet di-π-methane rearrangement of substituted dibenzobarrelenes. Our results suggest that, when using an electron-withdrawing group as substituent, the activation energy for the rate-determining step is directly modulated by the stabilization of the biradical intermediate on the triplet surface. This mechanistic feature was thoroughly analyzed and discussed within the conceptual framework provided by the diabatic model of intermediate stabilization (DMIS).

Exploring the Nature of Interaction and Stability between Water-Soluble Arsenic Pollutants and Metal-Phosphorene Hybrids: A Density Functional Theory Study.

To search for new uptake platforms for the removal of highly toxic and mobile arsenite [or trivalent arsenic, As(OH)3], we theoretically investigate the adsorption properties of intrinsic and metal-doped phosphorene nanoadsorbents. The doping of phosphorene with Ni or Cu atoms remarkably increases the uptake stability of arsenite at water environments compared to intrinsic phosphorene, with a weak competition of H2O molecules by the adsorption sites, where the adatom doping of phosphorene allows obtaining better uptake performance compared to the substitutional doping. The uptake is explained by a strong inner-sphere surface complexation, which is dominated by permanent electrostatic physical effects. Hydroxide anions show strong competitive adsorption compared to H2O and arsenite; thus, the straightforward recovery of the nanoadsorbents could be reached after removal by treatment at high pH solutions. Therefore, metal-doped phosphorene hybrids could serve as superior nanoadsorbents for arsenic separation from water by adsorption in solid phases.

Theoretical insights into the E1cB/E2 mechanistic dichotomy of elimination reactions.

E1cB and E2 eliminations have been described as competing mechanisms that can even share a common pathway when the E1cB/E2 borderline mechanism operates. A suitable case study evincing such a mechanistic dichotomy corresponds to the elimination reaction of ß-phenylmercaptoethyl phenolate, since its mechanism has been thought to be an E2 elimination. Nonetheless, according to the computational assessment of the substituents on the leaving group, we demonstrate that the reaction proceeds via a borderline E1cB mechanism. Stabilization of the carbanion was provided not only by substituent effects tuning the nucleofugality of the leaving group, but also by a base, since distortion/interaction-activation strain and Natural Bond Order (NBO) analyses suggest a stabilizing interaction between the base and Cß of the E1cB intermediate. In order to gain insights into these results in a more general context, we have rationalized them with a qualitative picture of the E1cB/E2 mechanistic dichotomy using simple relationships between diabatic parabolas modeling the potential wells of reactants, intermediates, and products. In this Diabatic Model of Intermediate Stabilization (DMIS), the borderline E1cB mechanism for the elimination reaction of ß-phenylmercaptoethyl phenolate was discussed in terms of bonding and dynamic stepwise processes. The conceptual model presented herein should be useful for the analysis of any reaction comprising competing one- and two-step mechanisms.

Mechanistic details of ethylene polymerization reaction using methallyl nickel(ii) catalysts.

The mechanism of ethylene polymerization by means of neutral methallyl-Ni(ii) complexes has been studied by quantum chemical calculations. Two isomer complexes having different ligand functionalization at the ortho or para position, and co-activated with trispentafluorephenylborane [B(C6F5)3], were studied according to the Cossee-Arlman's mechanism. Comparison of the reaction mechanism of both isomers shows that energy barriers strongly depend on ligand-functionalization and are mostly due to structural rearrangements. In addition, it was found that para-functionalization can be distinguished by favorable σ-donation whereas ortho-functionalization is more prone to a π back-donation process. Our results concerning the polymerization process for ortho and para isomers not only provide a theoretical perspective of available experimental data, but also explain the experimentally observed higher molecular weight of the methallyl-Ni(ii) ortho isomer co-catalyzed by B(C6F5)3, revealing the role of ligand-functionalization in polyethylene production.

Oxidized and Si-doped graphene: emerging adsorbents for removal of dioxane.

Graphene-based materials have emerged as new potential adsorbents for the adsorption and removal of persistent pollutants, and they could play a key role in the remediation of 1,4-dioxane. In this framework, a quantum chemistry study was carried out to rationalize the sorption properties of oxidized graphene (GO) and Si-doped graphene (SiG) nanosheets for use in 1,4-dioxane removal, taking into account that these adsorbents are experimentally available. Dispersion corrected PBE-D3/SVP calculations show that GO and SiG adsorbs dioxane through non-covalent and covalent interactions, respectively, with adsorption energies of up to ∼0.9 eV, which represents an important improvement with respect to the adsorption onto intrinsic graphene. The adsorption strength was also rationalized in terms of natural bond orbitals, atoms-in-molecules and energy decomposition analyses. In the case of GO, a high content of hydroxyl and carboxyl functional groups enhances the removal efficiency, and they are responsible for the high adsorption stability in aqueous environments and at room temperature (300 K). In addition, explicit/implicit solvent calculations and molecular dynamics trajectories show that the SiG-dioxane interaction is highly stable at 300 K, without pollutant diffusion; besides, the SiG-dioxane interaction is stabilized in the presence of H2O molecules. All the analyses suggest that GO and SiG should be considered as new remarkable candidates for sorption technologies related to the removal, control and remediation of 1,4-dioxane, where the sorption efficiency is sorted as SiG > GO ≫ G.

The Role of Co-Activation and Ligand Functionalization in Neutral Methallyl Nickel(II) Catalysts for Ethylene Oligomerization and Polymerization.

A detailed quantum chemical study that analyzed the mechanism of ethylene oligomerization and polymerization by means of a family of four neutral methallyl NiII catalysts is presented. The role of the boron co-activators, BF3 and B(C6 F5 )3 , and the position of ligand functionalization (ortho or para position of the N-arylcyano moiety of the catalysts) were investigated to explain the chain length of the obtained polymers. The chain initialization proceeded with higher activation barriers for the ortho-functionalized complexes (≈19 kcal mol-1 ) than the para-substituted isomers (17-18 kcal mol-1 ). Two main pathways were revealed for the chain propagation: The first pathway was favored when using the B(C6 F5 )3 co-activated catalyst, and it produced long-chain polymers. A second pathway led to the ß-hydrogen complexes, which resulted in chain oligomerization; this pathway was preferred when the BF3 co-activated catalysts were used. Otherwise, the termination of longer chains occurred via a stable hydride intermediate, which was formed with an energy barrier of about 14 kcal mol-1 for the B(C6 F5 )3 co-activated catalysts. Significant new insights were made into the reaction mechanism, whereby neutral methallyl NiII catalysts act in oligomerization and polymerization processes. Specifically, the role of co-activation and ligand functionalization, which are key information for the further design of related catalysts, were revealed.

The catalytic effect of the NH3 base on the chemical events in the caryolene-forming carbocation cascade.

Caryolene formation occur asynchronously in a concerted way through carbocationic rearrangements involving the generation of a secondary or a tertiary carbocation whether the reaction proceeds in the absence or in the presence of NH3 , respectively. Both caryolene formation mechanisms are analyzed within the general framework of the reaction force; the reaction force constant is used to gain insights into the synchronicity of the mechanisms and the reaction electronic flux helps to characterize the electronic activity taking place during the reaction. DFT calculations at the B3LYP/6-31+G(d,p) level show a clear difference in the mechanisms of the base promoted or base free caryolene formation reactions.

The performance of methallyl nickel complexes and boron adducts in the catalytic activation of ethylene: a conceptual DFT perspective.

In this work, global and local descriptors of chemical reactivity and selectivity are used to explain the differences in reactivities toward ethylene of methallyl nickel complexes and their B(C6F5)3 and BF3 adducts. DFT calculations were used to explain why nickel complexes alone are inactive in ethylene polymerization while their boron adducts can activate it. It is shown that chemical potential, hardness, electrophilicity and molecular electrostatic potential surfaces describe fairly well the reactivity and selectivity of these organometallic systems toward ethylene. Experimental data indicates that addition of a borane molecule to nickel complexes changes dramatically their reactivity-behavior that is confirmed computationally. Our results show that bare complexes are unable to activate ethylene-a Lewis base-because they also behave as Lewis bases. The addition of the co-catalyst-a Lewis acid-turns the adducts into Lewis acids, making them active towards ethylene.

A detailed analysis of the mechanism of a carbocationic triple shift rearrangement.

The mechanism of a carbocationic triple shift rearrangement is analyzed within the conceptual framework of the reaction force. All the systems were characterized computationally using DFT through B3LYP/6-31+G(d,p) methodology. A complete description of the electronic activity taking place during the reaction emerged through the use of the reaction electronic flux that, together with an NBO Wiberg bond order, produces a detailed picture of the reaction mechanism in terms of chemical events that drive the reaction during the different stages of the process. It is found that a carbocation triple shift occurs asynchronously although in a concerted way.

Why do five-membered heterocyclic compounds sometimes not participate in polar Diels-Alder reactions?

The reactions of bicyclic enone (BCE, 1) with cyclopentadiene (Cp, 2) and the five-membered heterocyclic compounds (FHCs) furan 3 and N-methyl pyrrole 4 for the construction of polycyclic heterocyclic compounds have been studied at the B3LYP/6-31G* level. No reaction takes place in the absence of Lewis acid (LA) catalysts as a consequence of the high activation energy associated with these reactions. Electrophilic activation of BCE 1 by formation of a complex with the BF3 LA, 1-BF3, and solvent effects favor the reactions. However, a different reactivity is manifested by Cp 2 and FHCs 3 and 4. Thus, while the reaction of 1-BF3 with Cp 2 yields the expected exo [4 + 2] cycloadduct, the reactions of these FHCs yield Michael adducts. In any case, the reactions are characterized by the nucleophilic/electrophilic interaction between the most nucleophilic centers of these dienes and the most electrophilic center of complex 1-BF3. The greater ability of FHCs 3 and 4 to stabilize positive charges opposed to Cp 2 favors a stepwise mechanism with formation of a zwitterionic intermediate. Although in most stepwise Diels-Alder reactions, the subsequent ring closure has unappreciable barriers, in these FHCs the abstraction of a proton with regeneration of the aromatic ring becomes competitive. Thermodynamic calculations suggest that the exergonic character of the formation of the Michael adducts could be the driving force for the reactions involving FHCs.