Anti-Electrostatic Pi-Hole Bonding: How Covalency Conquers Coulombics

Intermolecular bonding attraction at π-bonded centers is often described as “electrostatically driven” and given quasi-classical rationalization in terms of a “pi hole” depletion region in the electrostatic potential. However, we demonstrate here that such bonding attraction also occurs between closed-shell ions of like charge, thereby yielding locally stable complexes that sharply violate classical electrostatic expectations. Standard DFT and MP2 computational methods are employed to investigate complexation of simple pi-bonded diatomic anions (BO−, CN−) with simple atomic anions (H−, F−) or with one another. Such “anti-electrostatic” anion–anion attractions are shown to lead to robust metastable binding wells (ranging up to 20–30 kcal/mol at DFT level, or still deeper at dynamically correlated MP2 level) that are shielded by broad predissociation barriers (ranging up to 1.5 Å width) from long-range ionic dissociation. Like-charge attraction at pi-centers thereby provides additional evidence for the dominance of 3-center/4-electron (3c/4e) nD-π*AX interactions that are fully analogous to the nD-σ*AH interactions of H-bonding. Using standard keyword options of natural bond orbital (NBO) analysis, we demonstrate that both n-σ* (sigma hole) and n-π* (pi hole) interactions represent simple variants of the essential resonance-type donor-acceptor (Bürgi–Dunitz-type) attraction that apparently underlies all intermolecular association phenomena of chemical interest. We further demonstrate that “deletion” of such π*-based donor-acceptor interaction obliterates the characteristic Bürgi–Dunitz signatures of pi-hole interactions, thereby establishing the unique cause/effect relationship to short-range covalency (“charge transfer”) rather than envisioned Coulombic properties of unperturbed monomers.

Removal Performance and Mechanism of Benzo(b)Fluorathene Using MnO2 Nanoflower/Graphene Oxide Composites

High-ring polycyclic aromatic hydrocarbons (PAHs, Benzo[b]fluorathene (BbFA), etc.) are difficult to biodegrade in the water environment. To address this issue, an innovative method for the preparation of MnO2 nanoflower/graphene oxide composite (MnO2 NF/GO) was proposed for adsorption removal of BbFA. The physicochemical properties of MnO2 NF/GO were characterized by SEM, TEM, XRD, and N2 adsorption/desorption and XPS techniques. Results show that the MnO2 NF/GO had well-developed specific surface area and functional groups. Batch adsorption experiment results showed that adsorption capacity for BbFA was 74.07 mg/g. The pseudo-second-order kinetic model and Freundlich isotherm model are fitted well to the adsorption data. These show electron-donor-acceptor interaction; especially π-π interaction and π complexation played vital roles in BbFA removal onto MnO2 NF/GO. The study highlights the promising potential adsorbent for removal of PAHs.

Biochars derived from bamboo and rice straw for sorption of basic red dyes

The primary purpose of this study is to eliminate Basic Red 46 dye from aqueous solutions utilizing batch experiments by adsorption on biochars prepared from bamboo and rice straw biomass. Biochars prepared from bamboo (B), and rice straw (R) was pyrolyzed at 500°C (B500 and R500). Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FTIR) Spectroscopy, X-ray Diffraction (XRD), and surface area and porosity analyzers were used to characterize the B500 and R500 samples. The characterization results indicated that the biochars possessed an amorphous porous structure with many functional groups consisting primarily of silicates. The adsorption rate of BR46 was evaluated using two kinetic models (pseudo-first-order and pseudo-second-order), and the results indicated that the pseudo-second-order model fitted to the experimental data well (R2>0.99). Nearly 24 h was sufficient to achieve equilibrium with the dye adsorption for the two biochars. R500 had a greater adsorption efficiency than B500. As pH levels increased, the dye’s adsorption capability increased as well. The Langmuir and Freundlich isotherm models were used to investigate the equilibrium behavior of BR46 adsorption, and the equilibrium data fitted well with the Langmuir model (R2>0.99) compared to the Freundlich model (R2>0.89). The maximum adsorption capacities of BR46 are 9.06 mg/g for B500 and 22.12 mg/g for R500, respectively. Additionally, adsorption capacity increased as temperature increased, indicating that adsorption is favored at higher temperatures. The electrostatic interaction is shown to be the dominant mechanism of BR46 adsorption, and BR46 acts as an electron-acceptor, contributing to n-π EDA (Electron Donor-Acceptor) interaction. Thermodynamic parameters for the dye-adsorbent system revealed that the adsorption process is spontaneous and feasible. The values of the adsorption coefficient (Kd) were on the order of 102−103. Kd of R500 was greater than that of B500, indicating that R500 had a greater adsorption capacity. The results showed that R500 could be used as a low-cost alternative adsorbent for removing BR46 from effluents.

Study on the Uptake of Aqueous Zinc Ion Onto a Novel Organic Acid Modified Biosorbent

Alhagi Graecorum (AG) is an invasive plant with a massive/robust root structure that can grow up to 12 feet into the ground. The present study exploited the rich cellulosic content in this ‘AG’ root for the synthesis of a novel biosorbent (‘MA’). This low-cost biosorbent, with high carboxyl content of 447.22 (m. eq /100 g sample) was utilised for aqueous zinc ion sequestration. The surface functional groups and textural characteristics required for an efficient heavy metal binding was identified on ‘MA’ using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Sip isotherm emerged as the model of best fit for equilibrium studies, hence, Zn (II) ion sorption onto ‘MA’ is believed to occur via a hybrid blend of homogenous monolayer and heterogeneous multilayer adsorption. Meanwhile, the Elovich (SNE = 1.0429), intraparticle diffusion (SNE = 1.0205) and pseudo-first-order (SNE = 1.0455) provided the best fitting for 200, 400 and 600 mg/L adsorption system, respectively. The maximum adsorption capacity of 188.67 mg/g was recorded at optimum adsorption conditions, with the predominance of the electrostatic and electron-donor-acceptor interaction mechanism. The abundant surface oxygenous functional groups on ‘MA’ positively influenced its adsorption capacity; thus, making it a promising biosorbent for aqueous Zn (II) uptake.

Bonding in M(NHBMe)2 and M[Mn(CO)5]2 complexes (M=Zn, Cd, Hg; NHBMe=(HCNMe)2B): divalent group 12 metals with zero oxidation state

Quantum chemical studies using density functional theory were carried out on M(NHBMe)2 and M[Mn(CO)5]2 (M=Zn, Cd, Hg) complexes. The calculations suggest that M(NHBMe)2 and M[Mn(CO)5]2 have D2d and D4d symmetry, respectively, with a 1A1 electronic ground state. The bond dissociation energies of the ligands have the order of Zn > Cd > Hg. A thorough bonding analysis using charge and energy decomposition methods suggests that the title complexes are best represented as NHBMe⇆M0⇄NHBMe and Mn(CO)5⇆M0⇄Mn(CO)5 where the metal atom M in the electronic ground state with an ns2 electron configuration is bonded to the (NHBMe)2 and [Mn(CO)5]2 ligands through donor–acceptor interaction. These experimentally known complexes are the first examples of mononuclear complexes with divalent group 12 metals with zero oxidation state that are stable at ambient condition. These complexes represent the rare situation where the ligands act as a strong acceptor and the metal center acts as strong donor. The relativistic effect of Hg leads to a weaker electron donating strength of the 6s orbital, which explains the trend of the bond dissociation energy.

Adsorption of 17α-Ethinyl Estradiol and Bisphenol A to Graphene-Based Materials: Effects of Configuration of Adsorbates and the Presence of Cationic Surfactant

Endocrine-disrupting chemicals (EDCs) have attracted much attention in recent years. Graphene-based materials (GMs) have been deemed as excellent adsorbents for the removal of EDCs. The objective of the present study was to understand how the cationic surfactants (CTAB; cetyltrimethylammonium nitrate) affect the adsorption of EDCs (17α-ethinyl estradiol (EE2) and bisphenol A (BPA)) on graphene oxide (GO), reduced graphene oxides (RGOs), and the few-layered commercial graphene (CG). It was observed that the presence of CTAB showed different effects on the adsorption of EDCs to different GMs. The adsorption of EDCs on GO was enhanced because of the enhanced hydrophobicity of GMs after the adsorption of CTAB and the newly formed hemimicelles by the adsorbed CTAB, which could serve as the partition phase for EDCs. Moreover, the electron donor-acceptor interaction and cation bridging effect of the –NH4+ group of the adsorbed CTAB between EDCs and GMs could also enhance the adsorption of EDCs to GMs. With the increase of the extent of GM reduction, the adsorption enhancement by the presence of CTAB weakened. This could be attributed to the competition and pore blockage effect caused by the adsorbed CTAB. It is worth noting that the enhancement of CTAB on the adsorption of BPA to GMs was more profound than that of EE2. This is likely because the pore blockage effect plays a less important role in the adsorption of BPA due to its smaller molecular diameter and deformable structure.

Sorption of Sulfamethoxazole on Inorganic Acid Solution-Etched Biochar Derived from Alfalfa

The properties of alfalfa-derived biochars etched with phosphoric (PBC) or hydrochloric acid (ClBC) compared with raw materials (BC) were examine in this paper. SEM, FT-IR, XRD, BET and elemental analysis were performed to characterize the micromorphology and chemical structure comprehensibly. The results showed that the porous structure was enhanced, and surface area was increased via etching with inorganic acids. Batch adsorption experiments were performed for sulfamethoxazole (SMX) to biochars. The experimental data showed that modified biochars exhibited higher adsorption capacity for SMX, i.e., the adsorption quantity of ClBC and PBC had risen by 38% and 46%. The impact on pH values suggested that the physisorption, including pore-filling and electrostatic interaction, might be applied to original biochar. In addition, chemisorption also played a role, including hydrogen bonding, π-π electron donor acceptor interaction (π-π EDA), and so on. Furthermore, both pH and coexisting ions also had a certain effect on sorption. Enhancement of the electrostatic attraction between biochar and SMX might also account for the enhanced capacity of SMX at pH < 7, and coexisting ions could decrease the amount of SMX adsorbed onto biochars, mainly because of competition for adsorption sites.

Computational study on optical and NLO properties of Donor-Acceptor interaction molecules containing triazolobenzothiadiazole or benzothiadiazole central acceptor core

In this paper, based on the reported OTTAB, OTTTB and eight newly designed compounds, the DFT/TD-DFT theory is utilized to study the influence of the electron-accepting ability of the central...