Geometrical and Electronic Characteristics of Au<i>_n</i>O₂^– (<i>n</i> = 2–7)

Most density functionals do not properly describe the characteristics of superoxide (O₂^–) (i.e., first two vertical electron detachment energies and the excitation energies of neutralized singlet state) of small even-numbered Au<i>_n</i>O₂^– clusters. However, the second-order Møller–Plesset theory (MP2) shows significant charge transfer from Au cluster anions to oxygen molecule and so provides proper electronic characteristics of superoxide of small even-numbered Au<i>_n</i>O₂^– clusters. This has allowed us to properly describe the properties of even-numbered Au<i>_n</i>O₂^– clusters. Even in the case of odd-numbered Au<i>_n</i>O₂^– clusters, we find that Au₅^– is a chemically O₂-adsorbed singlet state at 0 K, against a commonly accepted physisorbed triplet state. This is further evidenced by our extensive coupled cluster with single, double, and perturbative triple excitations [CCSD­(T)] calculations, including the relativistic effect. However, the entropy effect makes the physisorbed triplet state more stable than the chemisorbed singlet state at higher temperatures, consistent with the experiment. The weak O₂ binding by odd-numbered cluster anions (<i>n</i> = 3, 5, and 7) could be further weakened by the entropic effect, which results in van der Waals complexes at high temperatures. The present study reports the geometrical and electronic characteristics of small Au<i>_n</i>O₂^– (<i>n</i> = 2–7) clusters including isomers, which match the corresponding photoelectron spectra (PES)

Triazine-Based Microporous Polymers for Selective Adsorption of CO₂

Propeller-shaped triazine was used to synthesize microporous polycarbazole materials through an inexpensive FeCl₃-catalyzed reaction using direct oxidative coupling (PCBZ) and extensive cross-linking (PCBZL) polymerization routes. PCBZL has a Brunauer–Emmett–Teller specific surface area of 424 m² g^–1 and shows larger CO₂ uptake (64.1 mg g^–1 at 273 K, 1 atm). Selective adsorption of CO₂ over N₂ calculated using the ideal adsorbed solution theory shows that both PCBZ (125) and PCBZL (148) exhibit selectivity at 298 K, which is significantly higher than PCBZ (110) and PCBZL (82) at 273 K. These values of selectivity are among the highest reported for any triazine-based microporous material. By introducing the electron-rich carbazole structure into the nitrogen fertile triazine-based system, the adsorption enthalpy is increased drastically, which in turn contributes to high selective adsorption values. The larger existing binding energy between CO₂ and propeller specifies more stable and favorable interactions between adsorbent and adsorbate, which transforms into reasonable adsorption capacity at low pressure and eventually high selectivity. These polymeric networks also show moderate working capacity with high regenerability factors. The combination of a simple inexpensive synthesis approach, high thermal/chemical stability, and reasonable selective adsorption make these materials potential candidates for CO₂ storage and separation applications

Anion Binding by Electron-Deficient Arenes Based on Complementary Geometry and Charge Distribution

Extended electron-deficient arenes are investigated as potential neutral receptors for polyanions. Anion binds via σ interaction with extended arenes, which are composed solely of C and N ring atoms and CN substituents. As a result, the positive charge on the aromatic C is enhanced, consequently maximizing binding strength. Selectivity is achieved because different charge distributions can be obtained for target anions of a particular geometry. The halides F^– and Cl^– form the most stable complex with <b>6</b>, while the linear N₃^– interacts most favorably with <b>7</b>. The trigonal NO₃^– and tetrahedral ClO₄^– fit the 3-fold rotational axis of <b>6</b> but do not form stable complexes with <b>5</b> and <b>7</b>. The Y-shaped HCOO^– forms complexes with <b>4</b>, <b>5</b>, and <b>7</b>, with the latter being the most stable. Thus, the anion complexes exhibit strong binding and the best geometrical fit between guest and host, reminiscent of Lego blocks

CO₂ Capturing Mechanism in Aqueous Ammonia: NH₃-Driven Decomposition−Recombination Pathway

Capturing CO₂ by aqueous ammonia has recently received much attention due to its advantages over other state-of-the-art CO₂-capture technology. Thus, understanding this CO₂-capturing mechanism, which has been causing controversy, is crucial for further development toward advanced CO₂ capture. The CO₂ conversion mechanism in aqueous ammonia is investigated using ab initio calculations and kinetic simulations. We show full details of all reaction pathways for the NH₃-driven conversion mechanism of CO₂ with the pronounced effect of microsolvation. Ammonia performs multiple roles as reactant, catalyst, base, and product controller. Both carbamic and carbonic acids are formed by the ammonia-driven trimolecular mechanism. Ammonia in microsolvation makes the formation of carbamic acid kinetically preferred over carbonic acid. As the concentration of CO₂ increases, the dominant product becomes carbonic acid. The conversion from carbamic acid into carbonic acid occurs through the decomposition−recombination pathway. This understanding would be exploited for the optimal CO₂ capture technology

Halogen−π Interactions between Benzene and X₂/CX₄ (X = Cl, Br): Assessment of Various Density Functionals with Respect to CCSD(T)

Various types of interactions between halogen (X) and π moiety (X−π interaction) including halogen bonding play important roles in forming the structures of biological, supramolecular, and nanomaterial systems containing halogens and aromatic rings. Furthermore, halogen molecules such as X₂ and CX₄ (X = Cl/Br) can be intercalated in graphite and bilayer graphene for doping and graphene functionalization/modification. Due to the X−π interactions, though recently highly studied, their structures are still hardly predictable. Here, using the coupled-cluster with single, double, and noniterative triple excitations (CCSD­(T)), the Møller–Plesset second-order perturbation theory (MP2), and various flavors of density functional theory (DFT) methods, we study complexes of benzene (Bz) with halogen-containing molecules X₂ and CX₄ (X = Cl/Br) and analyze various components of the interaction energy using symmetry adapted perturbation theory (SAPT). As for the lowest energy conformers (S1), X₂–Bz is found to have the T-shaped structure where the electropositive X atom-end of X₂ is pointing to the electronegative midpoint of CC bond of the Bz ring, and CX₄–Bz has the stacked structure. In addition to this CX₄–Bz (S1), other low energy conformers of X₂–Bz (S2/S3) and CX₄–Bz (S2) are stabilized primarily by the dispersion interaction, whereas the electrostatic interaction is substantial. Most of the density functionals show noticeable deviations from the CCSD­(T) complete basis set (CBS) limit binding energies, especially in the case of strongly halogen-bonded conformers of X₂–Bz (S1), whereas the deviations are relatively small for CX₄–Bz where the dispersion is more important. The halogen bond shows highly anisotropic electron density around halogen atoms and the DFT results are very sensitive to basis set. The unsatisfactory performance of many density functionals could be mainly due to less accurate exchange. This is evidenced from the good performance by the dispersion corrected hybrid and double hybrid functionals. B2GP-PLYP-D3 and PBE0-TS­(Tkatchenko-Scheffler)/D3 are well suited to describe the X−π interactions adequately, close to the CCSD­(T)/CBS binding energies (within ∼1 kJ/mol). This understanding would be useful to study diverse X−π interaction driven structures such as halogen containing compounds intercalated between 2-dimensional layers