Stability Changes in Iridium Nanoclusters via Monoxide Adsorption: A DFT Study within the van der Waals Corrections.

Small iridium nanoclusters are prominent subnanometric systems for catalysis-related applications, mainly because of a large surface-to-volume ratio, noncoalescence feature, and tunable properties, which are completely influenced by the number of atoms, geometry, and molecular interaction with the chemical environment. Herein, we investigate the interaction between Irn nanoclusters (n = 2-7) and polluting molecules, CO, NO, and SO, using van der Waals D3 corrected density functional theory calculations. Starting from a representative structural set, we determine the growth pattern of the lowest energy unprotected Irn nanoclusters, which is based on open structural motifs, and from the adsorption of a XO (X = C, N, and S) molecule, the preferred high-symmetric adsorption sites were determined, dominated by the onefold top site. For protected systems, 4XO/Ir4 and 6XO/Ir6, we found a reduction in the total magnetic moment, while the equilibrium bonds of the nanoclusters expanded (contracted) due to mCO and mNO (mSO) adsorption, with exceptions for systems with large structural distortions (4SO/Ir4 and 6NO/Ir6). Meanwhile, the C-O and N-O (S-O) bond strength decreases (increases) following an increase (decrease) in the C-O and N-O (S-O) distances upon adsorption. We show, through energetic analysis, that for the different chemical environments, relative stability changes occur from the most stable unprotected nanoclusters, planar square (Ir4), and prism (Ir6) to higher energy isomers. The change in the stability order between the two competing protected systems is feasible if the balance between the interaction energy (additive term) and distortion energies (nonadditive terms) compensates for the relative total energies of the unprotected configurations. For all systems, the interaction energy is the main reason responsible for stability alterations, except for 4SO/Ir4, where the main contribution is from a small penalty due to Ir4 distortions upon adsorption, and for 4NO/Ir4, where the energetic effects from the adsorption do not overcome the difference between the binding energies of the unprotected nanoclusters. Finally, from energy decomposition and Hirshfeld charge analysis, we find a predominant covalent nature of the physical contributions in mOX···Irn interactions with a cationic core (Irn) and an anionic shell (XO coverage).

Bare versus protected tetrairidium clusters by density functional theory.

The tetrairidium (Ir4) clusters are subnanometric systems vastly applied in catalysis, especially, because of the higher activity than mononuclear Ir complexes, intrinsic and controllable stability in relation to supports, and non-coalescence properties. The main catalytic properties of nanoclusters (activity and selectivity) are directly associated with their size, shape, and interactions with the environment, whose understanding requires study at the atomistic level. Here, the Ir4 clusters are studied considering the energetic stability for different chemical environments, bare versus protected, using density functional theory calculations within the generalized gradient approximation with van der Waals corrections and spin-orbit coupling, employing the all-electron projected augmented wave method. The square planar isomer is confirmed for the bare case as the lowest energy configuration considering semilocal and non-local exchange-correlation functionals, however, for different chemical environments (Ir4 protected by CO, O2, PH3, and SH2 ligands) the energy stability scenario is different; for CO, O2, and PH3 ligands the tetrahedron is the most stable isomer, in agreement with experimental insights, while for SH2 ligands the square motif is the most stable isomer. To improve the understanding of these systems, structural and electronic analysis were performed, in addition to energy decomposition analysis, to explore the bonding situation in Ir4 compounds. Our results showed an important relationship between the geometrical behavior and the nature and magnitude of Ir2Ir2 interactions, showing how the chemical environment affects the Ir4 nanoclusters. In general, the compounds with tetrahedron motifs showed a weakening of the σ and π bonds in relation to the square ones.

Tuning Heterocalixarenes to Improve Their Anion Recognition: A Computational Approach.

We have explored and analyzed the physical factors through which noncovalent interactions in anion sensing based on calixarene-type hosts can be tuned, using dispersion-corrected DFT and Kohn-Sham molecular orbital (KS-MO) theory in conjunction with a canonical energy decomposition analysis (EDA). We find that the host-guest interaction can be enhanced through the introduction of strongly electron-withdrawing groups at particular positions of the arene and triazine units in the host molecule as well as by coordination of a metal complex to the arene and triazine rings. Our analyses reveal that the enhanced anion affinity is caused by increasing the electrostatic potential in the heterocalixarene cavities. This insight can be employed to further tune and improve their selectivity for chloride ions.

Ultrastructural analysis of salivary glands in a phytophagous stink bug revealed the presence of unexpected muscles.

The exceptional abilities of stink bugs (Hemiptera: Pentatomidae) to colonize a diverse group of plants have been attributed to the feeding behaviors and the functions of the salivary complex of these insects. Here, we describe the ultrastructure of the salivary glands of the Neotropical brown stink bug, Euschistus heros, which is a major component of the pentatomid pest complex on soybeans, Glycine max, in the neotropics. Our results revealed a salivary gland complex consisting of two lobes (i.e., anterior and posterior), with a constriction between them (i.e., the hilum), in which the salivary and accessory gland ducts are inserted. The principal gland epithelium has a single layer of cells lining an enlarged lumen filled with saliva, and these cells are cuboidal, rich in rough endoplasmic reticulum and secretory vesicles, with well-developed nuclei, all of which are typical features of protein-secreting cells. We report, for the first time in insects, the presence of a layer of muscle cells surrounding the columnar hilum epithelium. The accessory salivary gland cells are cuboidal with nuclei containing condensed chromatin and cytoplasm rich in vacuoles and rough endoplasmic reticulum, indicating the potential involvement of these glands in water transport/secretion. The lumen content of each lobe of the principal gland suggests that the lobes produce different compounds. Thus, our results suggest that the E. heros salivary complex might have unconventional mechanisms to mix/release saliva, which might help explain the polyphagous abilities of these insects.

Competition between the phytophagous stink bugs Euschistus heros and Piezodorus guildinii in soybeans.

BACKGROUND: The abundance and contribution of the neotropical brown stink bug, Euschistus heros (F.), and the redbanded stink bug, Piezodorus guildinii (West.), to the composition of insect pests of soybean, Glycine max (L.), fields have changed both spatially and temporally in neotropical soybean production areas. Therefore, we assessed the competitiveness of each species in direct competition experiments following an additive series. We performed mixed (adult) insect infestations in soybean plants and evaluated the fitness of each species and the soybean yield. RESULTS: While the competitive ability of E. heros was significantly compromised by increments in conspecifics and heterospecifics (i.e. P. guildinii), the competitive ability of P. guildinii was compromised by the presence of heterospecifics (i.e. E. heros). The reproductive output of P. guildinii remained unaffected by increments in E. heros or of P. guildinii. Intriguingly, despite the fact that P. guildinii apparently lost the competition with E. heros, almost no pod production was observed in any plant colonised by the former. CONCLUSIONS: The higher abundance of E. heros in neotropical soybean fields seems to result from higher competitive ability than its heterospecific competitor P. guildinii, which may prevent the higher losses caused by P. guildinii. © 2016 Society of Chemical Industry.