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Covalent organic frameworks (COFs) provide a unique platform with tunable structures allowing precise control of pore sizes, shapes and functions. The key to synthesizing COFs with desired structures is to precisely control the conformation and geometry of building blocks as well as the growth direction of COFs. To achieve this, steric effects are noteworthy that may have a significant impact on the assembly of COFs. Specifically, the introduction of sterically demanding substituents or bulky groups into monomers of COFs will lead to intramolecular conformational changes and intermolecular repulsions, which induce structural changes in COFs, including changes in torsion angles, interlayer distances, stacking modes and topologies of 2D COFs, and changes in spatial nodes, interpenetration and topologies of 3D COFs. This review will help to understand the impacts of steric effects on the structures of COFs and to take them into extensive consideration in the design and synthesis of COFs with novel functionalities and structural attributes.

The effect of the lithium salt LiTFSI molality into the PEO-based electrolyte on the ionic conductivity was investigated in this work. To begin, the experimental evolution of LiTFSI-PEO electrolyte's ionic conductivity as a function of molality was analyzed, and hypotheses were put out to explain how ionic transport occurs in a heterogeneous microstructure of polymer electrolytes. To forecast the ionic conductivity in the electrolyte LiTFSI-PEO as a function of molality, an empirical mathematical model was then proposed, taking the phenomena predicted to occur into account. The proposed theoretical model presupposes that lithium salt's solvation state (total or partial solvation) and the steric effect brought on by heterogeneous areas are related to the conduction of ions (at the microscopic scale). The model is adjusted to accurately depict the entire experimental curve, which is thought to have three distinct domains. In the first domain, where the molality m is less than 1 mol/kg, the oxygen atoms completely solvate the lithium salt. The conduction is believed to be favorable in this situation since the released solvated lithium ions linearly fluctuate against the molality. The partial solvation of lithium corresponds to the second domain (1 mol/kg ≾ m ≾ 2 mol/kg), where the conductivity of the polymer electrolyte slightly increases with increasing molality. In the third domain, where m is greater than 2 mol/kg, the loss in ionic conductivity is caused by steric effects, as some of the lithium salt (LiTFSI) does not ionize and becomes immobilized, obstructing the transport pathway. Finally, the model was expanded to include how temperature affects ionic conductivity. The prediction model was effectively validated when it was put up against the findings of experiments conducted by various authors. Graphical abstract

Molecular structures of siloxane materials should be highly controlled for achieving advanced functionalities. However, it is still difficult to precisely control the structure of siloxane materials by the sol–gel processing. In the present study, we focused on the silanol groups in the intermediate oligomers and resultant siloxane materials as a key structural unit for controlling the molecular structure. Thermal stability and chemical reactivity of silanol groups were found to be highly dependent on the steric effects of the surrounding side chains and siloxane skeletons. The present work suggests that controlling the steric effects around silanol groups in the intermediate oligomers allows modulating the crosslink density of siloxane skeletons. The selective molecular modification tunes the structure and chemical properties of the resultant siloxane materials. Highlights Hydroxyl groups in oraganically modified siloxane oligomers exhibit a diffrent reactivity depending on the local environments. Molecular structures of siloxane materials should be highly controlled for achieving advanced functionalities. The selective molecular modification tunes the structure and chemical properties of the resultant siloxane materials.

Naphthyl-a-diimine nickel complexes with systematically varied ligand sterics, activated by modified methylaluminoxane (MMAO), were tested in the polymerization of higher a-olefin (1-hexene, 1-decene and 1-hexadecene) under suitable conditions. The polymerization results indicated the possibility of precise microstructure control, depending on catalyst structure, polymerization temperature, monomer concentration and types of monomers, which in turn strongly affects the resultant polymer properties. Naphthyl-a-diimine nickel complex bearing chiral bulky sec-phenethyl groups in the o-naphthyl position showed good catalytic activity, and resulted in branched polymers (42-88/1000C) with high molecular weights (Mn: (4.3-15.2) × 104 g·mol-1) and narrow molecular weight distribution (Mw/Mn = 1.13-1.29, RT), which suggested a living polymerization. The increasing steric hindrance of catalyst leads to enhance insertion for 2,1-insertion of a-olefin and the chain-walking reaction.

The classical Poisson-Boltzmann model can only work when ion concentrations are very dilute, which often does not match the experimental conditions. Researchers have been working on the modification of the model to include the steric effect of ions, which is non-negligible when the ion concentrations are not dilute. Generally the steric effect was modeled to correct the Helmholtz free energy either through its internal energy or entropy, and an overview is given here. The Bikerman model, based on adding solvent entropy to the free energy through the concept of volume exclusion, is a rather popular steric-effect model nowadays. However, ion sizes are treated as identical in the Bikerman model, making an extension of the Bikerman model to include specific ion sizes desirable. Directly replacing the ions of non-specific size by specific ones in the model seems natural and has been accepted by many researchers in this field. However, this straightforward modification does not have a free energy formula to support it. Here modifications of the Bikerman model to include specific ion sizes have been developed iteratively, and such a model is achieved with a guarantee that: (1) it can approach Boltzmann distribution at diluteness; (2) it can reach saturation limit as the reciprocal of specific ion size under extreme electrostatic conditions; (3) its entropy can be derived by mean-field lattice gas model.

Hydrogen has the potential to serve as a new energy resource, reducing greenhouse gas emissions that contribute to climate change. Natural hydrogenases exhibit impressive catalytic abilities for hydrogen production, but they often lack oxygen tolerance. Oxygen-tolerant hydrogenases can work under oxygen by reacting with oxygen to form inactive states, which can be reactivated to catalytic states by oxygen atom removal. Herein, we synthesized three NiFeSe complexes: (NiSe(CH3)FeCp, NiSe(CH3)FeCp* and NiSe(PhNMe2)FeCp) with features of active sites of [NiFeSe]-H2ases, which are the oxygen-tolerant hydrogenases, and we investigated the influence of electronic and steric factors on the oxygen reaction of these “biomimetic” complexes. In our research, we found that they react with oxygen, forming 1-oxygen species, which is related to the O2-damaged [NiFeSe] active site. Through a comparative analysis of oxygen reactions, we have discovered that electronic factors and steric hindrance on Se play a significant role in determining the oxygen reactivity of NiFe complexes related to hydrogenases’ active sites.

The electrostatic attraction between ions and water is the primary reason for the change in ion bare diameter, which plays a crucial role in saltwater transportation. This study utilizes molecular dynamics (MD) to analyze saltwater transport through a nanoporous graphene membrane by pressure-driven flow. In this work, we describe the impact of pore diameter atomic boundary position on single-ion transportation and signify the steric effect of ions on the water mass flow rate and velocity profile. Due to hydration layer formation, ions hinder the water molecules from their regular velocity, which also decreases the flow rate of water molecules. Interestingly, a significant deviation for different atomic boundary positions is observed for ion rejection for pore diameters less than 1 nm. However, for larger pore diameters, the ion rejection closely matches the atomic boundary position specified by a 2 % water density drop inside the nanopore.

[M4O4] (M = 3d transition metal) represents an interesting class of compounds featuring cubane-type molecular structures, and particularly, [Mn4O4] cubanes or their derivatives attract much attention by virtue of their potential applications as single-molecule magnets (SMMs) or catalysts. However, the rational design of desired cubane-related structures is still a challenging subject due to the lack of readily available methods to effectively tune the construction patterns of the molecule assembly. In this work, we report the employment of different alcohols to prepare three cubane-related molecules, Mn2(dhd)4(iPrOH)2 (1), Mn4(dhd)4(OEt)4(EtOH)4 (2) and Mn4(dhd)6(OMe)2(MeOH)2 (3) (dhd = 5,5-dimethyl-2,4-hexanedione). Interestingly, the bulkiest iPrOH leads to simple rhombic dimer molecule 1. It can be deemed a rudimentary structure oftetranuclear [Mn4O4] cubane 2, which can be realized by the use of less bulky EtOH. In addition, the least bulky MeOH promotes the assembly of the cubanes, eventually bringing about defective dicubane molecular cluster 3. The accurate crystal structures of 1–3 were modeled by single-crystal X-ray diffraction, and their electronic structures were investigated through absorption spectroscopy coupled with theoretical calculations. Overall, this work demonstrates a systematic study on controlling cubane-type structures of Mn-based compounds by applying different solvents, which provides a new means to design functional molecules for specific applications.

The present work analyzes the combined viscoelectric and steric effects on electroosmotic flow in a soft channel with polyelectrolyte coating. The structured channel surface, which controls the electric potential, creates two different flow regions: the electrolyte flow within the permeable polyelectrolyte layer (PEL) and the bulk electrolyte. Thus, this study discusses the interaction of various electrostatic effects to predict the electroosmotic flow field. The nonlinear governing equations describing the fluid flow are the modified Poisson–Boltzmann equation for the electric potential distribution, the mass conservation equation, and the modified Navier–Stokes equations for the flow field, which are solved numerically using a one-dimensional (1D) scheme. The results indicate that the flow enhances when increasing the electric potential magnitude across the channel cross-section via the rise in different dimensionless parameters, such as the PEL thickness, the steric factor, and the ratio of the electrokinetic parameter of the PEL to that of the electrolyte layer. This research demonstrates that the PEL significantly enhances control over electroosmotic flow. However, it is crucial to consider that viscoelectric effects at high electric fields and the friction generated by the grafted polymer brushes of the PEL can reduce these benefits.

The reactions of newly designed lithiated triamidoamines Li3LR (R = iPr, Pen, and Cy2) with VCl3(THF)3 under N2 yielded dinitrogen–divanadium complexes with a μ-N2 between vanadium atoms [V(LR)2(μ-N2)] (R = iPr (1) and Pen (2)) for the former two, while not dinitrogen–divanadium complexes but a mononuclear vanadium complex with a vacant site, [V(LCy2)] (R = Cy2 (3)), were obtained for the third ligand. The V–NN2 and N–N distances were 1.7655(18) and 1.219(4) Å for 1 and 1.7935(14) and 1.226(3) Å for 2, respectively. The ν(14N–14N) stretching vibrations of 1 and 2, as measured using resonance Raman spectroscopy, were detected at 1436 and 1412 cm–1, respectively. Complex 3 reacted with potassium metal in the presence of 18-crown-6-ether under N2 to give a hetero-dinuclear vanadium complex with μ-N2 between vanadium and potassium, [VK(LCy2)(μ-N2)(18-crown-6)] (4). The N–N distance and ν(14N–14N) stretching for 4 were 1.152(3) Å and 1818 cm−1, respectively, suggesting that 4 is more activated than complexes 1 and 2. The complexes 1, 2, 3, and 4 reacted with HOTf and K[C10H8] to give NH3 and N2H4. The yields of NH3 and N2H4 (per V atom) were 47 and 11% for 1, 38 and 16% for 2, 77 and 7% for 3, and 80 and 5% for 4, respectively, and 3 and 4, which have a ligand LCy2, showed higher reactivity than 1 and 2.