Influence Of Steric Effects Research Articles

Electroosmotic flow in graphene nanochannels considering steric effects

Graphene nanochannels are excellent channels for electroosmotic flow (EOF) due to their larger slip length. In this study, the fully developed EOF in graphene nanochannels is investigated numerically, where the influence of surface charge mobility on the Navier-slip boundary conditions and the influence of steric effect on the electric potential distribution are considered. In addition, an analytical solution is provided for the scenario with low zeta potential. Detailed investigations are conducted on the impact of slip length, surface charge density, surface charge mobility, effective ion size, solution concentration, and channel height on velocity profiles. The findings indicate that the velocity increases with slip length, surface charge density, and effective ion size. Yet, accounting for surface charge mobility (αs = 0.815) leads to a reduction in slip velocity. It is noteworthy that our investigation focuses on quantifying the velocity decline due to surface charge mobility, as well as the velocity enhancement resulting from the steric effect. By adjusting parameters, such as channel height, bare slip length, and solution concentration, we achieve a maximum velocity increase of approximately 48%. These insights are valuable for optimizing the design of efficient electro-osmotic pumping systems.

Influence of Steric Effects on the Emission Behavior of Pyrene-Based Blue Luminogens.

Pyrene-based derivatives have been widely deployed in organic luminescent materials because of their bright fluorescence, high charge carrier mobility, and facile modification. Nevertheless, the fluorescence output of conventional pyrenes is prone to quenching upon aggregation due to extensive intermolecular π-π stacking interactions. To address this issue, a set of new Y-shaped pyrene-containing luminogens are synthesized from a new bromopyrene chemical precursor, 2-hydroxyl-7-tert-butyl-1,3-bromopyrene, where the bromo and hydroxyl groups at the pyrene core can be readily modified to obtain the target products and provide great flexibility in tuning the photophysical performances. When the hydroxy group at the 2-position of pyrene was replaced by a benzyl group, the steric hindrance of the benzyl group not only efficiently inhibits the detrimental intermolecular π-π stacking interactions but also rigidifies the molecular conformation, resulting in a narrow-band blue emission. Moreover, the TPE-containing compounds 2c and 3c possessed characteristic aggregation-induced emission (AIE) properties with fluorescence quantum yields of up to 66% and 38% in the solid state, respectively. Thus, this article has methodically investigated the factors influencing the optical behavior, such as intermolecular interactions, and the steric effects of the substituent group, thereby opening up the potential to develop narrow-band pyrene-based blue emitters for OLED device applications.

Role of Steric Effects on Rates of Hydrogen Atom Transfer Reactions.

The influence of steric effects on the rates of hydrogen atom transfer (HAT) reactions between oxyradicals and alkanes is explored computationally. Quantum chemical density functional theory computations of transition states show that activation barriers and reaction enthalpies are both influenced by bulky substituents on the radical but very little by substituents on the alkane. The activation barriers remain roughly correlated with reaction enthalpies via the Evans-Polanyi relationship even when steric repulsion effects become important, although dispersion effects sometimes stabilize transition states. By making comparisons to previously developed Evans-Polanyi and modified Roberts-Steel relationships, we find that HAT reactions between bulky molecules remain well-described by these relationships.

Heat transfer and entropy generation analysis of electroosmotic flows in curved rectangular nanochannels considering the influence of steric effects

The heat transfer characteristics and entropy generation of an electroosmotic flow in curved rectangular nanochannels under high zeta potential conditions are investigated numerically with consideration of the steric effects. The numerical solutions of the Nusselt number (Nu) and the local entropy generation rate (SG) are derived under constant wall heat flux assumption. The variation trends of Nu and SG with corresponding parameters, including the curvature ratio of the curved nanochannel (δ), the effective ionic size (a0) and the wall heat flux (qw) are investigated. Results reveal that, for small values of qw (qw < 10), the consideration of the steric effects (corresponding to a non zero assumption for a0) leads to a lower SG and an enhanced Nu simultaneously. In this case, to further reduce entropy generation, a curved nanochannel with a large curvature ratio δ is adopted. However, for relatively large values of qw, the steric effects will have a negative effect on entropy generation minimization, especially for a curved nanochannel with large curvature ratio. Our results may be useful in designing the efficient thermal nano-equipment.

The infrared spectra of amides. Part 1. The stretching vibrations of primary carboxamides

The infrared spectra of more than 150 primarycarboxamides in the NH stretching and the carbonyl region have been measured. Comparison is made with Bellamys analysis of the stretching modes of amines. The offset to higher frequencies of amides compared with amines is attributed to the widening of the bond angle. The symmetric and antisymmetric stretching of the NH2 bonds is analysed via a rearrangement of the Linnett equations. It is shown thereby that changes of bond strength vary the symmetric and antisymmetric frequencies approximately proportionately whilst deviations from that relationship are due either to changes of the bond angle, steric hindrance or to hydrogen bonding. The spectral changes of m- and p-substituted amides are correlated with Hammett sigma functions, both in the carbonyl and the NH2 stretching region. Intensities and the influence of steric effects are also discussed.

Influence of Steric Effect on the Pseudo-Multicomponent Synthesis of N-Aroylmethyl-4-Arylimidazoles.

A pseudo-three-component synthesis of N-aroylmethylimidazoles 3 with three new C–N bonds formed regioselectively under microwave conditions was developed. Products were obtained by reacting two equivalents of aroylmethyl bromide (ArCOCH2Br, 1) with the appropriate amidine salt (RCN2H3.HX, 2) and with K2CO3 as a base in acetonitrile. The bicomponent reaction also occurred, giving the expected 4(5)-aryl-1H-imidazoles 4. Notably, the ratio of products 3 and 4 is governed by steric factors of the amidine 2 (i.e., R = H, CH3, Ph). Therefore, a computational study was carried out to understand the reaction course regarding product ratio (3/4), regioselectivity, and the steric effects of the amidine substituent group.

Immortal polymerization of LA: the influence of steric effects, electronic effects and pKaon chain transfer agents

In this paper, we systematically investigated the iROP of l-LA based Ca[N(SiMe3)2]2(THF)2/CTAs binary catalytic systems for the first time. The steric effect, electronic effect and pKa of CTAs all had great influences on the iROP of LA.

Steric Effects on Space Electroosmotic Thrusters in Soft Nanochannels

The influence of steric effects on the performances of space electroosmotic thrusters (EOTs) was numerically delineated in soft nanochannels for which its walls are covered with polyelectrolyte materials. The size effect of the ionic species, namely the steric effect, is neglected in many previous research studies, but it has vital influences on electrostatic potential and electroosmotic velocity, which is further introduced into the present study in order to understand and improve the exploration of nano electroosmotic thrusters with soft channels. The thruster’s thrust, specific impulse, total input power, thruster efficiency and thrust-to-power ratio are computed based on finite difference methods. It is found that the thruster’s thrust and specific impulse increase with the steric parameter while the efficiency and thrust-to-power ratio possess opposite trends due to the enhancement of Joule heating dissipation. For real situations with the consideration of ion size, although the thruster’s thrust could be promoted, the efficiency is only 30–70%, and the peak values of thrust-to-power ratio fade away.

Influence of steric effects on the kinetics of cyclic-carbonate vegetable oils aminolysis

Polyurethane polymer is more and more famous because of its versatile properties. The current production process requires the use of isocyanate, which is a hazardous chemical. An alternative eco-friendly route of production is the aminolysis reaction between a cyclic-carbonate, from carbonated vegetable oil, and a diamine. Nevertheless, the kinetics of this reaction system is slow, which might be due to steric hindrance. In order to predict and evaluate this steric hindrance, different amines were tested for the aminolysis of carbonated methyl oleate. Four amines were used: n-butylamine, methylbutylamine, ethylbutylamine and dibutylamine. Kinetic models for the aminolysis with each amine were developed. A relationship between structure and reactivity of each substituent was found through the concept of Linear Free Energy Relationships.

Networking and Dynamic Switches in Biological Condensates

Cellular liquid-liquid phase separation (LLPS) plays a key role in the dynamics and function of RNA-protein condensates like stress granules. In this issue ofCell, Yang et al., Guillén-Boixet et al., and Sanders et al. use a combination of experiment and modeling to provide an exciting mechanistic insight into the relationship between stress granules and LLPS, for example, in the context of protein disorder, switchable interactions, graph theory, and multiple interacting dense phases.

A theoretical study of Fe(PMe3)4-catalyzed anti-Markovnikov addition of aromatics to alkenes to provide linear alkylation products

The mechanisms and regioselectivities of Fe(0)-catalyzed alkylation of aromatic compounds with alkenes were explored by DFT calculations. Our calculations show that these systems tend to undergo anti-Markovnikov addition. The influence of steric effects on the reaction mechanism has been investigated. The results indicate that the reaction is more likely to provide the linear product with dissociation of the 1PMe3 ligand. The reductive elimination is the rate-determining step of the whole process, such that the electrostatic interactions of the reaction site and the steric hindrance of the trimethylsilyl groups are favorable for the anti-Markovnikov rather than the Markovnikov addition pathway. Our calculations provide insights into the regioselective origin of the alkylation of aromatic compounds mediated by Fe(PMe3)4.

Influence of steric effects in solid-phase aza-peptide synthesis

Aza-peptides are peptide analogs with potential applications as drug candidates. However, due to difficulties associated with the synthesis of these compounds, information regarding their bioactivity is very limited. Herein, we identify steric hindrance as one reason for the slowness of the aza-peptide bond formation reaction. The steric effect of the side group of amino acids in their coupling with the semicarbazide moiety in the synthesis of a model peptide, H-AA-AzAla-Phe-NH2, was studied and quantified using COMU as a coupling reagent. Characterization of the role of this structural factor in aza-peptide bond synthesis is essential for outlining a new and efficient synthesis protocol.

Influence of Steric Effect on the Rapid Sea Level Rise at Jeju Island, Korea

ABSTRACT Lim, C.; Park, S.-H.; Kim, D.-Y.; Woo, S.-B., and Jeong, K.-Y., 2017. Influence of steric effect on the rapid sea level rise at Jeju Island, Korea. In: Lee, J.L.; Griffiths, T.; Lotan, A.; Suh, K.-S., and Lee, J. (eds.), The 2nd International Water Safety Symposium. Journal of Coastal Research, Special Issue No. 79, pp. 189–193. Coconut Creek (Florida), ISSN 0749-0208. According to a report by the Korea Hydrographic and Oceanographic Agency (KHOA), the sea level rise in west coast (1.31 mm/yr), east coast (2.69 mm/yr) and south coast (2.05 mm/yr) of Korea are different. Especially, the seas around Jeju Island show one of the most rapid increase of sea level rise in recent years. This investigation on the data from Jeju and Seogwipo tidal station indicated that the sea level rises around Jeju and near Seogwipo were 5.63 mm/yr and 3.75 mm/yr, respectively. This shows that the northern and southern parts of the island have different temporal rates of sea level rise. The purpose of this research is t...

The influence of steric effects on the kinetics and mechanism of SNAr reactions of 1‐phenoxy‐nitrobenzenes with aliphatic primary amines in acetonitrile

AbstractRate constants are reported for the reactions of 1‐phenoxy‐dinitrobenzenes, 3, 1‐phenoxy‐dinitrotrifluoromethylbenzenes, 4, with n‐propylamine, and 1‐methylheptylamine in acetonitrile as solvent. The results are compared with results reported previously for n‐butylamine, pyrrolidine, and piperidine. Decreasing ring activation leads to lower values of k1 for nucleophilic attack although this may be mediated by reduced steric congestion around the reaction centre. Specific steric effects, leading to rate retardation, are noted for the ortho‐CF3 group. In general, reactant‐bearing ortho‐CF3 group were subject to base catalysis irrespective of the amine nucleophile and values of kAm/k−1 are reduced as the size of the amine get bulkier. This is likely to reflect increases in values of k−1 coupled with decreases in values of kAm as the proton transfer from zwitterionic intermediates to catalysing amine becomes less thermodynamically favourable.

Further insight into the electrocatalytic water oxidation by macrocyclic nickel(ii) complexes: the influence of steric effect on catalytic activity

Macrocyclic nickel(ii) complexes with axially oriented methyl groups can impose a steric effect on the axial position of the in situ formed NiIII center, which results in higher NiIII/II oxidation potentials and suppresses the axial coordination of phosphate anions with the NiIII center.

Structure property relationships affecting the proton conductivity in imidazole loaded Al-MOFs.

The structures of the imidazole loaded derivatives of Al-MIL-53 [Al(OH)(1,4-BDC-(CH3)x)] (x = 0, 1, 2) and CAU-11 ([Al(OH)(SDBA)]) (1,4-H2BDC = terephthalic acid; H2SDBA = 4,4'-sulfonyldibenzoic acid) were determined from powder X-ray diffraction data. Impedance spectroscopy measurements were carried out to evaluate their proton conductivities under anhydrous conditions at temperatures up to 110 °C. In Al-MIL-53-(CH3)x_HIm (x = 0, 1, 2) the formation of hydrogen bonds between the framework and the guest molecules results in a decrease in proton conductivity (x0 = 1.7 × 10-6, x1 = 1.9 × 10-8 and x2 = 1.7 × 10-9 S cm-1 at 110 °C and Eact = 0.42, 0.41 and 0.46 eV, for 0, 1 and 2 CH3-groups, respectively). The highest conductivity has been measured for CAU-11_HIm with 3.0 × 10-4 S cm-1 at 110 °C (Eact = 0.19 eV), where no host-guest hydrogen bonding interactions are observed.

The influence of steric effects on intramolecular secondary bonding interactions; cytotoxicity in gold(iii) bithiazole complexes

The reaction of two sterically different bithiazole ligands with gold(III) was investigated. Our results show that, depending on the ligand used, different coordination geometries around the gold(III) center were achieved. Where sterically unhindered 4bt ligand was used, regular square-planar compound [Au(4bt)Cl2][AuCl4] (1) is isolated, while in the case of sterically hindered dm4bt ligand, the interplay between the steric effect and the intramolecular secondary bonding interaction leads to the formation of disordered square-pyramidal geometry in [Au(dm4bt)Cl3] (2). Furthermore, the steric influence of the methyl group in the ligand plays an important role in the cytotoxicity of the compound in different cultures. Interestingly, compound 1 is more potent to kill a breast cancer cell line than cisplatin (13 times), and its cytotoxicity arises from the cationic part, [Au(4bt)Cl2](+).

The Molecular and Crystal Structures of Galvinol Derivatives

Galvinols are interesting, sterically hindered compounds that serve as precursors for the generation of stable galvinoxyl radicals. In order to elucidate their basic structural chemistry and the influence of steric effects on their conformation a comparative analysis of several galvinol derivatives was undertaken. The aryl and quinoid subunits could clearly be identified, and substituents at the connecting methine bridge were found to influence the conformation of the molecules. As a result of the sterically hindered residues the molecules pack mainly through weak van der Waals interactions without formation of hydrogen bonds. The observation of different crystal forms and packing for galvinols and their conformational flexibility will impact current solid-state applications and provides unambiguous structural data for theoretical calculations.

Influence of steric effect on the structural aspects of N,N′,N″-triarylguanidine derived six-membered [C,N] palladacycles

Guanidine derived six-membered [C,N] palladacycles of the types [(C,N)Pd(μ-OC(O)R)]2 (1a–d), [(C,N)Pd(μ-Br)]2 (2a,b), cis-[(C,N)PdBr(L)] (3a–d, 4, and 5), and ring contracted guanidine derived five-membered [C,N] palladacycle, [(C,N)PdBr(CNXy)] (6) were prepared in high yield following the established methods with a view aimed at understanding the influence of the substituents on the aryl rings of the guanidine upon the solid state structure and solution behaviour of palladacycles. Palladacycles were characterised by microanalytical, IR, NMR and mass spectral data. The molecular structures of 1a, 1c, 2a, 2b, 3a, 3c, 3d, and 4–6 were determined by single crystal X-ray diffraction data. Palladacycles 1a and 1c were shown to exist as a dimer in transoid in–in conformation in the solid state but as a mixture of a dimer in major proportion and a monomer (κ2-O,O′-OAc) in solution as deduced from 1H NMR data. Palladacycles 2a and 2b were shown to exist as a dimer in transoid conformation in the solid state but the former was shown to exist as a mixture of a dimer and presumably a trimer in solution as revealed by a variable temperature 1H NMR data in conjunction with ESI-MS data. The cis configuration around the palladium atom in 3a, 3c, and 3d was ascribed to steric influence of the aryl moiety of NAr unit and that in 4–6 was ascribed to antisymbiosis. The solution behaviour of 3d was studied by a variable concentration (VC) 1H NMR data.

Intermolecular steric hindrance in 7-acylamino-[1H]-2-oxo-1,8-naphthyridines: NMR, ESI-MS, IR, and DFT calculation studies

Intermolecular interactions of 7-(RCONH)-[1H]-2-oxo-1,8-naphthyridines (R = Me, Et, i-Pr, t-Bu, 1-adamantyl (1-Ad), CF3, and C2F5) containing ADAD quadruple hydrogen bonding motif were studied by liquid and solid state NMR, ESI-MS, IR, and DFT calculations. 1H NMR was used to determine the dimerization constants of i-Pr and 1-Ad congeners in CDCl3. 13C and 15N cross-polarization (CP) magic angle spinning (MAS) NMR data suggest that compounds possess similar solid state structures. Further, mass spectral data reveal that in gas phase both Me and 1-Ad derivatives form also multimers due to lack of competitive solvent interactions. The structures of the gas phase multimers depend on the size of the alkyl group. These results are in agreement with quantum chemical calculations. Geometry optimization and 1H NMR spectra show that in dimers that carry bulky alkyl groups (t-Bu and 1-Ad) certain hydrogen bonds are weaker than in Me, Et, and i-Pr derivatives while strong electron acceptors, CF3 and C2F5, deshields hydrogen bonded protons but creates significant electronic F/O repulsion yielding lowering of the energy of interaction. The influence of steric effect on dimerization of quadruply hydrogen bonded dimers was correlated with the Taft E s values.