Path to achieving molecular dispersion in a dense reactive mixture

A uniform dispersion of reactants is necessary to achieve a complete reaction involving multicomponents. In this study, we have examined the role of plasticizer in the reaction of two seemingly unlikely reactants: a highly crystalline hexamethylenetetramine (HMTA) and a strongly hydrogen bonded phenol formaldehyde resin. By combining information from NMR, infrared spectroscopy and differential scanning calorimetry, we were able to determine the role of specific intermolecular interactions necessary for the plasticizer to dissolve the highly crystalline HMTA and to plasticize the phenol formaldehyde resin in this crosslinking reaction. The presence of the plasticizer increased the segmental mobility, disrupted the hydrogen bonded matrix, and freed the hydroxyl units, which further increased the solubility of the HMTA. Both the endothermic and exothermic transitions are accounted for in the calorimetric data obtained. For the first time, it is possible to obtain the effective molar ratio of each component needed to complete the crosslinking reaction efficiently. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1519–1526     

不同变质程度煤的高分辨率透射电镜分析

利用高分辨率透射电子显微镜(HRTEM)分析了三种不同变质程度煤样的结构特征。基于傅里叶-反傅里叶变换方法,并结合Matlab、Arcgis和Auto CAD软件,通过图像分析技术,获得了HRTEM照片的晶格条纹参数。结果表明,三种煤样的晶格条纹呈现不同特征,按条纹长度分别归属于1×1-8×8共计八个类型。以3×3为临界点,在1×1和2×2中,ML-8中芳香层片的比例高于DP-4和XM-3;在3×3-8×8中,ML-8中芳香层片的比例低于DP-4和XM-3。对比HRTEM和XRD参数d002发现,随着镜质组反射率的增加d002都呈现递减趋势。     

锡苯和铅苯二聚反应的理论研究

采用密度泛函理论B3LYP方法,研究了锡苯和铅苯的[2+2],[4+2]及[4+4]二聚反应的微观机理和势能剖面,考察了Sn(Pb)原子上的2,4,6-三甲基苯基(Mes)取代基对反应势能剖面的影响.研究结果表明,所有反应均为协同过程,且大多数情况下,2个C—Sn(Pb)键同步形成.[2+2]和[4+2]反应在热力学和动力学上均比相应的[4+4]反应容易进行,而[4+2]反应在动力学上比相应的[2+2]反应有利.Sn(Pb)原子上的Mes取代基在热力学和动力学上均不利于反应的进行.铅苯的动力学稳定性与锡苯相当,但其热力学稳定性高于锡苯.     

Insights into the Cascade Reaction of CO and Heteroallenes Mediated by Dinitrogen Hafnocene Complexes: The Indirect Effect of Nitride's Nucleophilicity

A DFT mechanistic exploration of the reactivity of the dinitrogen hafnocene complex, [{(η⁵‐C₅H₂‐1,2,4‐Me₃)₂Hf}₂(μ₂‐N₂)], towards mixtures of CO/CO₂ and CO/OCNtBu is reported. The crucial role of the nitride intermediate is highlighted, as well as the importance of the bridging mode of the cyanate ligand between the two Hf metal atoms throughout the process. Interestingly, the CO₂ addition to the nitride intermediate occurs through an outer‐sphere transition state, whereas the addition of the heteroallene is governed by the steric congestion imposed by cyclopentadienyl ligands.     

Bio‐Inspired Transition Metal–Organic Hydride Conjugates for Catalysis of Transfer Hydrogenation: Experiment and Theory

Taking inspiration from yeast alcohol dehydrogenase (yADH), a benzimidazolium (BI⁺) organic hydride‐acceptor domain has been coupled with a 1,10‐phenanthroline (phen) metal‐binding domain to afford a novel multifunctional ligand ( L ^BI+) with hydride‐carrier capacity ( L ^BI++H^?? L ^BIH). Complexes of the type [Cp*M( L ^BI)Cl][PF₆]₂ (M=Rh, Ir) have been made and fully characterised by cyclic voltammetry, UV/Vis spectroelectrochemistry, and, for the Ir^III congener, X‐ray crystallography. [Cp*Rh( L ^BI)Cl][PF₆]₂ catalyses the transfer hydrogenation of imines by formate ion in very goods yield under conditions where the corresponding [Cp*Ir( L ^BI)Cl][PF₆] and [Cp*M(phen)Cl][PF₆] (M=Rh, Ir) complexes are almost inert as catalysts. Possible alternatives for the catalysis pathway are canvassed, and the free energies of intermediates and transition states determined by DFT calculations. The DFT study supports a mechanism involving formate‐driven Rh?H formation (90 kJ mol^?1 free‐energy barrier), transfer of hydride between the Rh and BI⁺ centres to generate a tethered benzimidazoline (BIH) hydride donor, binding of imine substrate at Rh, back‐transfer of hydride from the BIH organic hydride donor to the Rh‐activated imine substrate (89 kJ mol^?1 barrier), and exergonic protonation of the metal‐bound amide by formic acid with release of amine product to close the catalytic cycle. Parallels with the mechanism of biological hydride transfer in yADH are discussed.     

Experimental and Theoretical Studies on the Rearrangement of 2‐Oxoazepane α,α‐Amino Acids into 2′‐Oxopiperidine β2,3,3‐Amino Acids: An Example of Intramolecular Catalysis

Enantiopure β‐amino acids represent interesting scaffolds for peptidomimetics, foldamers and bioactive compounds. However, the synthesis of highly substituted analogues is still a major challenge. Herein, we describe the spontaneous rearrangement of 4‐carboxy‐2‐oxoazepane α,α‐amino acids to lead to 2′‐oxopiperidine‐containing β^2,3,3‐amino acids, upon basic or acid hydrolysis of the 2‐oxoazepane α,α‐amino acid ester. Under acidic conditions, a totally stereoselective synthetic route has been developed. The reordering process involved the spontaneous breakdown of an amide bond, which typically requires strong conditions, and the formation of a new bond leading to the six‐membered heterocycle. A quantum mechanical study was carried out to obtain insight into the remarkable ease of this rearrangement, which occurs at room temperature, either in solution or upon storage of the 4‐carboxylic acid substituted 2‐oxoazepane derivatives. This theoretical study suggests that the rearrangement process occurs through a concerted mechanism, in which the energy of the transition states can be lowered by the participation of a catalytic water molecule. Interestingly, it also suggested a role for the carboxylic acid at position 4 of the 2‐oxoazepane ring, which facilitates this rearrangement, participating directly in the intramolecular catalysis.     

Mechanistic Aspects of a Highly Active Dinuclear Zinc Catalyst for the Co‐polymerization of Epoxides and CO2

The dinuclear zinc complex reported by us is to date the most active zinc catalyst for the co‐polymerization of cyclohexene oxide (CHO) and carbon dioxide. However, co‐polymerization experiments with propylene oxide (PO) and CO₂ revealed surprisingly low conversions. Within this work, we focused on clarification of this behavior through experimental results and quantum chemical studies. The combination of both results indicated the formation of an energetically highly stable intermediate in the presence of propylene oxide and carbon dioxide. A similar species in the case of cyclohexene oxide/CO₂ co‐polymerization was not stable enough to deactivate the catalyst due to steric repulsion.     

Enhanced Catalytic Activity in Liquid‐Exfoliated FeOCl Nanosheets as a Fenton‐Like Catalyst

A facile liquid‐phase exfoliation method to prepare few‐layer FeOCl nanosheets in acetonitrile by ultrasonication is reported. The detailed exfoliation mechanism and generated products were investigated by combining first‐principle calculations and experimental approaches. The similar cleavage energies of FeOCl (340 mJ m^?2) and graphite (320 mJ m^?2) confirm the experimental exfoliation feasibility. As a Fenton reagent, FeOCl nanosheets showed outstanding properties in the catalytic degradation of phenol in water at room temperature, under neutral pH conditions, and with sunlight irradiation. Apart from the increased surface area of the nanosheets, the surface state change of the nanosheets also plays a key role in improving the catalytic performance. The changes of charge density, density of states (DOS), and valence state of Fe atoms in the exfoliated FeOCl nanosheets versus plates illustrated that surface atomistic relationships made the few‐layer nanosheets higher activity, indicating the exfoliation process of the FeOCl nanosheets also brought about surface state changes.     

Ligand Displacement Reaction Paths in a Diiron Hydrogenase Active Site Model Complex

The mechanism and energetics of CO, 1‐hexene, and 1‐hexyne substitution from the complexes (SBenz)₂[Fe₂(CO)₆] (SBenz=SCH₂Ph) ( 1 ‐CO), (SBenz)₂[Fe₂(CO)₅(η²‐1‐hexene)] ( 1 ‐(η²‐1‐hexene)), and (SBenz)₂[Fe₂(CO)₅(η²‐1‐hexyne)] ( 1 ‐(η²‐1‐hexyne)) were studied by using time‐resolved infrared spectroscopy. Exchange of both CO and 1‐hexyne by P(OEt)₃ and pyridine, respectively, proceeds by a bimolecular mechanism. As similar activation enthalpies are obtained for both reactions, the rate‐determining step in both cases is assumed to be the rotation of the Fe(CO)₂L (L=CO or 1‐hexyne) unit to accommodate the incoming ligand. The kinetic profile for the displacement of 1‐hexene is quite different than that for the alkyne and, in this case, both reaction channels, that is, dissociative (S_N1) and associative (S_N2), were found to be competitive. Because DFT calculations predict similar binding enthalpies of alkene and alkyne to the iron center, the results indicate that the bimolecular pathway in the case of the alkyne is lower in free energy than that of the alkene. In complexes of this type, subtle changes in the departing ligand characteristics and the nature of the mercapto bridge can influence the exchange mechanism, such that more than one reaction pathway is available for ligand substitution. The difference between this and the analogous study of (μ‐pdt)[Fe(CO)₃]₂ (pdt=S(CH₂)₃S) underscores the unique characteristics of a three‐atom S?S linker in the active site of diiron hydrogenases.