A quantum chemical study on the polycondensation reaction of polyesters: The mechanism of catalysis in the polycondensation reaction

We carried out a theoretical study on the mechanism of catalysis in the poly(ethylene terephthalate) (PET) polycondensation reaction. Transesterification reaction of diethylterephthalate with ethanol is investigated as a model system by using the B3LYP level of theory, and Sb(OEt)₃, Ge(OEt)₄ and Ti(OEt)₄ are adopted as model catalysts. We found that the metal center of metal alkoxides coordinates to the carbonyl oxygen atom of the ester, and the alkoxy oxygen atom of alkoxy ligands attacks to the carbonyl carbon atom of the ester to form the four-centered transition state. The activation energy for tetraethoxy titanium catalyzed reaction in vacuo is 15.47 kcal/mol; this is comparable to the experimental result of 11.2 kcal/mol for poly(butylene terephthalate)/Ti(OBu)₄. Because the other mechanisms gave much higher activation energies, this is the most convincing mechanism of PET polycondensation catalysis by antimony, germanium and titanium alkoxides.     

Theoretical Spectroscopy of the NiII Intermediate States in the Catalytic Cycle and the Activation of [NiFe] Hydrogenases

[NiFe] hydrogenases catalyze the reversible oxidation of dihydrogen. The corresponding catalytic cycle involves a formidable number of redox states of the Ni‐Fe active site; these can be distinguished experimentally by the IR stretching frequencies of their CN and CO ligands coordinated to iron. These spectroscopic fingerprints serve as sensitive probes for the intrinsic electronic structure of the metal core and, indirectly, for the structural composition of the active site. In this study, density functional theory (DFT) was used to calculate vibrational frequencies, by focusing on the EPR‐silent intermediate states that contain divalent metal centers. By using the well‐characterized Ni‐C and Ni‐B states as references, we identified candidates for the Ni‐SI_r, Ni‐SI_a, and Ni‐R states by matching the predicted relative frequency shifts with experimental results. The Ni‐SI_r and Ni‐SI_a states feature a water molecule loosely bound to nickel and a formally vacant bridge. Both states are connected to each other through protonation equilibria; that is, in the Ni‐SI_a state one of the terminal thiolates is protonated, whereas in Ni‐SI_r this thiolate is unprotonated. For the reduced Ni‐R state two feasible models emerged: in one, H₂ coordinates side‐on to nickel, and the second features a hydride bridge and a protonated thiolate. The Ni‐SU state remains elusive as no unequivocal correspondence between the experimental data and calculated frequencies of the models was found, thus indicating that a larger structural rearrangement might occur upon reduction from Ni‐A to Ni‐SU and that the bridging ligand might dissociate.     

Fluorescent Amino Acids: Modular Building Blocks for the Assembly of New Tools for Chemical Biology

Fluorescence spectroscopy is a powerful tool for probing complex biological processes. The ubiquity of peptide–protein and protein–protein interactions in these processes has made them important targets for fluorescence labeling, and to allow sensitive readout of information concerning location, interactions with other biomolecules, and macromolecular dynamics. This review describes recent advances in design, properties and applications in the area of fluorescent amino acids (FlAAs). The ability to site‐selectively incorporate fluorescent amino acid building blocks into a protein or peptide of interest provides the advantage of closely retaining native function and appearance. The development of an array of fluorescent amino acids with a variety of properties, such as environment sensitivity, chelation‐enhanced fluorescence, and profluorescence, has allowed researchers to gain insights into biological processes, including protein conformational changes, binding events, enzyme activities, and protein trafficking and localization.     

医用橡胶瓶塞生产工艺对其物化性能影响

介绍了在生产医用橡胶瓶塞过程中混炼和后处理等工艺对产品物化性能的影响。     

加强化工见习改革及基地建设,培养学生工科意识

通过加强对化工见习的改革与基地建设,建立产学研合作示范基地,改革考核方式,充分调动学生的积极性,不仅增强了学生工科意识的培养力度,也探索了服务地方的理科类创业型人才培养模式。文章从化工见习的重要性、建立化工见习基地、考核方式等方面对高师化学专业化工见习提出了几点建议,为更好的开展化工见习做一些有益的探讨。     

A modified method for barium titanate nanoparticles synthesis

In this research, a modified, cost effective sol-gel procedure applied to synthesize BaTiO₃ nanoparticles. XRD and electron microscopy (SEM and TEM) applied for microstructural characterization of powders. The obtained results showed that the type of precursors, their ratio and the hydrolysis conditions had a great effect on time, temperature and therefore the costs of the synthesis process. By selection, utilization of optimized precursor's type, hydrolysis conditions, fine cubic BaTiO₃ nanoparticles were synthesized at low temperature and in short time span (1 h calcination at 800 °C). The proposed procedure seems to be more preferable for mass production.The result indicated that the polymorphic transformation to tetragonal (ferroelectric characteristic) occurred at 900 °C, which might be an indication of being nanosized.     

探索基础化学实验中分析化学实验教学的新策略

介绍了基础化学实验中分析化学实验教学存在的一些问题,探索了些新的策略,建立合理的分析实验课程体系,科学合理安排实验教学内容,改革实验教学方法和教学手段;全方位提升分析化学的实验的教学质量和水平。     

Dark zones of solid propellant flames: Critically assessed datasets, quantitative model comparison, and detailed chemical analysis

The formation of propellant dark zone (DZ) structures in the gaseous flames above many solid propellants has been a subject of recurrent interest to us for about 20 years. The DZ structure is controlled by small molecule chemistry. The DZ chemistry is very important in controlling both the flame structure at low pressure (10–100 atm) and burning rates at high pressure (above ∼500 atm), even though DZs collapse at higher pressures. We developed a detailed, frequently updated mechanism to model it. This report reviews prior work, introduces our most recent modeling results, and documents the first quantitative tests of several key assumptions. All relevant experimental literature is critically assessed to identify datasets for testing our model. Comparison of predictions and experimental results shows reasonable agreement, and thus we advocate use of our mechanism. But the precision of both experiments and predictions is not tight, nor are there many test datasets. Further experimentation is needed, and types of study that would be of greatest benefit are suggested. A detailed discussion of the chemistry that controls DZ structure is presented.     

Synthesis and thermal stability of Re2Zr2O7, (Re = La,Gd) and La2(Zr1−xCex)2O7−δ compounds under reducing and oxidant atmospheres for thermal barrier coatings

Rare-earth zirconates and cerates have attracted particular interest for thermal barrier coating (TBC) applications due to their advantageous thermal properties, such as a low conductivity and efficient phase stability at elevated temperatures. This study focuses on synthesising La₂Zr₂O₇, Gd₂Zr₂O₇, La₂Ce₂O_7?γ and La₂(Zr_0.7Ce_0.3)O_7?γ compounds via two soft chemistry processes, alkoxide and citrate synthesis. Thermal analysis, X-ray diffraction (XRD) and scanning electron microscope observations were used to analyse the powder after calcinations under air. Chemical reactivity tests under a reducing atmosphere were performed at 1400 °C and investigated by XRD analysis. It was found that the lanthanum and gadolinium zirconates are the most stable and interesting materials under an Ar_(g)/3%H_2(g) atmosphere.