The Interplay between Steric and Electronic Effects in SN2 Reactions

Myths of steric hindrance : In contrast with current opinion, energy decomposition analysis shows that the presence of bulky substituents at carbon leads to the release of steric repulsion in the transition state shown in the graphic. It is rather the weakening of the electrostatic attraction, and in particular the loss of attractive orbital interactions, that are responsible for the activation barrier.

     

A model of the chemical bond must be rooted in quantum mechanics, provide insight, and possess predictive power

In this response to the preceding paper by Bader, we show that the core arguments and statements presented in the latter are flawed. We argue that it is insufficient for a model of the chemical bond to be rooted in quantum mechanics. A good model must in addition provide insight and possess predictive power. Our molecular orbital (MO) model of the chemical bond, in particular, the associated energy-decomposition approach satisfies all these conditions. On the other hand, Atoms-in-Molecules (AIM) theory is only rooted in quantum mechanics as far as its mathematical framework is concerned. The physical status of its central concepts is not so clear. In particular, "bond paths" and "bond critical points" are once more confirmed not to be indicators of a stabilizing interaction. Moreover, AIM theory lacks any predictive power. We also address specific questions raised in the preceding paper. Finally, interpreting chemical bonding implies choosing a perspective on this phenomenon. That there are many perspectives is a matter of fact and this is in no way unphysical. What is unscientific is to claim uniqueness and truth for one of these choices, namely AIM, and to dismiss on this ground all other approaches.     

Thorpe-Ingold效应及其在有机成环反应中的应用

本文首先介绍了Thorpe-Ingold效应及其相关理论和实验研究进展,然后综述了近年来Thorpe-Ingold效应在形成三、四、五和六元环产物的环化反应中的应用。Thorpe-Ingold效应可以有效地促进分子内和分子间环化,提高环化的产率和速率。该效应主要通过空间效应、电子效应或者两者协同起作用,在某些情况下,该效应还会受催化剂和溶剂等影响。利用该效应可以促使一些难以发生的环化反应顺利进行,并能获得较好的产率。     

Influence of Steric Hindrance Between Hydrogen Atoms of Linkage Groups and Adjacent Phenyls on Properties of Polyimide

A diamine monomer 4,4′-methylenedianiline(MDA) was introduced to modify the polyimide of pyromellitic dianhydride(PMDA) and 4,4′-oxydianiline(ODA) by polycondensation. A series of polyamic acids was synthesized from MDA and ODA of different molar ratios with PMDA of sum mole of moles of MDA and ODA, and polyimide films were obtained by thermal imidization. Polyimide(PI) films were characterized by tensile testing, dynamic mechanical analysis(DMA), thermal gravimetry analysis(TGA), Fourier transform infrared spectroscopy (FTIR), wide X-ray diffraction(WAXD) and molecular simulation. With the increase of MDA content, the tensile strength and thermal decomposition temperature remained generally stable compared with those of PMDA/ODA polyimide. Unexpectedly, the glass transition temperature(T_g) and Young’s modulus increased from 388.7 ℃ and 2.37 GPa to 408.3 ℃ and 5.74 GPa, respectively. The results of WAXD and molecular simulation indicate the steric hindrance among hydrogen atoms of the linkage groups and adjacent phenyls enhanced the properties of the polyimide modified with MDA.     

聚乙二醇/羟基磷灰石纳米杂化材料的制备及表征

以聚乙二醇单甲醚(MPEG)为原料, 采用先磷酰化再水解的方法合成了聚乙二醇单甲醚磷酸酯(P-MPEG). 以P-MPEG为空间位阻剂, 采用共沉淀法合成了内核为纳米羟基磷灰石(nHA)、 壳层为MPEG链的纳米杂化材料. 用傅里叶变换红外光谱(FTIR)、 X射线衍射(XRD)、 透射电子显微镜(TEM)和激光粒度分析(LPSA)对材料结构进行了表征. 结果表明, 所合成的杂化材料不仅能在水中再分散, 而且可以在甲醇和二甲基甲酰胺(DMF)等有机溶剂中再分散.     

1H NMR探究空间效应对表面活性剂复配体系中协同作用的影响

通过核磁共振氢谱（¹H NMR）实验测量了温度为298.0 K时3种表面活性剂——十六烷基三甲基溴化铵（CTAB）、辛基苯基聚氧乙烯醚（TX-100）和十二烷基硫酸钠（SDS）组成的复配体系CTAB/TX-100、SDS/TX-100和CTAB/TX-100/SDS中各组分的化学位移,得到了各混合体系中CTAB、TX-100和SDS的临界胶束浓度（CMC）.对不同体系中各组分CMC随摩尔分数的变化曲线进行比较,发现了空间效应对表面活性剂复配体系中协同作用的影响在一定的情况下并不逊于静电相互作用：表面活性剂分子的截面积越大,对体系中其他组分的协同作用越强;当组分间存在弱的静电相互作用时,空间效应对协同作用起到更主要的影响.     

六角星形MoSe_2双层纳米片的制备及其光致发光性能

采用化学气相沉积法,以三氧化钼作为钼源,硒粉作为硒源,在H_2/Ar气氛下生长出硒化钼纳米片.扫描电镜、X射线衍射表征结果表明,MoSe_2产物呈六角星状,横向尺寸约10μm,具有很好的晶体质量和结构.拉曼光谱表征其结构,确定其为双层纳米片.研究表明,高温反应时间对双层纳米片的生长具有重要的影响.通过对双层纳米片的生长机理的探究,推测其经历了3个生长过程:在高温下,Mo源和Se源被气化成气态分子并发生硒化反应形成晶核;晶核呈三角形外延生长;当反应时间持续增加,在空间位阻效应的影响下,晶体以中心原子岛为核,外延耦合生长出第二层三角形,最终形成六角星状双层纳米片.光致发光光谱结果表明,六角星状MoSe_2双层纳米片在1.53 eV处具有直接带隙和1.78 eV处具有间接带隙,其较宽范围的激发光谱响应预测其在光电探测器件领域具有潜在的应用前景.     

Pauli blocking potential applied to heavy-ion fusion reactions

In this study, the Pauli blocking potential between two colliding nuclei in the density overlapping region is applied to describe the heavy nuclei fusion process. Inspired by the Pauli blocking effect in the \begin{document}$ \alpha $\end{document}-decay of heavy nuclei, the Pauli blocking potential of single nucleon from the surrounding matter is obtained. In fusion reactions with strong density overlap, the Pauli blocking potential between the projectile and target can be constructed using a single folding model. By considering this potential, the double folding model with a new parameter set is employed to analyze the fusion processes of 95 systems. A wider Coulomb barrier and shallower potential pocket are formed in the inner part of the potential between the two colliding nuclei, compared to that calculated using the Akyüz-Winther potential. The fusion hindrance phenomena at deep sub-barrier energies are described well for fusion systems \begin{document}$ ^{16} $\end{document}O + \begin{document}$ ^{208} $\end{document}Pb and \begin{document}$ ^{58} $\end{document}Ni + \begin{document}$ ^{58} $\end{document}Ni.     

Metal complexes of porphyrin–anthraquinone hybrids: DNA binding and photocleavage specificities

A cationic porphyrin–anthraquinone hybrid and its Zn(II), Cu(II), Co(III), and Mn(III) complexes were synthesized and their interactions with duplex DNA were studied using a combination of absorption, fluorescence titration, circular dichroism spectroscopy, thermal DNA denaturation, viscosity measurements, gel electrophoresis, and gas chromatography. Metal coordination induces intermolecular steric hindrance and thus the metal hybrids have smaller DNA binding affinities than the free base hybrid; the steric hindrance could relax the intermolecular aggregation and increase ¹O₂ generation of the metal hybrids. The DNA photocleavage abilities of these hybrids were investigated.     

氨基酸分子构象稳定性与光电离解离动力学的氢键效应*

本文综述了氨基酸分子内氢键种类和数目对分子构象稳定性和光电离解离动力学的各种效应：得出分子内含有N···H–O和C=O···H–O氢键的构象具有较高的热力学稳定性; N···H–O氢键导致分子第一电离能数值的增加,同时可以导致电离过程中的质子转移以及后续二氧化碳释放的解离过程。氨基酸分子正离子的稳定性或碳碳化学键断裂解离动力学也不尽相同,丝氨酸分子在电离阈值附近解离过程中表现出奇特的立体效应。