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1.
Crystal Effects on Mesobilirubin: A Combined NMR Spectroscopic and Density Functional Theory Study 下载免费PDF全文
We report solid‐state NMR investigations of crystal effects in powdered mesobilirubin‐IXα, an open‐chain tetrapyrrole that is structurally related to bilirubin‐IXα but hydrogenated at the 3‐ and 18‐vinyl groups. 13C and 15N cross‐polarization magic‐angle spinning (CP/MAS) NMR experiments were performed on the compound at natural abundance. To facilitate the spectral analysis, density functional calculations were carried out at the B3LYP/6‐311G(d,p) level of theory, using an enneameric cluster to simulate the solid. The 1H, 13C and 15N chemical shift data calculated for the enneamer are in a good agreement with those observed in the experimental spectra, and the relative order of the calculated resonances was thus used to confirm the tentative assignments obtained mainly from the heteronuclear correlation spectra. The observed signal splittings of a small subset of the 13C resonances in the peripheral regions of the two terminal rings provide evidence for microcrystalline heterogeneity of the powdered compound. 相似文献
2.
Palladium‐Catalyzed Desymmetrization of Silacyclobutanes with Alkynes to Silicon‐Stereogenic Silanes: A Density Functional Theory Study 下载免费PDF全文
Jin Zhang Jin‐Zhou Xu Dr. Zhan‐Jiang Zheng Dr. Zheng Xu Dr. Yu‐Ming Cui Dr. Jian Cao Prof. Dr. Li‐Wen Xu 《化学:亚洲杂志》2016,11(20):2867-2875
The palladium‐catalyzed desymmetrization of silacyclobutanes using electron‐deficient alkynes proceeds with high enantioselectivity in the presence of a chiral P ligand; this provides a facile approach for the synthesis of novel silicon‐stereogenic silanes. In this work, we used hybrid density functional theory (DFT) to elucidate the mechanism of the palladium‐catalyzed desymmetrization of silacyclobutanes with dimethyl acetylenedicarboxylate. Full catalytic cycle including two different initiation modes that were proposed to be a possible initial step to the formation of the 1‐pallada‐2‐silacyclopentane/alkyne intermediate—the oxidative addition of the palladium complex to the silacyclobutane Si?C bond (cycle MA) or coordination of the Pd0 complex with the alkyne C≡C bond (cycle MB)—have been studied. It was found that the ring‐expansion reaction began with cycle MB is energetically more favorable. The formation of a seven‐membered metallocyclic PdII intermediate was found to be the rate‐determining step, whereas the enantioselectivity‐determining step, oxidative addition of silacyclobutane to the three‐membered metallocyclic PdII intermediate, was found to be quite sensitive to the steric repulsion between the chiral ligand and silacyclobutane. 相似文献
3.
采用密度泛函方法对3-甲硫基-4-氰基-5-氨基吡唑与碘甲烷反应的机理进行了研究. 提出了两种可能的反应途径: 反应途径Ⅰ为反应物先脱去吡唑上的质子, 生成阴离子中间物, 然后碘甲烷分别进攻中间物吡唑环上的2个氮原子, 生成两种异构产物; 反应途径Ⅱ为反应物通过分子间氢转移存在两种异构体, 碘甲烷直接进攻每个异构反应物吡唑上的氮原子, 形成中间物, 然后脱去碘化氢, 生成产物. 计算结果表明, 途径Ⅱ应为主要反应途径. 还找出了两种异构产物间甲基迁移反应的过渡态, 得出该反应的活化能为278.5 kJ/mol, 在常温下甲基迁移反应不容易进行. 相似文献
4.
Filipe J. S. Duarte Dipl.‐Chem. Eurico J. Cabrita Prof. Dr. Gernot Frenking Prof. Dr. A. Gil Santos Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(7):1734-1746
A rationalization of stereoselectivity : The mechanisms of proline‐catalyzed and imidazole‐co‐catalyzed intramolecular Baylis–Hillman reactions have been studied by using density functional theory methods. The computational data has allowed us to rationalize the experimental outcome, validating some of the mechanistic steps proposed in the literature, as well as to propose new ones that considerably change and improve our understanding of the full reaction path (see scheme).
5.
Danilo Cuccato Evangelos Mavroudakis Marco Dossi Davide Moscatelli 《Macromolecular theory and simulations》2013,22(2):127-135
In this work, secondary reactions involved in the free radical polymerization of butyl acrylate are investigated using quantum chemistry. First, various backbiting reactions are studied by adopting a simplified molecular model suitable for treating long polymer chains. The predicted reaction kinetics suggest the possibility of a radical migration along the poly(butyl acrylate) (PBA) chain as a consequence of subsequent j:j + 4 hydrogen abstractions, which are characterized by a low activation energy. Moreover, branching propagation and β‐scission reactions originating from mid‐chain radicals are investigated using a complete PBA model composed of five monomer units. The reaction kinetics involving short‐branch radicals are also examined, and a novel backbiting step leading to the formation of short branches is proposed.
6.
为了解决年龄衰老、基因突变和癌症等问题, 理解DNA的氧化损伤机理非常重要. 本文利用密度泛函方法和极化连续介质模型在液相条件下研究了羟基自由基夺取鸟嘌呤-胞嘧啶(GC)碱基对上5 个氢原子的反应机理. 研究结果表明, 所有的脱氢反应路径都是放热过程, 热力学上五个脱氢反应路径形成自由基的稳定性顺序是(H2b-GC)·>(GC-H4b)·>(GC-H6)·>(GC-H5)·~(H8-GC)·, 其中H2b反应路径的能量变化最大, 说明该反应平衡时的转化率最高. 动力学上, 相对于反应复合物的局部反应能垒大小顺序是H2b 相似文献
7.
On the Importance of Decarbonylation as a Side‐Reaction in the Ruthenium‐Catalysed Dehydrogenation of Alcohols: A Combined Experimental and Density Functional Study 下载免费PDF全文
Dr. Nicolas Sieffert Romain Réocreux Patrizia Lorusso Prof. Dr. David J. Cole‐Hamilton Prof. Dr. Michael Bühl 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(14):4141-4155
We report a density functional study (B97‐D2 level) of the mechanism(s) operating in the alcohol decarbonylation that occurs as an important side‐reaction during dehydrogenation catalysed by [RuH2(H2)(PPh3)3]. By using MeOH as the substrate, three distinct pathways have been fully characterised involving either neutral tris‐ or bis‐phosphines or anionic bis‐phosphine complexes after deprotonation. α‐Agostic formaldehyde and formyl complexes are key intermediates, and the computed rate‐limiting barriers are similar between the various decarbonylation and dehydrogenation paths. The key steps have also been studied for reactions involving EtOH and iPrOH as substrates, rationalising the known resistance of the latter towards decarbonylation. Kinetic isotope effects (KIEs) were predicted computationally for all pathways and studied experimentally for one specific decarbonylation path designed to start from [RuH(OCH3)(PPh3)3]. From the good agreement between computed and experimental KIEs (observed kH/kD=4), the rate‐limiting step for methanol decarbonylation has been ascribed to the formation of the first agostic intermediate from a transient formaldehyde complex. 相似文献
8.
Density Functional Theory Mechanistic Study of Boron‐Catalyzed N‐Alkylation of Amines with Formic Acid: Formic Acid Activation by Silylation Reaction 下载免费PDF全文
New methodology for the alkylation of amines is an intriguing issue in both academia and industry. Recently, several groups reported the metal‐free B(C6F5)3‐catalyzed N‐alkylation of amines, but the mechanistic details of these important reactions are unclear. Herein, a computational study was performed to elucidate the mechanism of the N‐alkylation of amines with formic acid catalyzed by the Lewis acid B(C6F5)3 in the presence of hydrosilane. We found that the reaction started with the activation of formic acid through a novel model. Then, the high electrophilicity of the C center of the formic acid unit and the nucleophilic character of the amine resulted in a C?N coupling reaction. Finally, two sequential silyl‐group and H? transfer steps occurred to generate the final product. Upon comparing the reaction barrier and the hydrogenation of indole, our mechanism is more favorable than that proposed by the group of Yu and Fu. 相似文献
9.
Alexander F. Khlebnikov Martin S. Seyfried Heinz Heimgartner 《Helvetica chimica acta》2016,99(2):110-115
It has been shown previously that the reaction of diazomethane with 5‐benzylidene‐3‐phenylrhodanine ( 1 ) in THF at ?20° occurs at the exocyclic C?C bond via cyclopropanation to give 3a and methylation to yield 4 , respectively, whereas the corresponding reaction with phenyldiazomethane in toluene at 0° leads to the cyclopropane derivative 3b exclusively. Surprisingly, under similar conditions, no reaction was observed between 1 and diphenyldiazomethane, but the 2‐diphenylmethylidene derivative 5 was formed in boiling toluene. In the present study, these results have been rationalized by calculations at the DFT B3LYP/6‐31G(d) level using PCM solvent model. In the case of diazomethane, the formation of 3a occurs via initial Michael addition, whereas 4 is formed via [3+2] cycloaddition followed by N2 elimination and H‐migration. The preferred pathway of the reaction of 1 with phenyldiazomethane is a [3+2] cycloaddition, subsequent N2 elimination and ring closure of an intermediate zwitterion to give 3b . Finally, the calculations show that the energetically most favorable reaction of 1 with diphenyldiazomethane is the initial formation of diphenylcarbene, which adds to the S‐atom to give a thiocarbonyl ylide, followed by 1,3‐dipolar electrocyclization and S‐elimination. 相似文献
10.
Rudolph A. Marcus 《Angewandte Chemie (International ed. in English)》1993,32(8):1111-1121
Since the late 1940s, the field of electron transfer processes has grown enormously, both in chemistry and biology. The development of the field, experimentally and theoretically, as well as its relation to the study of other kinds of chemical reactions, presents to us an intriguing history, one in which many threads have been brought together. In this lecture, some history, recent trends, and my own involvement in this research are described. 相似文献
11.
Density functional theory calculations are performed to study the addition mechanism of e‐rich moieties such as triethyl phosphite to a carbonyl group on the rim of a fullerene orifice. Three possible reaction channels have been investigated. The obtained results show that the reaction of a carbonyl group on a fullerene orifice with triethyl phosphite most likely proceeds along the classical Abramov reaction; however, the classical product is not stable and is converted into the experimental product. An attack on a fullerene carbonyl carbon will trigger a rearrangement of the phosphate group to the carbonyl oxygen as the conversion transition state is stabilized by fullerene conjugation. This work provides a new insight on the reactivity of open‐cage fullerenes, which may prove helpful in designing new switchable fullerene systems. 相似文献
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13.
Which nucleophiles are capable of attacking the allyl ligand of the Pd‐stabilized allyl cation 1 ? This question is answered by the electrophilicity parameter of 1 which is derived from kinetic investigations. 相似文献
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15.
Extending the Row of Lanthanide Tetrafluorides: A Combined Matrix‐Isolation and Quantum‐Chemical Study 下载免费PDF全文
Thomas Vent‐Schmidt Dr. Zongtang Fang Zachary Lee Prof. Dr. David Dixon Prof. Dr. Sebastian Riedel 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(7):2406-2416
Only the neutral tetrafluorides of Ce, Pr, and Tb as well as the [LnF7]3? anions of Dy and Nd, with the metal in the +IV oxidation state, have been previously reported. We report our attempts to extend the row of neutral lanthanide tetrafluorides through the reaction of laser‐ablated metal atoms with fluorine and their stabilization and characterization by matrix‐isolation IR spectroscopy. In addition to the above three tetrafluorides, we found two new tetrafluorides, 3NdF4 and 7DyF4, both of which are in the +IV oxidation state, which extends this lanthanide oxidation state to two new metals. Our experimental results are supported by quantum‐chemical calculations and the role of the lanthanide oxidation state is discussed for both the LnF4 and [LnF4]? species. Most of the LnF4 species are predicted to be in the +IV oxidation state and all of the [LnF4]? anions are predicted to be in the +III oxidation state. The LnF4 species are predicted to be strong oxidizing agents and the LnF3 species are predicted to be moderate to strong Lewis acids. 相似文献
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17.
Density Functional Theory Study of Mechanism of Cycloaddition Reaction Between Dimethyl-Silylene Carbene and Acetone 下载免费PDF全文
The mechanism of the cycloaddition reaction between singlet dimethyl-silylene carbene and acetone has been investigated with density functional theory, From the potential energy profile, it can be predicted that the reaction has two competitive dominant reaction pathways. The presented rule of this reaction: the [2+2] cycloaddition effect between the πorbital of dimethyl-silylene carbene and the π orbital of π-bonded compounds leads to the formation of a twisty four-membered ring intermediate and a planar four-membered ring product; The unsaturated property of C atom from carbene in the planar four-membered ring product,resulting in the generation of CH3-transfer product and silicic bis-heterocyclic compound. 相似文献
18.
Reaction products of laser‐ablated praseodymium atoms with fluorine in excess neon, argon, krypton, or neat fluorine at 4—10 K are investigated by IR spectroscopy and quantum chemical DFT and coupled‐cluster calculations. 相似文献
19.
Frank Meyer‐Wegner Josef H. Wender Konstantin Falahati Timo Porsch Tanja Sinke Dr. Michael Bolte Prof. Dr. Matthias Wagner Prof. Dr. Max C. Holthausen Dr. Hans‐Wolfram Lerner 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(16):4681-4690
The reaction of 4‐chloro‐1,2‐dimethyl‐4‐supersilylsila‐1‐cyclopentene ( 2 a ) with Li[NiPr2] at ?78 °C results in the formation of the formal 1,4‐addition product of the silacyclopentadiene derivative 3,4‐dimethyl‐1‐supersilylsila‐1,3‐cyclopentadiene ( 4 a ) with 2,3‐dimethyl‐4‐supersilylsila‐1,3‐cyclopentadiene ( 5 a ). In addition the respective adducts of the Diels–Alder reactions of 4 a + 4 a and 4 a + 5 a were obtained. Compound 4 a , which displays an s‐cis‐silacyclopentadiene configuration, reacts with cyclohexene to form the racemate of the [4+2] cycloadduct of 4 a and cyclohexene ( 9 ). In the reaction between 4 a and 2,3‐dimethylbutadiene, however, 4 a acted as silene as well as silacyclopentadiene to yield the [2+4] and [4+2] cycloadducts 10 and 11 , respectively. The constitutions of 9 , 10 , and 11 were confirmed by NMR spectroscopy and their crystal structures were determined. Reaction of 4‐chloro‐1,2‐dimethyl‐4‐tert‐butyl‐4‐silacyclopent‐1‐ene ( 2 c ) with KC8 yielded the corresponding disilane ( 12 ), which was characterized by X‐ray crystal structure analysis (triclinic, P$\bar 1$ ). DFT calculations are used to unveil the mechanistic scenario underlying the observed reactivity. 相似文献