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1,1‐Dilithioethylene: Toward Spectroscopic Identification of the Definitive Singlet Ground Electronic State of a Peculiar Structure 下载免费PDF全文
Yameng Liu Xiao Wang Prof. Yongdong Liu Prof. Rugang Zhong Dr. Yaoming Xie Prof. Henry F. Schaefer III 《Chemphyschem》2016,17(11):1623-1629
1,1‐Dilithioethylene is a prototypical carbon–lithium compound that is not known experimentally. All low‐lying singlet and triplet structures of interest were investigated by using high‐level theoretical methods with correlation‐consistent basis sets up to pentuple ζ. The coupled cluster methods adopted included up to full triple excitations and perturbative quadruples. In contrast to earlier studies that predicted the twisted C2v triplet to be the ground state, we found a peculiar planar Cs singlet ground state in the present research. The lowest excited electronic state of 1,1‐dilithioethylene, the twisted Cs triplet, was found to lie 9.0 kcal mol?1 above the ground state by using energy extrapolation to the complete basis set limit. For the planar Cs singlet and twisted Cs triplet states of 1,1‐dilithioethylene, anharmonic vibrational frequencies were reported on the basis of second‐order vibrational perturbation theory. The remarkably low (2050 cm?1) C?H stretching fundamental (the C?H bond near the bridging lithium) of the singlet state was found to have very strong infrared intensity. These highly reliable theoretical findings may assist in the long‐sought experimental identification of 1,1‐dilithioethylene. Using natural bond orbital analysis, we found that lithium bridging structures were strongly influenced by electrostatic effects. All carbon–carbon linkages corresponded to conventional double bonds. 相似文献
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Pseudo‐Contact NMR Shifts over the Paramagnetic Metalloprotein CoMMP‐12 from First Principles 下载免费PDF全文
Dr. Ladislav Benda Dr. Jiří Mareš Dr. Enrico Ravera Prof. Dr. Giacomo Parigi Prof. Dr. Claudio Luchinat Prof. Dr. Martin Kaupp Prof. Dr. Juha Vaara 《Angewandte Chemie (International ed. in English)》2016,55(47):14713-14717
Long‐range pseudo‐contact NMR shifts (PCSs) provide important restraints for the structure refinement of proteins when a paramagnetic metal center is present, either naturally or introduced artificially. Here we show that ab initio quantum‐chemical methods and a modern version of the Kurland–McGarvey approach for paramagnetic NMR (pNMR) shifts in the presence of zero‐field splitting (ZFS) together provide accurate predictions of all PCSs in a metalloprotein (high‐spin cobalt‐substituted MMP‐12 as a test case). Computations of 314 13C PCSs using g‐ and ZFS tensors based on multi‐reference methods provide a reliable bridge between EPR‐parameter‐ and susceptibility‐based pNMR formalisms. Due to the high sensitivity of PCSs to even small structural differences, local structures based either on X‐ray diffraction or on various DFT optimizations could be evaluated critically by comparing computed and experimental PCSs. Many DFT functionals provide insufficiently accurate structures. We also found the available 1RMZ PDB X‐ray structure to exhibit deficiencies related to binding of a hydroxamate inhibitor. This has led to a newly refined PDB structure for MMP‐12 (5LAB) that provides a more accurate coordination arrangement and PCSs. 相似文献
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Reactions of the Donor‐Stabilized Silylene Bis[N,N′‐diisopropyl‐benzamidinato(−)]silicon(II) with Brønsted Acids 下载免费PDF全文
Konstantin Junold Katharina Sinner Johannes A. Baus Dr. Christian Burschka Dr. Célia Fonseca Guerra Prof. Dr. F. Matthias Bickelhaupt Prof. Dr. Reinhold Tacke 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(50):16462-16466
Reaction of the donor‐stabilized silylene 1 (which is three‐coordinate in the solid state and four‐coordinate in solution) with [HMCp(CO)3] (M=Mo, W; Cp=cyclopentadienyl) leads to the cationic five‐coordinate silicon(IV) complexes 2 and 3 , respectively, and reaction of 1 with CH3COOH yields the neutral six‐coordinate silicon(IV) complex 4 . Compounds 2 – 4 were structurally characterized by crystal structure analyses and multinuclear NMR spectroscopic studies in the solid state and in solution. The formation of 2 – 4 can be formally described in terms of a Brønsted acid/base reaction, coupled with a redox process (SiII→SiIV, H+→H?). 相似文献
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The Structure and Torsional Dynamics of Two Methyl Groups in 2‐Acetyl‐5‐methylfuran as Observed by Microwave Spectroscopy 下载免费PDF全文
The molecular‐beam Fourier transform microwave spectrum of 2‐acetyl‐5‐methylfuran is recorded in the frequency range 2–26.5 GHz. Quantum chemical calculations calculate two conformers with trans or cis configuration of the acetyl group, both of which are assigned in the experimental spectrum. All rotational transitions split into quintets due to the internal rotations of two nonequivalent methyl groups. By using the program XIAM, the experimental spectra can be simulated with standard deviations within the measurement accuracy, and yield well‐determined rotational and internal rotation parameters, inter alia the V3 potentials. Whereas the V3 barrier height of the ring‐methyl rotor does not change for the two conformers, that of the acetyl‐methyl rotor differs by about 100 cm?1. The predicted values from quantum chemistry are only on the correct order of magnitude. 相似文献
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Rong‐Min Wei Ting Liu Jing Li Xiuling Zhang Yuting Chen Yi‐Quan Zhang 《化学:亚洲杂志》2019,14(11):2029-2035
By using paramagnetic [Fe(CN)6]3? anions in place of diamagnetic [Co(CN)6]3? anions, two field‐induced mononuclear single‐molecular magnets, [Nd(18‐crown‐6)(H2O)4][Co(CN)6] ? 2 H2O ( 1 ) and [Nd(18‐crown‐6)(H2O)4][Fe(CN)6] ? 2 H2O ( 2 ), have been synthesized and characterized. Single‐crystal X‐ray diffraction analysis revealed that compounds 1 and 2 were ionic complexes. The NdIII ions were located inside the cavities of the 18‐crown‐6 ligands and were each bound by four water molecules on either side of the crown ether. Magnetic investigations showed that these compounds were both field‐induced single‐molecular magnets. By comparing the slow relaxation behaviors of compounds 1 and 2 , we found significant differences between the direct and Raman processes for these two complexes, with a stronger direct process in compound 2 at low temperatures. Complete active space self‐consistent field (CASSCF) calculations were also performed on two [Nd(18‐crown‐6)(H2O)4]3+ fragments of compounds 1 and 2 . Ab initio calculations showed that the magnetic anisotropies of the NdIII centers in complexes 1 and 2 were similar to each other, which indicated that the difference in relaxation behavior was not owing to the magnetic anisotropy of NdIII. Our analysis showed that the magnetic interaction between the NdIII ion and the low‐spin FeIII ion in complex 2 played an important role in enhancing the direct process and suppressing the Raman process of the single‐molecular magnet. 相似文献
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On Ammonia Binding to the Oxygen‐Evolving Complex of Photosystem II: A Quantum Chemical Study 下载免费PDF全文
Johannes Schraut Prof. Dr. Martin Kaupp 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(24):7300-7308
A recent EPR study (M. Perrez Navarro et al., Proc. Natl. Acad. Sci. 2013 , 110, 15561) provided evidence that ammonia binding to the oxygen‐evolving complex (OEC) of photosystem II in its S2 state takes place at a terminal‐water binding position (W1) on the “dangler” manganese center MnA. This contradicted earlier interpretations of 14N electron‐spin‐echo envelope modulation (ESEEM) and extended X‐ray absorption fine‐structure (EXAFS) data, which were taken to indicate replacement of a bridging oxo ligand by an NH2 unit. Here we have used systematic broken‐symmetry density functional theory calculations on large (ca. 200 atom) model clusters of an extensive variety of substitution patterns and core geometries to examine these contradictory pieces of evidence. Computed relative energies clearly favor the terminal substitution pattern over bridging‐ligand arrangements (by about 20–30 kcal mol?1) and support W1 as the preferred binding site. Computed 14N EPR nuclear‐quadrupole coupling tensors confirm previous assumptions that the appreciable asymmetry may be accounted for by strong, asymmetric hydrogen bonding to the bound terminal NH3 ligand (mainly by Asp61). Indeed, bridging NH2 substitution would lead to exaggerated asymmetries. Although our computed structures confirm that the reported elongation of an Mn–Mn distance by about 0.15 Å inferred from EXAFS experiments may only be reproduced by bridging NH2 substitution, it seems possible that the underlying EXAFS data were skewed by problems due to radiation damage. Overall, the present data clearly support the suggested terminal NH3 coordination at the W1 site. The finding is significant for the proposed mechanistic scenarios of OEC catalysis, as this is not a water substrate site, and effects of this ammonia binding on catalysis thus must be due to more indirect influences on the likely substrate binding site at the O5 bridging‐oxygen position. 相似文献
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Resolving the Differences Between the 1.9 Å and 1.95 Å Crystal Structures of Photosystem II: A Single Proton Relocation Defines Two Tautomeric Forms of the Water‐Oxidizing Complex 下载免费PDF全文
Dr. Simon Petrie Prof. Ron J. Pace Prof. Rob Stranger 《Angewandte Chemie (International ed. in English)》2015,54(24):7120-7124
Great progress has been made in characterizing the water‐oxidizing complex (WOC) in photosystem II (PSII) with the publication of a 1.9 Å resolution X‐ray diffraction (XRD) and recently a 1.95 Å X‐ray free‐electron laser (XFEL) structure. However, these achievements are under threat because of perceived conflicts with other experimental data. For the earlier 1.9 Å structure, lack of agreement with extended X‐ray absorption fine structure (EXAFS) data led to the notion that the WOC suffered from X‐ray photoreduction. In the recent 1.95 Å structure, Mn photoreduction is not an issue, but poor agreement with computational models which adopt the ‘high’ oxidation state paradigm, has again resulted in criticism of the structure on the basis of contamination with lower S states of the WOC. Here we use DFT modeling to show that the distinct WOC geometries in the 1.9 and 1.95 Å structures can be straightforwardly accounted for when the Mn oxidation states are consistent with the ‘low’ oxidation state paradigm. Remarkably, our calculations show that the two structures are tautomers, related by a single proton relocation. 相似文献
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Visible‐Light Photocatalysis of C(sp3)‐H Fluorination by the Uranyl Ion: Mechanistic Insights 下载免费PDF全文
Dr. Liangliang Wu Dr. Xiaoyan Cao Prof. Xuebo Chen Prof. Weihai Fang Prof. Michael Dolg 《Angewandte Chemie (International ed. in English)》2018,57(36):11812-11816
The uranyl dication shows photocatalytic activity towards C(sp3)?H bonds of aliphatic compounds, but not towards those of alkylbenzenes or cyclic ketones. Theoretical insights into the corresponding mechanisms are still limited. Multi‐configurational ab initio calculations including relativistic effects reveal the inherent electron‐transfer mechanism for the uranyl catalyzed C?H fluorination under blue light. Along the reaction path of the triplet state it was found that the hydrogen atom abstraction triggered by the electron‐rich oxygen of the uranyl moiety is the rate‐limiting step. The subsequent steps, that is, N?F and O?H bond breakage in a manner of concerted asynchronicity, generation of the targeted fluorinated product, and recovery of the photocatalyst are nearly barrierless. Moreover the single electron transfer between the reactive substrates plays a fundamental role during the whole photocatalytic cycle. 相似文献
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Corrigendum: Transport,Electrochemical and Thermophysical Properties of Two N‐Donor‐Functionalised Ionic Liquids 下载免费PDF全文
Thomas Rüther Kenneth R. Harris Michael D. Horne Mitsuhiro Kanakubo Theo Rodopoulos Jean‐Pierre Veder Lawrence A. Woolf 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(10):3203-3203
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Dr. Oscar Martinez Jr. Prof. Kyle N. Crabtree Dr. Carl A. Gottlieb Prof. John F. Stanton Dr. Michael C. McCarthy 《Angewandte Chemie (International ed. in English)》2015,54(6):1808-1811
The phenyl radical (C6H5.) is the prototypical σ‐type aryl radical and one of the most common aromatic building blocks for larger ring molecules. Using a combination of rotational spectroscopy of singly substituted isotopic species and vibrational corrections calculated theoretically, an extremely accurate molecular structure has been determined. Relative to benzene, the phenyl radical has a substantially larger C‐Cipso‐C bond angle [125.8(3)° vs. 120°], and a shorter distance [2.713(3) Å vs. 2.783(2) Å] between the ipso and para carbon atoms. 相似文献
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Dr. Gerald Knizia Dr. Johannes E. M. N. Klein 《Angewandte Chemie (International ed. in English)》2015,54(18):5518-5522
The “curly arrow” of Robinson and Ingold is the primary tool for describing and rationalizing reaction mechanisms. Despite this approach’s ubiquity and stellar success, its physical basis has never been clarified and a direct connection to quantum chemistry has never been found. Here we report that the bond rearrangements expressed by curly arrows can be directly observed in ab initio computations, as transformations of intrinsic bond orbitals (IBOs) along the reaction coordinate. Our results clarify that curly arrows are rooted in physical reality—a notion which has been challenged before—and show how quantum chemistry can directly establish reaction mechanisms in intuitive terms and unprecedented detail. 相似文献
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E. Daiann Sosa Carrizo Prof. Dr. F. Matthias Bickelhaupt Dr. Israel Fernández 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(41):14362-14369
Trends in reactivity of β‐chloride and β‐hydride elimination reactions involving Group 10 transition‐metal complexes have been computationally explored and analyzed in detail by DFT. These reactions do not require the initial formation of a vacant coordination site; they proceed concertedly without a prior ligand‐dissociation step. Whereas β‐chloride elimination is associated with relatively moderate activation barriers, the high barriers calculated for analogous β‐hydride eliminations suggest that the latter process is unfeasible for this type of compounds. This differential behavior is analyzed within the activation strain model, which provides quantitative insight into the physical factors controlling these β‐elimination reactions. The effects of the nature of the Group 10 transition metal (Ni, Pd, Pt), as well as the substituents attached to the β‐eliminating fragment (R2C?CR2X; R, X=H, Cl) on the transformation have also been considered and are rationalized herein. 相似文献
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《Angewandte Chemie (International ed. in English)》2017,56(33):9732-9735
We report X‐ray crystallographic and 19F NMR studies of the G‐protein RhoA complexed with MgF3−, GDP, and RhoGAP, which has the mutation Arg85′Ala. When combined with DFT calculations, these data permit the identification of changes in transition state (TS) properties. The X‐ray data show how Tyr34 maintains solvent exclusion and the core H‐bond network in the active site by relocating to replace the missing Arg85′ sidechain. The 19F NMR data show deshielding effects that indicate the main function of Arg85′ is electronic polarization of the transferring phosphoryl group, primarily mediated by H‐bonding to O3G and thence to PG. DFT calculations identify electron‐density redistribution and pinpoint why the TS for guanosine 5′‐triphosphate (GTP) hydrolysis is higher in energy when RhoA is complexed with RhoGAPArg85′Ala relative to wild‐type (WT) RhoGAP. This study demonstrates that 19F NMR measurements, in combination with X‐ray crystallography and DFT calculations, can reliably dissect the response of small GTPases to site‐specific modifications. 相似文献