1,721,488 research outputs found
Theoretical studies on reactivity of organoboron compounds, transition metal boryl complexes, and acyclic silylene
No full textTheoretical studies on the reactivity of organoboron compounds, transition-metal boryl complexes, and acyclic silylene have been reported with the aid of DFT calculations in this thesis. We have unraveled reaction mechanisms based on experimental results and have made useful predictions on some cases. For organoboron compounds, the reactivity is dependent on the Lewis acidity of the empty p orbital of the boron center. Once the empty p orbital of the boron center is occupied by a nucleophile, the remaining three bonds around the boron center could be activated and undergo further migrations/transformations. This important insight helps us understand phosphine-catalyzed trans-hydroboration of alkynoate esters (Chapter 2), the reactivity of diborane compounds toward organic azides (Chapter 3), the reactivity of borole compounds toward epoxides (Chapter 4), and the reactivity of borole cycloaddition reactions (Chapter 5). For transition-metal boryl complexes, we have reported theoretical studies on the reactivity of a (NHC)Au-diarylboryl toward polar multiple bond molecule (Chapter 6) and Rh-catalyzed deoxygenative borylation of ketones (Chapter 7). Based on these studies and a series of previous work of our group on the reactivity of (NHC)Cu-boryl complexes, we have systematically compared the reactivities of different metal-boryl complexes toward phenyl aldehyde (Chapter 8), and pointed out that in addition to that the metal-boryl σ-bonding electrons could play the nucleophilic role, the electrophilicity of “empty” p orbital on the boron center of a boryl ligand, and the nucleophilicity of d electron pairs of the metal center could initiate a reaction as well. Apart from boron chemistry, theoretical studies on the reactivity of a silylene compound toward small gas molecule (CO, CO2 and N2O) activation is also reported (Chapter 9), our study has provided an in-depth understanding of the acyclic silylene’s single-site ambiphilic reactivity.</p
Mechanistic studies on hydrogenation reactions catalyzed by transition metal complexes
No full textThe development of catalysts relying on earth-abundant st-row transition metals is highly desirable. The unique electronic properties of 1st-row transition metal would give rise to more unexpected and surprising reactivities for homogeneous hydrogenation reactions. In addition, novel hydrogenation mechanisms distinguished from the classical “inner-sphere” and “outer-sphere” mechanisms are needed for the hydrogenation of some challenging substrates. In this thesis, we focus on the understanding of Co chemistry and the chemoselective issues related to the hydrogenation of C=C and C=O catalyzed by Co complexes (Chapters 2-5). Furthermore, we discussed two interesting examples of hydrogenation including (NHC)Cu(I)H-guanidine bifunctional catalysis for hydrogenation of esters (Chapter 6) and ionic hydrogenation of oxocarbenium ions (Chapter 7). Co(I), as one of the 1st-row transition metals with a d8 electron configuration, prefers 5-coordinated Co (I) complexes due to its relatively smaller d-p gap when compared to other heavier d8 transition metal centers. When Co(I) combines with a carbene ligand, it was found that trans-dihydride complexes could be stabilized, which are important intermediates frequently proposed in transition metal catalyzed hydrogenation reactions. Our theoretical studies indicate that via an “inner-sphere” mechanism, the Co(I) catalyst [Co(I)(H2)(PR3)] was able to selectively hydrogenate C=C but not C=O, while the Co(I)/tetraphosphine catalyst [(P4N2)Co(I)H] catalyzed selective reduction of C=O rather than the C=C via an “outer-sphere” mechanism. Besides the classical “inner-sphere” and “outer-sphere” mechanisms, novel hydrogenation mechanisms are interesting and can give surprising results and have the potential for the hydrogenation of challenging substrates. Related to this aspect, this thesis also covers two novel hydrogenation mechanisms: (1) a combination of a super hydride donor NHCCu(I)H and an organic guanidine pendant in the ligand is powerful to reduce the relative inert ester group (Chapter 6) and (2) a far less explored ionic hydrogenation mechanism was reported for the enantioselective hydrogenation of oxocarbenium ions (Chapter 7).</p
Exploring the mechanisms of nickel/rhodium-catalyzed cross-coupling and rhodium-catalyzed olefin hydroboration reactions, and rhenium/tantalum metalacyclic complex formation : a theoretical study
No full textThis thesis presents a series of studies investigating the mechanisms of various catalytic reactions and metalacyclic complex formation through the use of density functional theory (DFT) calculations. Specifically, the mechanisms of nickel (Ni)/rhodium (Rh)-catalyzed cross-coupling and rhodium-catalyzed olefin hydroboration reactions, as well as rhenium (Re)/tantalum (Ta) metalacyclic complex formation. In Chapter 2, the Ni-catalyzed cross-coupling reaction of 2,2-difluorovinyl benzoate and arylboronic acid was investigated through DFT calculations. It was found that the base exchange step after oxidative addition can facilitate the following transmetalation. Chapter 3 discussed the Ni-catalyzed asymmetric ring-opening reaction of dinaphthofuran forming C-C. It was discovered that migration of the phenyl nucleophile to the Ni(0) metal center prior to oxidative addition of a C-O bond to Ni(0) plays a significant role in the mechanism for asymmetric selectivity. In Chapter 4, the mechanisms of Rh(III)-catalyzed intramolecular cross-coupling reaction of two carbon-carbon double bond to form macrocycles was investigated. Theoretical studies revealed that the formation of unobserved olefination macrocyclization product needs to undergo β-H elimination, which requires a high Gibbs energy barrier due to the stabilization of the C-H bond by the electron-withdrawing group (-CF3). Chapter 5 discussed the mechanisms of Rh-catalyzed regiodivergent hydroboration of alkenes for the synthesis of linear and branched alkylboronates. DFT calculations revealed that the very different steric effects of two ligands are responsible for product selectivity. In Chapter 6, DFT calculations were performed to study Rh-catalyzed asymmetric hydroboration of silyl enol ethers using two new phosphine ligands to construct valuable chiral boronic esters. The study found that the ligand can create an excellent chiral pocket for the asymmetric hydroboration. Chapter 7 investigated the reactivity of rhenacyclobutadiene with allenes was studied using DFT calculations. It was found that the initial nucleophilic attack of allenes on either ReC(OR) or ReC(Ar) is important for the product selectivity. In Chapter 8, the mechanism on reactions of metallacyclobutadienes with aminoalkynes to synthesize Dewar metallabenzenes was investigated with DFT calculations. The mechanism of the reaction and the nature of Dewar metallabenzenes were discussed. Finally, in Chapter 9, the mechanism for the synthesis of tantallapyridinium complex using tantalum alkyne complexes with isocyanide was studied with the aid of DFT calculations. The tantallapyridinium ring was found to be aromatic based on various experimental and theoretical results. Overall, this thesis provides valuable insights into the mechanisms of various catalytic reactions and metalacyclic complex formations, which can aid in the development of new catalysts and synthetic method.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Theory of lone pair induced dynamical instability in halide perovskites
No full textWe study halide perovskites materials, which have lately emerged as potential materials for low-cost, high-efficiency solar cells due to their ease of synthesis and relatively abundant constituent elements, as well as their intriguing photo-physical features. Increasing evidences has showed that the drivers of their remarkable properties is related to a temperature-activated symmetry-lowering distortion effect, which was originally named emphanisis in the context of compounds like rock-salt group IV chalcogenides. Here we study a class of ABX3 compounds [A+ = Cs or CH3NH3 (MA) or CH(NH2)2 (FA); B2+ = Sn or Pb; X- = Br or I] to understand the driving forces behind emphanisis or dynamical off-centering instability. We show that electron-phonon interaction is the driving force with the covalency of BX3 framework and the dipole fluctuations of the A-site cation plays an essential role in determining the off-center temperature. We propose a general formula that capture the observed off-centering temperature dependence on aforementioned factors. Our theory suggests that A-site driven polaron-like model might be beneficial to understand the off-centering phenomenon in halide perovskites.</p
Exploring the dual nature of boracyclic compound with ring strain and Lewis acidity : synthesis, properties, and reactivity studies
No full textThis thesis reports on experimental and theoretical studies of boracyclic compounds with ring strain and/or Lewis acidity. A series of three-, four-, five-, and six-membered boracycles containing a three-coordinate boron center were synthesized, exhibiting interesting properties and reactivity. The small ring compounds, i.e., the three-and four-membered boracycles, possess strong strain and can undergo ring-expansion and ring-opening rearrangement reactions. Additionally, by replacing phenylene with ortho-carboranyl, three-dimensional analogues of 9-borafluorene and 9,10-boraanthracene were reported as Lewis superacids. These five-and six-membered boracycles have great potential in catalysis and the synthesis of low-valent species. In Chapter 2, four-membered boracyclic mono-BN isosteres of butafulvene, which are highly strained isomers of azaborines and B-amino boroles, have been synthesized via hydrolysis of urana-borabicyclic complexes. Their 4-dimethylaminopyridine (DMAP) adducts can further isomerize to 1,2,4,6-multisubstituted BN-9,1-naphthalenes. Both NMR reaction monitoring and theoretical calculations point to a reaction mechanism involving dearomative insertion of DMAP followed by two consecutive 1,2-hydrogen shifts. In Chapter 3, Density Functional Theory studies show among the parent borirane, benzoborirene and ortho-dicarbadodecaborane-fused borirane, the latter possesses the highest ring strain and the highest Lewis acidity. The o-carborane arylborirane, which possesses both features, not only allows the 1,2-insertion of P=O and C=O under milder conditions, but also enables the heretofore unknown dearomative 1,4-insertion of Ar-(C=O)- into a B–C bond. In Chapter 4, the thermally robust B-carborane iminoboranes (Me3Si)−Cb−B≡N−R (R = SiMe3, tBu) have been successfully synthesized by THF promoted isomerization from the corresponding o-carborane-fused aminoboriranes. Kinetics study revealed a bimolecular mechanism between borirane and THF, which is in good agreement with the computationally proposed reaction pathway. The reactivity of B-carborane iminoborane towards isonitriles and azides were studied as well. In Chapter 5, reactivity study of o-carborane-fused bis(trimethylsilyl)aminoborirane towards three different organic azides, i.e., aryl, alkyl, and silyl azides, are represented. The reaction with DippN3 (Dipp = 2,6-iPr2C6H4) resulted in the cycloaddition of DippN3 to the borirane BN unit accompanied by silyl migration. Conversely, in the reaction with BnN3, only the BnN3:borirane 1:2 ring expansion product was obtained. Finally, the reaction with Me3SiN3 resulted in a formally nitrene insertion product under thermal conditions. Furthermore, the mechanism of these substituent-dependent reactions was studied using DFT calculations. In Chapter 6, the Lewis acidity of three-dimensional analogues of 9,10–diboraanthracene, B2R2(C2B10H10)2 (R = Cl, Br, Me, Ph, H (only exists in silane adduct), N3), were studied by experimental (CV and Gutmann-Beckett method) and computational (fluoride ion affinity, hydride ion affinity and LUMO energies) methods. The parent species was isolated only as a neutral borane-silane adduct, which exhibits silane dissociation-association equilibrium in solution. In the thermally stable azido species, (C2B10H10)2(BN3)2, the combination of the Lewis superacidity of boron and the Lewis basicity of the adjacent azido Nα atom enabled the cooperative E–H (E = B, Si, Ge) activation at the B–N bond. In Chapter 7, the three-dimensional analogues of 9-borafluorenes, (C2B10H10)2BR (R = Cl, Br, Ph, Mes, H (merely as silane adduct)) were synthesized. Gutmann–Beckett and computational fluoride/hydride ion affinity (FIA/HIA) studies confirmed their Lewis superacidity, with the Lewis acidity of Cl, Br and parent species being higher than that of the corresponding 3D analogues of 9,10-diboraanthracenes (DBA). The mesityl species, which has a bulkier aryl group than phenyl species, exhibits distinct properties, such as air-stable and bluish-violet fluorescent arising from the intramolecular charge-transfer (ICT) transition. Additionally, the three-dimensional analogue of N-heterocyclic carbene (NHC) stabilized 9-borafluorene radical was synthesized, and its distinct boron-centered radical character was corroborated by DFT calculations.</p
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Computational material chemistry studies on stability of 2D phosphorus, conductivity of conjugated 2D COF and phase transformation of inorganic perovskite
No full textComputational studies on chemical properties of solid-state materials and important solid-phase chemical processes have been reported in thesis. Chapter Two reports a systematic study about the influence of gauche effect of P-P bonds on the relative stability among 2D phosphorus allotropes. This work revealed that the better stability of 2D violet phosphorus over other 2D phosphorus allotropes comes from the fact that 2D violet phosphorus has the highest proportion of P-P bonds under gauche conformation, the most energetically favorable P-P bond conformation, than other 2D phosphorus allotropes. And through orbitalwise integrated crystal orbital Hamilton population analysis, this work verified that the stability of gauche P-P bonds results from the minimized repulsion between the two neighboring lone pairs when the bond torsion is around 70~90°. Chapter Three presents a research work to investigate the reasons leading to the high conductivity of a newly synthesized 2D Covalent-Organic framework (COF) film from the perspective electronic structure. Our results indicated that the improved matching between the LUMO of the metal-porphyrin moiety and the LUMO of the applied linkage moiety leads to the formation of an enhanced donor–acceptor system which allows the excited electrons in metal-porphyrin center to facilely flow into the unoccupied orbitals of the applied linkage moieties. Bonding analysis showed that, apart from interlayer π-π stacking, interlayer metal-metal bonds also exist, and that 2D COFs with different kinds of metal centers have some subtle differences in terms of the characteristics of the interlayer metal-metal bonds. Chapter Four covers our metadynamics simulation of the δ- to α-CsPbI3 transformation at elevated temperature. The results of our simulation clearly showed that, throughout the process, δ-CsPbI3 underwent two stages of movements, the intrachain rearrangement stage and then the interchain connection stage, to transform into α-CsPbI3. The dynamics of different types of atoms of the Pb-I framework in these two stages were elucidated clearly through our detailed statistical analysis. The dynamic behavior of Cs+ cations was observed to possibly facilitate the interchain connection stage of the transformation.</p
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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