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We report the first computational study on a nickel hydride HAT-initiated catalytic reaction, a novel hydrodefluorination of CF3-substituted aryl alkenes to afford gem-difluoroalkenes. This study provides detailed mechanistic insights into the reaction, including HAT from NiH to C＝C, a carbon radical rebound to nickel to facilitate chemoselective defluorination, and a two-state reactivity of Ni(II) enabling σ-bond metathesis with PhSiH3 to regenerate the catalyst. The findings can have implications for developing new metal hydride HAT-initiated reactions. [ABSTRACT FROM AUTHOR]

A late Middle Pleistocene mandible from Baishiya Karst Cave (BKC) on the Tibetan Plateau has been inferred to be from a Denisovan, an Asian hominin related to Neanderthals, on the basis of an amino acid substitution in its collagen. Here we describe the stratigraphy, chronology, and mitochondrial DNA extracted from the sediments in BKC. We recover Denisovan mitochondrial DNA from sediments deposited ~100 thousand and ~60 thousand years ago (ka) and possibly as recently as ~45 ka. The long-term occupation of BKC by Denisovans suggests that they may have adapted to life at high altitudes and may have contributed such adaptations to modern humans on the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Denisovans, well known for their high-coverage genetic sequences, are considered to be a new human species genetically distinct from Neanderthals and modern humans [1]. As a close sister group of Neanderthals, Denisovans not only interbred with Neanderthals [2], but they also contributed genetically to extant humans, including modern populations in parts of Oceania and Asia, aboriginal Americans and high-altitude modern Tibetans [[3]]. According to these two studies, the oldest Denisovan fossils may have been as early as 195 ka (Denisova 2) [8], while archaeological remains and sedimentary DNA extracts indicate that the cave may have been occupied by Denisovans since around 300 ka (or, conservatively, around 200 ka) until 50 ka [6], overlapping with occupations of Neanderthals from 193 to 97 ka [8]. (a) Denisova Cave and Baishiya Karst Cave where Denisovan fossils have been discovered (red), and Middle Pleistocene archaeological sites with archaic Homo sapiens fossils in East Asia (blue). [Extracted from the article]

A series of Holocene charcoal concentrations (CC) records from a slope sedimentary section in Gonghe Basin, northeast Tibetan Plateau, are analyzed to investigate fire characteristics (e.g., frequency or magnitude) and their linkages with paleoclimate change and ancient human activities. The results reveal that changes in fire indicators are well correlated with paleoclimatic change during the past 10.0 ka BP. This pattern is characterized by lower CC in the warm and humid middle Holocene, and higher total CC in the cold, dry climate of the early and late Holocene. Meanwhile, an event characterized by abnormally high CC values at ∼3.6 ka BP, distinct from the above trend, indicates an anomalously high fire frequency which cannot easily be explained as the result of a climatic event. Alternatively, this event is noted to correspond closely with the time of rapid human occupation of the high elevation Tibetan Plateau. Many basic domestic and agricultural activities (e.g., slash-and-burn farming, cooking or heating.) have required fire in the hostile environment of the Tibetan Plateau; this is reflected in the higher CC following the spread of human habitation. The analysis of CC might be a useful alternative method for investigating human activities when no direct evidence is available in the Holocene Tibetan Plateau records. [ABSTRACT FROM AUTHOR]

Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are highly toxic to humans and the environment. Developing efficient methods to detect or remove these pollutants is particularly important and urgent. Boron nitride nanotubes (BNNTs) with low dimension and high surface-to-volume ratio might be one of promising materials for the adsorption of PCDD/Fs. Here we present a density functional theory (DFT) study on the interaction of the pristine and Ni doped (8,0) single-walled BNNTs with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the most toxic congener among PCDD/F family. The calculated results show that the pristine BNNT intrinsically interacts with TCDD via physisorption with π-π stacking interaction, in contrast, the Ni-doped BNNT presents much higher reactivities toward TCDD. The impurity Ni atom plays a crucial role for capturing TCDD molecule. We also find that the Ni doping introduces the local electronic states within the band gap of the BNNT and induces magnetism in the doped systems. The present results are expected to provide useful guidance for the potential application of BNNTs as adsorption materials for detecting or removing dioxin pollutants. [ABSTRACT FROM AUTHOR]

Prehistoric human history on the Tibetan Plateau is a hotly debated topic. Archaeological research on the plateau during the past few decades has enormously improved our understanding of the topic and makes it possible for us to consider the processes and mechanisms of prehistoric human migration to the region. By reviewing the published archaeological research on the Tibetan Plateau, we propose that the first people on the plateau initially spread into the He-Huang region from the Chinese Loess Plateau, and then moved to the low elevation Northeastern Tibetan Plateau and perhaps subsequently to the entire plateau. This process consisted of four stages. (1) During the climatic amelioration of the Last Deglacial period (15-11.6 ka BP), Upper Paleolithic hunter-gatherers with a developed microlithic technology first spread into the Northeastern Tibetan Plateau. (2) In the early-mid Holocene (11.6-6 ka BP), Epipaleolithic microlithic hunter-gatherers were widely distributed on the northeastern plateau and spread southwards to the interior plateau, possibly with millet agriculture developed in the neighboring low elevation regions. (3) In the mid-late Holocene (6-4 ka BP), Neolithic millet farmers spread into low elevation river valleys in the northeastern and southeastern plateau areas. (4) In the late Holocene (4-2.3 ka BP), Bronze Age barley and wheat farmers further settled on the high elevation regions of the Tibetan Plateau, especially after 3.6 ka BP. Finally, we suggest that all of the reported Paleolithic sites earlier than the LGM on the Tibetan Plateau need further examination. [ABSTRACT FROM AUTHOR]

G-rich aptamers have been widely applied to develop various sensors for detecting proteins, small molecules, and cations, which is based on the target-induced conformational transfer from single strand to G-quadruplex. However, the transforming process is unclear. Here, with PW17 as an aptamer example, the forming process of G-quadruplex induced by K+ is investigated by circular dichroism spectroscopy, electrospray ionization mass spectroscopy, and native gel electrophoresis. The results demonstrate that PW17 undergoes a conformational transforming process from loose and unstable to compact and stable G-quadruplex, which is strictly K+ concentration-dependent. The process contains three stages: (1) K+ (<0.5 mM) could induce PW17 forming a loose and unstable G-quadruplex; (2) the compact and stable K+-stabilized G-quadruplex is almost formed when K+ is equal to or larger than 7 mM; and (3) when K+ ranges from 0.5 mM to 7 mM, the transformation of K+-stabilized PW17 from loose and unstable to compact and stable occurs. Interestingly, dimeric G-quadruplex through 5'-5' stacking is involved in the forming process until completely formed at 40 mM K+. Moreover, the total process is thermodynamically controlled. With PW17 as a sensing probe and PPIX as a fluorescent probe for detection of K+, three linear fluorescent ranges are observed, which corresponds to the three forming stages of G-quadruplex. Clarifying the forming process provides a representative example to deeply understand and further design aptamer-based biosensers and logic devices. [ABSTRACT FROM AUTHOR]

Highlights: [•] The reactivities of Ge doped BNNT towards CO and NO are theoretically investigated. [•] Toxic CO and NO molecules present strong chemisorption on the Ge doped BNNT. [•] The doping of the Ge atom improves the electronic property of the BNNT. [•] Ge doped BNNT is expected to be a potential sensor for detecting toxic CO and NO. [ABSTRACT FROM AUTHOR]

Although chlorophenols (CPs) areconsidered to be the most important and direct precursors of polychlorinateddibenzo-p-dioxins and dibenzofurans (PCDD/Fs), ourunderstanding of the formation mechanism of PCDD/Fs is exclusivelylimited to an invariable idea that chlorophenoxy radicals (CPRs) areonly necessary intermediates. The present work presents a systematictheoretical study that aims at providing new insight into the homogeneousformation of PCDD/Fs from CPs. Two different types of radicals fromCPRs, i.e., substituted phenyl radicals and phenoxyl diradicals, areproposed to serve as potential sources contributing to the formationof PCDD/Fs. The thermodynamic and kinetic properties of reactionsof 2-chlorophenol (2-CP), as a representative of CP congeners, withatomic H to produce various potential radicals forming PCDD/Fs arestudied by performing density functional theory calculations and directkinetics studies. The newly proposed radicals, especially substitutedphenoxyl diradicals (the most direct intermediates of PCDD/Fs), canbe formed via reactions of 2-CP with atomic H with small barriersand large reaction energies. They should be expected to be responsiblefor the homogeneous formation of PCDD/Fs under high temperature. Severaltypical PCDD/F products are predicated through direct self- and cross-couplingsof the newly proposed radicals. The radical coupling patterns proposedin the present work expand our understanding of the formation mechanismof PCDD/Fs from CP precursors. [ABSTRACT FROM AUTHOR]

By performing density functional theory calculations, we studied methanol decomposition promoted by neutral, anionic and cationic Au trimers, which represent the three simplest prototypes of Au-cluster-based catalysts with different charge states. The results show that the Au3- and Au3+-mediated reactions proceed via four successive single dehydrogenation steps, CH3OH→CH3O→CH2O→CHO→CO, while the Au3 --mediated reaction occurs through two double dehydrogenation steps, CH3OH→CH2O→CO. The additional negative charge reduces remarkably the binding capability of CO (the completely dehydrogenated product of methanol) on the cluster, and is thus beneficial in reducing poisoning of the catalyst by CO. In contrast, the neutral and positively charged clusters present strong interactions with CO, making the catalyst readily poisoned by CO. Furthermore, the reaction promoted by the cationic cluster shows a much higher energy barrier than those by the neutral and anionic clusters. So selecting suitable substrates that make Au nanoparticles negatively charged may be a promising strategy for promoting methanol oxidation. [ABSTRACT FROM AUTHOR]

Abstract: Density functional theory (DFT) calculations show a new concerted mechanism of formic acid (HCOOH) oxidation on Pt (111), which involves the simultaneous formation of CO2 and CO via the HCOOH dimer in an elementary step. The newly proposed mechanism rationalizes the easy CO poisoning of Pt-based catalysts and improves our understanding for the mechanism of catalytic HCOOH oxidation. [Copyright &y& Elsevier]

The mechanism and products of the reaction of 2-methyl-3-buten-2-ol (MBO232) with Cl atoms in the presence of O2 have been elucidated by performing high-level quantum chemistry calculations. The geometries of the reactants, intermediates, transition states, and products are optimized at the MP2(full)/6-311G(d, p) level, and their single-point energies are refined at the CCSD(T)/6-311 + G(d, p) level. The potential energy surface profiles have been constructed at the CCSD(T)/6-311 + G(d, p)//MP2(full)/6-311G(d, p) + 0.95 × ZPE level of theory, and the possible channels involved in the reaction are also discussed. The calculations indicate that the reaction predominantly proceeds via the addition of Cl atoms to the double bond rather than the direct abstraction of the H atoms in MBO232. The nascent adducts (CH3)2C(OH)CHCH2Cl (IM1) and (CH3)2C(OH)CHClCH2 (IM2) do not undergo subsequent isomerization and dissociation reactions, but rather react with O2. The theoretical results show that the major products are CH2ClCHO and CH3C(O)CH3 for the reaction of MBO232 + Cl in the presence of O2, which is in good agreement with the experimental finding. [ABSTRACT FROM AUTHOR]

Density functional theory calculations were carried out to reveal the mechanistic details of aldehyde–alkyne reductive couplings with trialkylsilane/dialkylsilane. The reaction with trialkylsilane is found to proceed through oxidative cyclization, Si‐H/Ni‐O σ‐bond metathesis, and C(sp2)‐H reductive elimination, leading to silylated allylic alcohols. The steric hindrance between the n‐pent group of alkyne and iPr group of the NHC ligand determines the regioselectivity. While for the reaction with dialkylsilane, the present calculations propose a new mechanism, which consists of oxidative cyclization, Si−H/Ni−O σ‐bond metathesis, Ni−C/Si−H σ‐bond metathesis, and dehydrogenation, resulting in oxasilacyclopentenes. The calculated energy profiles rationalize the experimentally observed chemodivergence. [ABSTRACT FROM AUTHOR]

The aromatic CC bond cleavage by a tungsten complex reported recently by Sattler and Parkin15 offers fresh opportunities for the functionalization of organic molecules. The mechanism of such a process has not yet been determined, which appeals to computational assistance to understand how the unstrained CC bond is activated at the molecular level.16, 17 In this work, by performing density functional theory calculations, we studied various possible mechanisms of cleavage of the aromatic CC bond in quinoxaline (QoxH) by the W-based complex [W(PMe3)4(η2-CH2PMe2)H]. The calculated results show that the mechanism proposed by Sattler and Parkin involves an overall barrier of as high as 42.0 kcal mol−1 and thus does not seem to be consistent with the experimental observation. Alternatively, an improved mechanism has been presented in detail, which involves the removal and recoordination of a second PMe3 ligand on the tungsten center. In our new mechanism, it is proposed that the CC cleavage occurs prior to the second CH bond addition, in contrast to Sattler and Parkin's mechanism in which the CC bond is broken after the second CH bond addition. We find that the rate-determining step of the reaction is the ring-opening process of the tungsten complex with an activation barrier of 28.5 kcal mol−1 after the first PMe3 ligand dissociation from the metal center. The mono-hydrido species is located as the global minimum on the potential-energy surface, which is in agreement with the experimental observation for this species. The present theoretical results provide new insight into the mechanism of the remarkable CC bond cleavage. [ABSTRACT FROM AUTHOR]

To understand the mechanism of allene formation throughthe rearrangement of cyclopropenes catalyzed by PtCl2,we have performed a detailed density functional theory calculationstudy on a representative substrate, 1-(trimethylsilyl)-2-(phenylethyl)cyclopropene.Three reaction pathways proposed in the original study have been examined;however the calculated results seem not to completely rationalizethe experimental findings. Alternatively, by performing an exhaustivesearch on the potential energy surface, we present a novel mechanismof PtCl2, which is fixed appropriately on the cyclopropene/alleneto form the linear Cl–Pt–Cl disposition, a vital configurationfor catalyzing the rearrangement of cyclopropene. The newly proposedmechanism involves an SN2-type C–C bond activationof the cyclopropene by PtCl2fixed on a cyclopropene moleculevia the d−π interaction between the metal center andthe substrate to form the product precursor PtCl2-allenewith the metal center coordinated to the external CC bondin the allene framework. Once formed, the PtCl2-alleneimmediately serves as a new active center to catalyze the rearrangementreaction rather than directly dissociating into the allene productand the PtCl2catalyst due to its high stability. Duringthe catalytic cycle, an allene-PtCl2-allene sandwich compoundis identified as the most stable structure on the potential energysurface, and its direct dissociation results in the formation of theproduct allene and the regeneration of the catalytically active centerPtCl2-allene with an energy demand of 24.4 kcal/mol. Thisprocess is found to be the rate-determining step of the catalyticcycle. In addition, to understand the experimental finding that theH-substituted cyclopropenes do not provide any allenes, we have alsoperformed calculations on the H-substituted cyclopropene system andfound that the highest barrier to be overcome during the catalyticcycle amounts to 35.2 kcal/mol. This high energy barrier can be attributedto the fact that the C–H bond activation is more difficultthan the C–Si bond activation. The theoretical results notonly rationalize well the experimental observations but provide newinsight into the mechanism of the important rearrangement reaction. [ABSTRACT FROM AUTHOR]

Abstract: The rapid enrichment and detection of trace polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are currently challenging issues in the field of environmental science. In this paper, by performing quantum chemistry (QM) calculations and molecular dynamics (MD) simulations, we studied the inclusion complexation of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a representative PCDD molecule, with β-cyclodextrin (β-CD), one of the widely used compounds in supramolecular chemistry. The calculated results reveal that the stable inclusion complex can be formed in both the gas phase and solvent, which proposes that β-CD may serve as a potential substrate enriching TCDD. The calculated vibrational spectra indicate that the infrared (IR) and Raman spectroscopy may be suitable for the detection of β-CD-modified TCDD. The present theoretical results may be informative to environmental scientists who are devoting themselves to developing effective methods for detection and treatment of POPs. [Copyright &y& Elsevier]

By performing density functional theory calculations, the Menshutkin reaction between the N-methyl imidazole with chloroethane is reexamined to rationalize the experimental discovery. The calculated results show that the reaction proceeds via a S2 mechanism with a barrier of 119.1 kJ mol which is much lower than that reported in previous literature according to a five-membered transition state mechanism. Moreover, it is found that the barrier is further reduced to 98.1 kJ mol in toluene solution. The present result validates the experimental finding that the Menshutkin reaction for synthesizing N-alkyl imidazolium halide salts proceed smoothly at lower heating temperature. [ABSTRACT FROM AUTHOR]

Abstract: The interactions of the cellulose molecule with several anions, including acetate , alkyl phosphate, tetrafluoroborate and hexafluorophosphate anions which are most commonly involved in the imidazolium ionic liquids (ILs), have been studied by performing density functional theory calculations. Based on calculated geometries, energies, IR characteristics, and electronic properties of the cellulose–anion complexes, it is found that the strength of interactions of anions with cellulose follows the order: acetate anion>alkyl phosphate anion>tetrafluoroborate anion>hexafluorophosphate anion, which is consistent with the experimentally observed solubility trend of cellulose in the corresponding imidazolium-based ILs. The present study may provide basic aids to some extent for understanding the dissolution behavior of cellulose in the imidazolium-based ILs. [ABSTRACT FROM AUTHOR]

To better understand the interactions between cellulose and imidazolium-based ionic liquids (ILs), quantum chemistry calculations have been performed on the systems composed of one cellulose unit with the anion, cation, and the ion pair of 1-butyl-3-methylimidazolium chloride ([bmim]Cl) by the density functional method. The relevant geometries, energies, electronic properties and IR characteristics have been systematically discussed. It is found that H-bond interaction is essential for the systems under consideration. The hydroxyls in cellulose bind to chloride anions strongly through H-bonds, which could be predominant to cellulose dissolution in ILs. Chloride anion prefers to occur between two adjacent hydroxyls in cellulose to form bridging OH⋯Cl⋯HO hydrogen bonds. In contrast, weak hydrogen bonds exist between the hydrogen atoms on the imidazolium cation and hydroxyl oxygen atoms of cellulose, which are too much weaker than the hydrogen bonds between the cellulose hydroxyls and chloride anions to be detected by the experiments. The phenomena of cellulose dissolution in ILs should be a result of the joint interactions of chloride anions and [bmim]+ cations with hydroxyls in cellulose. [ABSTRACT FROM AUTHOR]

This paper reports the recent excavation of Unit Dadiwan06 at the Dadiwan site in Qin'an County, Gansu. A 65 ka chronological framework is established for Dadiwan06 on the basis of absolute dating (AMS 14C and OSL), stratigraphy, climate change events and archaeology. Artifact distributions reveal patterns of human behavioral variation and adaptation over the past 60 ka, from primitive hunting and gathering to advanced hunting and gathering, to primitive Neolithic agriculture, and finally to advanced Neolithic agriculture. [ABSTRACT FROM AUTHOR]

By performing density functional theory calculations, we have investigated the Michael addition of acetylacetone to methyl vinyl ketone in the absence and presence of the ionic liquid 1-butyl-3-methylimidazolium hydroxide ([bmIm]OH). In the absence of ionic liquids, acetylacetone is firstly tautomerized to enol form and then takes place Michael addition to methyl vinyl ketone. As in the catalyzed Michael addition reaction, a bmIm+-OH- ion pair is introduced into the reaction system to model the effect of the ionic liquid environment on the reactivity. The calculated results show that the anion enhances nucleophilic ability of acetylacetone since the OH- anion captures a proton to form an acetylacetone anion-H2O complex, and the cation improves the electrophilic ability of methyl vinyl ketone by forming intermolecular hydrogen-bonds. Both the remarkable effects of the cation and anion on the reactivity of reactants promote this reaction, which take place more easily compared with uncatalyzed reaction. The calculated results show that the main product of the Michael addition is in its ketone form. Our study provides a detailed reaction mechanism of Michael addition catalyzed by basic ionic liquid [bmIm]OH and clearly reveal the catalytic role of ionic liquid in important chemical reaction. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [ABSTRACT FROM AUTHOR]

Existing theoretical results for formic acid (HCOOH) decomposition on Pt(1 1 1) cannot rationalize the easy CO poisoning of the catalysts in the gas phase. The present work reexamined HCOOH decomposition on Pt(1 1 1) by considering the effect of the initial adsorption structure of the reactant on the reactivity. Our calculations present a new adsorption configuration of HCOOH on Pt(1 1 1), from which the formation of CO is found to be competing with the formation of CO 2 . The newly proposed mechanism improves our understanding for the mechanism of HCOOH decomposition catalyzed by Pt-based catalysts. [ABSTRACT FROM AUTHOR]

Abstract: We have carried out a detailed theoretical study for the geometrical structures and electronic properties of the cation and the ion pair of the cycle tetramethylguanidinium nitrate ionic liquid. It is found that in the cation, the triazolium ring presents almost a plane and the NMe2 group is not coplanar with the ring. The central C2 atom carries high positive charge, which is expected to act as the most active site for the electrostatic attacking of an anion. For the ion pair, three most stable configurations have been located, where the anion lies on either side of the triazolium ring or between C5 and C6 in the cation via multiple hydrogen bonds. However, the calculated results show that the electrostatic interaction between cation and anion plays a crucial role for stabilizing the ion pair. Moreover, the charge-localized character of the cation effectively increases the electrostatic interaction between the cation and anion, which is mainly responsible for the high thermal stabilities of the ionic liquids. There is a close correlation between the charge on C2 and the relative stability of the ion pair: the more positive charge on C2, the more stable the ion pair. Natural bond orbital and frontier molecular orbital analyses reveal that the charge transfer from the anion to cation occurs mainly through the , or interactions. [Copyright &y& Elsevier]

Abstract: By performing DFT calculations, the reaction of 4-chlorophenol with OH is reexamined to reconcile an experimental finding with a recent theoretical result. The calculated results show that abstracting hydrogen atom in the hydroxyl of 4-chlorophenol by the OH is the most plausible process for forming 4-chlorocaechol intermediate, while adding OH to the aromatic ring is the dominant pathway for forming the hydroquinone. The 4-chlorophenoxyl radical plays a crucial role during the hydroxyl-initiated 4-CP degradation due to its energetic stability and the low barrier involved in the reaction, which supports the experimental finding but differs from the recent theoretical result. [Copyright &y& Elsevier]

Abstract: Geminal dicationic ionic liquids (ILs), a new category of IL family, have been developed recently and found to possess unique properties compared to conventional monocationic ILs. To establish a basis for understanding their novel properties, we studied the geometrical and electronic structures of the dication ([(mim)C3(mim)]2+) and the ion pair ([(mim)C3(mim)]2+-2Br−) in the geminal dicationic IL 1,3-bis[3-methylimidazolium-yl]propane bromide by performing density functional theory calculations. The geometrical structures and relative stabilities for the dication and the ion pair are discussed, and their electronic properties are analyzed in detail. The intrinsic interaction between the dication and Br anions in the most stable conformer was investigated by performing the natural bond orbital analyses. Results for the dication and the ion pair are compared with those of the corresponding monocation ([C4mim]+) and ion pair ([C4mim]+-Br−). 1H NMR spectroscopy for the most stable ion pair has been calculated and the general trend is found to be in fairly agreement with the experimental data. [Copyright &y& Elsevier]

Abstract: The reactivity of silica toward to water is an important issue in environmental science and materials science for many years. In this paper, we study the hydrolysis stabilities of silica molecular chains and molecular rings based on two-membered silica ring by performing density functional theory calculations. It is found that the hydrolysis for the linear molecular chains firstly takes place in the middle parts and carries on gradually to the ends. In contrast with the early conjecture that the molecular rings might have higher hydrolysis stability than the corresponding linear chains, we find that these fully coordinated molecular rings are less stable than the corresponding molecular chains with the presence of water molecule. [Copyright &y& Elsevier]

Abstract: The Michael addition of malononitrile to an α,β-unsaturated imide catalyzed by a bifunctional-thiourea catalyst, which represents a prototype of catalytic asymmetric C–C forming reactions, has been investigated by performing density functional theory calculations. Two enantioselective reaction channels, denoted as (R)- and (S)-channels, which refer to the (R)- and (S)-configurations of the Michael adducts, have been explored in detail. It was shown that the formation of the Michael adduct involves three elementary steps: the catalyst protonation, the C–C bond coupling, and the catalyst deprotonation. The C–C bond coupling step has been identified as the energetic bottleneck along each channel. The calculated barrier for each of the three elementary steps along the (R)-channel is considerably less energy demanding than the corresponding those along the (S)-channel. Factors contributing to the high enantioselectivity have been clarified. The present results provided by our calculations are in agreement with all experimental findings and allow a detailed and consistent view of the mechanistic details of this important reaction sequence. [Copyright &y& Elsevier]

Abstract: The geometrical structures and relative stabilities of boron-rich boron carbon clusters B n C (n =1–7) are investigated using density functional theory calculations. The planar multi-cyclic geometries for n =3–7 with C atom at the apex with most B–C bonds and most three-coordination boron atoms are the most stable structures. It is interesting that the C atoms reside in the apexes not doped in the B rings, that is the doped C atoms prefer to the surface of the B structures and do not change the frame structures of B clusters. The compact 3D structures and the linear isomers, however, are energetically more unfavorable for n =3–7 with the increase of the size. The calculated disproportionation energy, binding energy and HOMO–LUMO gaps show that the B2C and B6C clusters are magic clusters. [Copyright &y& Elsevier]

The theoretical studies of how room temperature ionic liquids control desired reactions are very scarce in contrast with their increasing applications in many fields as recyclable solvents, catalysts, and reaction mediums. The present work considers the Diels–Alder (D-A) reaction of cyclopentadiene with methacrolein in the presence of diethylimidazolium salts as the first prototype of our systemic studies about important organic synthesis reactions catalyzed by room temperature ionic liquids. We show the mechanism details of the D-A reactions with and without the dialkylimidazolium cation and rationalize the experimental findings based on the results from the quantum chemistry calculations at the AM1, HF/6-31G(d), and B3PW91/6-31G(d,p) levels of theory, respectively. It is found that the diethylimidazolium cation acts as a Lewis acid center to catalyze the D-A reaction, which decreases the barrier and increases the asynchronicity of the D-A reaction, but does not change the potential energy surface profile of the reaction compared to the noncatalyzed process. The present results rationalize the early experimental findings well and provide the first prototype for theoretically understanding the D-A reaction in the presence of dialkylimidazolium salts. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [ABSTRACT FROM AUTHOR]

Abstract: The tandem nitroso aldol/Michael reaction between nitrosobenzene and cyclohexenone with pyrrolidine-based catalyst has been recently reported to obtain O-nitroso Diels–Alder bicyclic products. We present here a theoretical study for the novel reaction to rationalize the experimental findings of the regioselectivity and bicyclic products of the reaction. By performing density functional theory calculations, we have identified the detailed mechanism of the title reaction and the pivotal factors controlling the regioselectivity of the reaction. Two regioselective channels (O- and N-selective) for the aldol/Michael reaction have been characterized in detail. The calculated results indicate that both the aldol reaction and the next Michael reaction for the O-selective channel are much more favorable in energy than the corresponding N-selective channel. Theoretical results account well for the regioselectivity and the formal nitroso Diels–Alder adducts observed in the recent experiment. [Copyright &y& Elsevier]

Abstract: The direct catalytic enantioselective α-amination reaction of carbonyl compounds, a powerful approach to asymmetric carbon–nitrogen bond-forming, has been extensively studied, however, our understanding of the mechanism is far from complete. A theoretical study is presented for the α-amination reaction of 2-acetylcyclopentanone with azodicarboxylate catalyzed by a urea-based chiral bifunctional organocatalyst. By performing density functional theory (DFT) calculations, we have identified a detailed mechanism of the reaction and the roles of the amino group and urea in the catalyst. The structures of the catalyst, substrates, intermediates, and transition states involved in the reaction have been located along four possible reaction channels. The rate-determining step is the C–N bond-forming step. The calculations show that the catalyst promotes the reaction by deprotonating 2-acetylcyclopentanone and forming hydrogen bonds with the substrates. The origin of enantioselectivity of the reaction is also discussed. [Copyright &y& Elsevier]

The NO migration from N-methyl-N-nitrosobenzene-sulfonamide to 3,6-dibromocarbazole was proposed in a recent literature to follow a stepwise reaction path. However, the present density functional theory calculations at the MP2/6–31G(d,p)//B3LYP/6–31G(d,p) level show that this reaction exclusively proceeds via a concerted mechanism involving a four-membered ring transition state. The calculated barrier is in good agreement with the experimental finding. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [ABSTRACT FROM AUTHOR]

Abstract: In order to explore a novel sensor to detect the gaseous molecules, we investigate reactivities of the intrinsic and aluminum-doped (Al-doped) single-walled (8,0) carbon nanotube (SWCNT) with CO using density functional theory (DFT) calculations. The Al-doped SWCNT presents high sensitivity to CO, compared with the intrinsic SWCNT, as indicated by the calculated geometrical structures and electronic properties for these systems. Al-doped SWCNTs are expected to be a potential candidate for detecting the presence of CO. [Copyright &y& Elsevier]

Abstract: The geometrical structures and relative stability of the various possible isomers of silicon–oxygen–sulfur ionic oligomers, and (n =1–6), were studied using quantum chemistry calculations. The ground state of the monomer is a triangular molecule, and that of the dimmer is a rhombic 4MR with two S atoms ending the same Si atom. The most stable isomers for the trimer and tetramer anionic clusters are simply extensions of the rhombic structure, while the tetramer cationic cluster becomes the linear 4MRs with one Si–S defect at the each side. However, for n =5 and n =6, the hybrid structures by the 4MRs and hexagonal six-membered ring (6MR) become energetically most favorable configurations. These distinctive structural geometries would provide guide for future experimental detections of these small Si–O–S ionic oligomers. [Copyright &y& Elsevier]

Abstract: The Michael reaction of nitroalkenes catalyzed by a bifunctional-urea is studied using density functional theory (DFT) calculations, to determine the detailed catalytic mechanism and key factors controlling the enantioselectivity. Four reaction channels, corresponding to the different approach modes of nitroalkenes to a chiral scaffold and different processes of second proton transfer, have been characterized. The rate determining step is proton transfer from the amino group of a catalyst to an α-carbon of nitronate, and the enantioselectivity is controlled by the steps involved in carbon–carbon bond formation. The calculated results provide a general model that explains the mechanism and enantioselectivity of the title reaction. [Copyright &y& Elsevier]

Abstract: In an effort to search for new inorganic fullerene-like structures, we designed a series of novel silicon–carbon cages, (SiC) n (n =6–36), based on the uniformly hybrid Si–C four- and six-membered-rings, and researched their geometrical and electronic structures, as well as their relative stabilities using the density function theory. Among these cages, the structures for n =12, 16, and 36 were found to been energetically more favorable. The calculated disproportionation energy and binding energy per SiC unit show that the (SiC)12 cage is the most stable one among these designed structures. The present calculations not only indicate that silicon–carbon fullerenes are promised to be synthesized in future, but also provide a new way for stabilizing silicon cages by uniformly doping carbon atoms into silicon structures. [Copyright &y& Elsevier]

Abstract: The geometrical structures and relative stability of the various possible isomers of silicon–oxygen–sulfur oligomers, (SiOS) n (n =1–6), were studied using quantum chemistry calculations. The linear monomer with Si atom bonding to both O and S atoms is the ground state isomer, and the rhombic chains of four-membered rings are the most stable configurations for n =2–4, while the hybrid structures by the 4MRs and hexagonal six-membered ring become energetically most favorable for n =5 and n =6. Vibrational frequency calculations show that the characteristic IR signatures occur at 1290cm−1 for n =1, at the vicinity of 900cm−1 for n =2–4, and at about 1080cm−1 for n =5–6. These distinctive spectra would provide guide for future experimental detections of these small Si–O–S oligomers. [Copyright &y& Elsevier]

The reactivities and bonding characteristics of carbon and nitrogen hydride species, CHx (x=1–4) and NHy (y=1–3), on wurtzite AlN(0001) surface have been investigated using ab initio molecular orbital calculations performed at HF/6-31G(d) level of theory in order to suggest a substrate pretreatment for chemical vapor deposition of diamond films on AlN substrate. It is shown that the ionic carbon and nitrogen hydride species possess higher chemical reactivity with AlN substrate than the corresponding neutral species. Both the carbon and nitrogen hydrides, especially for the nitrogen hydrides, prefer bonding to the top site of the aluminum atom to the three-fold hollow site above the nitrogen atom on the 〈0001〉 direction of w-AlN. NH2 species is found to form the most stable interfacial bond with the Al atom on the surface with a binding energy approximately 20 kcal mol−1 more than that of CH3. Our results imply that a surface pretreatment of AlN substrate with NH2 species before the diamond deposition would facilitate stable interfacial bonding and hence a good adhesion of the grown film with the substrate. [Copyright &y& Elsevier]

The reaction of Fe+ (6D) with ethane in the gas-phase was re-examined at B3LYP/6-311++G(3df,3pd)//B3LYP/6-311+G(d,p) level of theory to explore new characteristic of the C–C and C–H bond activations in alkanes. Two different mechanisms from those described in the early works are shown: one is the concerted elimination of CH4 against the consecutive addition–elimination mechanism, and the other is the 1,1-H2 elimination vs. general 1,2-H2 elimination. Although these alternative pathways are unimportant experimentally as they involve high-energy transition states, it may improve our understanding for the activation mechanism of the C–C and C–H bond in alkane. [Copyright &y& Elsevier]

In an effort to search for new tubular nanostructures made of aluminum nitride (AlN), four model compounds, a diamond nanowire (c-C54), a carbon nanotube (h-C54), an AlN nanowire (c-Al27N27) and an AlN nanotube (h-Al27N27), were studied by using first-principle method. A shapely beautiful AlN nanotube with a uniform diameter has been obtained in this work, which even rivals the carbon nanotube. The estimated band gap of this AlN nanotube is larger than that of the corresponding nanowire structure, different from those of carbon nanotubes with respect to diamond nanowires. The AlN nanotubes are expected to find novel applications in the nanotechnology area. [Copyright &y& Elsevier]

The reaction of Fe+ with CS2, which was selected as a representative system of the reactions of transition metals with CS2, has been investigated using density functional theory at B3LYP/6-311+G(d) level of theory. The geometries, energies and frequencies of reactant, intermediates, transition states and products were explored in detail on both the quartet and sextet potential energy surfaces of [Fe, C, S2]+. Our calculations indicate the reaction proceeds via an insertion–elimination mechanism but not a direct abstract mechanism. All of the observed products in the early experiment has been explained according to the cleavage of different bonds in the insertion intermediate S–Fe+–C–S. The connectivity in [Fe,C,S]+, which cannot be specified by experiment, was revealed as [S–C–Fe]+ in our study. By comparing the Fe++CS2 and V++CS2, we suppose all the reactions of first-row transition metal ions with CS2 could proceed according to the similar insertion–elimination mechanism. [Copyright &y& Elsevier]

Aurochs (Bos primigenius), once widely distributed in Afro-Eurasia, became extinct in the early 1600 s. However, their phylogeography and relative contributions to domestic cattle remain unknown. In this study, we analyzed 16 genomes of ancient aurochs and three mitogenomes of ancient bison (Bison priscus) excavated in East Asia, dating from 43,000 to 3,590 years ago. These newly generated data with previously published genomic information on aurochs as well as ancient/extant domestic cattle worldwide through genome analysis. Our findings revealed significant genetic divergence between East Asian aurochs and their European, Near Eastern, and African counterparts on the basis of both mitochondrial and nuclear genomic data. Furthermore, we identified evidence of gene flow from East Asian aurochs into ancient and present-day taurine cattle, suggesting their potential role in facilitating the environmental adaptation of domestic cattle. [ABSTRACT FROM AUTHOR]

The Paleolithic archaeological record of the Tibetan Plateau is crucial for understanding human ecological and genetic adaptation to life in high altitudes. Recent work on the Tibetan Plateau has documented hominin occupations by Denisovans at Baishiya Karst Cave (BKC) from at least ca. 160, and again around 100 and 60 thousand years ago (ka), followed by modern human occupation at Nwya Devu (ND) around 30–40 ka. However, with the exception of these two geographically distinct sites, there are very few Paleolithic sites with secure stratigraphy and reliable dates on the Tibetan Plateau. Thus, the spatial and temporal history of Paleolithic hominin occupation of the Tibetan Plateau remains poorly understood. Here we report a newly discovered well-stratified and well-dated Paleolithic site, Jiangjunfu 01 (JJF01), from the northeastern margin of the plateau. Optical dating of sediments from cultural layers shows that the site was occupied by hominin who employed simple core-and-flake technology, during warmer interglacial environments ∼90–120 ka. To date, JJF01 is one of the three oldest archaeological sites with secure stratigraphy and reliable dating from the Tibetan Plateau, further confirming that hominins, potentially Denisovans, occupied and inhabited the highest region of our planet at least by the early Upper Pleistocene. • A newly discovered well-stratified and well-dated Paleolithic site, JJF01, on the Tibetan Plateau (TP) is reported. • JJF01 was occupied ∼90–120 ka based on optical dating representing one of the three oldest sites on the TP. • It's further confirmed that hominins occupied the highest region of our planet at least by the early Upper Pleistocene. • JJF01 provides new evidence and insights into the technological capacity and adaptation of pre-modern human groups on the TP. • The low and gentle northeastern part of the TP might play a crucial role in human adaptation to high-altitude environments. [ABSTRACT FROM AUTHOR]

The authors would like to apologize that, due to their neglect, the affiliations of the second author of this paper, Huan Xia, were not correctly indicated. Huan Xia is a registered doctoral student at Lanzhou University in 2018 and is also supervised by Fahu Chen at the Institute of Tibetan Plateau Research, Chinese Academy of Sciences (CAS). Here, corrections are made by adding Huan Xia's first affiliation, Lanzhou University, to her name. [Extracted from the article]

Highlights • The original spatial pattern of the natural geographical attributes was changed by human activities. • Different ecosystem components have different responses to human activities. • The establishment of reserved areas has a positive effect on the protection of Marine biodiversity. Abstract Using the marine biological valuation method, we selected five ecosystem components, namely, phytoplankton, microzooplankton, macrozooplankton, macrobenthos, and nekton, to evaluate the marine biodiversity value of Jiangsu's offshore areas in 2006 (early-term coastal development) and 2014 (mid-term coastal development), and the impact of human activities on the values were analyzed based on the differences between the two time periods. In general, the biological value (BV) of the Jiangsu offshore area changed in the context of high-intensity human activities, disrupting the original spatial pattern of the natural geographical attributes indicated by the BV in 2006. From the perspective of various ecosystem components, the impact on macrobenthos is long-lasting and significant, the impact on plankton is temporary and recoverable, and the presence of offshore structures leads to the formation of new habitats that attract new species and more predators (fishes), with the BV of nekton in the radial sand ridge area being higher than that in other marine areas. The results of our approach confirm the importance of areas that were previously identified for protection (particularly in the central regions). Our results also highlight the value of currently unprotected sites, mainly in the northern and southern regions. Highlighting the impact of high-intensity human activities on the BV of Jiangsu lays a foundation for developing future marine spatial plans and provides a scientific reference and basis for administrative departments to develop marine management decisions. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The mechanism details for dehydration of fructose into HMF were calculated by DFT. • The calculated barriers were much lower than those located in previous literature. • The catalytic performance of ionic liquids on the acidity and nucleophilicity. Abstract DFT calculations have been performed to gain insight into the mechanism of production 5-hydroxymethylfurfural (HMF) from fructose catalyzed by a representative imidazolium-based ionic liquid (IL), 1-butyl-3-methyl-imidazolium bromide ([BMIM]Br). The overall barrier of the catalytic conversion is found to be 41.5 kcal/mol, which is in contrast to 75.6 kcal/mol located in previous literature. Different from the mechanism proposed in the literature, which Br anion of IL interacts with the hydroxyl group of the substrate via H-bonding to induce the dehydration process, while cation of IL acts as a spectator, the present calculations emphasize the collaboration of cation and anion of IL. [ABSTRACT FROM AUTHOR]

Climatic change that affects biological productivity is often argued to be a primary force influencing human activities during the glacial period. To test this assumption, we combine in-site pollen, paleoclimatic, and archaeological data from the Dadiwan site and nearby areas on the western Loess Plateau (WLP) that date to Marine Oxygen Isotope Stage (MIS) 3. Our comparison of multiple datasets suggests that regional human activities increased when the vegetation around the Dadiwan area shifted from forest steppe in the early MIS 3 (59–46.7 ka) to steppe in the middle to late MIS 3 (46.7–29.5 ka). Our results indicate that regional human activities increased again during the late MIS 3 when the amount of precipitation was higher, as indicated by the lower Artemisia proportion. We suggest that increased precipitation on the WLP enhanced the above-ground biomass production and may be responsible for an increase in human activity and population in this region. [ABSTRACT FROM AUTHOR]

Prehistoric projectile weapons are crucial for understanding the hunting behavior, survival strategies, and subsistence patterns of prehistoric humans. Arrowheads were the primary prehistoric projectile weapon, and here we review the spatio-temporal distribution and changes in the morphology, quantity, and materials composition of arrowheads in the northern regions of China from the Middle and Upper Paleolithic periods to the Bronze Age, together with the factors that influenced these changes. The results reveal that primitive stone arrowheads appeared sporadically in the Middle Paleolithic period. Additionally, the "broad spectrum revolution" and microblade technology during the Upper Paleolithic promoted the regional-scale diffusion of broad and thick stone arrowheads in northern China. During the Neolithic period, relatively narrow and thin stone and bone arrowheads spread rapidly across the entire Yellow River Basin and most of northeastern China. The dominance of these two types of arrowheads alternated, during the period of 7000–5000 yr BP, the number of arrowheads peaked, possibly closely related to changes in the role of hunting (fishing and hunting) and gathering in the subsistence economy, human population size, and the emergence and development of a grinding technology. During the Bronze Age, a significant number of bronze arrowheads appeared, and the total number of arrowheads increased again, and their distribution expanded to northwestern China. The emergence of a bronze smelting technology and inter-group conflicts may have been the primary factors influencing this trend. Our results demonstrate that arrowheads were always an essential component of prehistoric tool kits. Due to technological developments in subsistence strategies, the morphology, function, and popularity of arrowheads underwent substantial changes during different periods. Therefore, a comprehensive study of prehistoric arrowheads can help better understand the subsistence patterns and processes of economic and societal development of prehistoric humans. [ABSTRACT FROM AUTHOR]