Reactivity of the Indenyl Radical (C9H7) with Acetylene (C2H2) and Vinylacetylene (C4H4)

The reactions of the indenyl radicals with acetylene (C₂H₂) and vinylacetylene (C₄H₄) is studied in a hot chemical reactor coupled to synchrotron based vacuum ultraviolet ionization mass spectrometry. These experimental results are combined with theory to reveal that the resonantly stabilized and thermodynamically most stable 1-indenyl radical (C₉H₇^.) is always formed in the pyrolysis of 1-, 2-, 6-, and 7-bromoindenes at 1500 K. The 1-indenyl radical reacts with acetylene yielding 1-ethynylindene plus atomic hydrogen, rather than adding a second acetylene molecule and leading to ring closure and formation of fluorene as observed in other reaction mechanisms such as the hydrogen abstraction acetylene addition or hydrogen abstraction vinylacetylene addition pathways. While this reaction mechanism is analogous to the bimolecular reaction between the phenyl radical (C₆H₅^.) and acetylene forming phenylacetylene (C₆H₅CCH), the 1-indenyl+acetylene→1-ethynylindene+hydrogen reaction is highly endoergic (114 kJ mol⁻¹) and slow, contrary to the exoergic (−38 kJ mol⁻¹) and faster phenyl+acetylene→phenylacetylene+hydrogen reaction. In a similar manner, no ring closure leading to fluorene formation was observed in the reaction of 1-indenyl radical with vinylacetylene. These experimental results are explained through rate constant calculations based on theoretically derived potential energy surfaces.     

Aromaticity of giant polycyclic aromatic hydrocarbons with hollow sites: super ring currents in super-rings

We present a systematic theoretical study based on semi-empirical, Hartree-Fock (HF), and density functional theory (DFT) models of a series of polycyclic aromatic hydrocarbons (PAHs) that exhibit hollow sites. In this study we focus particularly on the magnetic criteria of aromaticity, namely (1)H NMR and nucleus-independent chemical shifts (NICS), and on their relationships with other electronic properties. The computed shifts and NICS indices indicate that an external magnetic field induces exceptionally strong ring currents in even-layered PAH doughnuts, in particular in the layer directly adjacent to the central hole of double-layered compounds. These exceptionally strong ring currents also correlate with particularly small HOMO-LUMO gaps and electronic excitation energies and to abnormally high polarizabilities, indicating in turn that these compounds have a more pronounced metallic character. Comparison is made with further depictions of aromaticity in these systems and in [18]-[66]annulene rings by employing topological, structural, and energetic criteria.     

Pushing the Limits of Acene Chemistry: The Recent Surge of Large Acenes

Acenes, consisting of linearly fused benzene rings, are an important fundamental class of organic compounds with various applications. Hexacene is the largest acene that was synthesized and isolated in the 20th century. The next largest member of the acene family, heptacene, was observed in 2007 and since then significant progress in preparing acenes has been reported. Significantly larger acenes, up to undecacene, could be studied by means of low-temperature matrix isolation spectroscopy with in situ photolytic generation, and up to dodecacene by means of on-surface synthesis employing innovative precursors and highly defined crystalline metal surfaces under ultrahigh vacuum conditions. The review summarizes recent experimental and theoretical advances in the area of acenes that give a significantly deeper insight into the fundamental properties and nature of the electronic structure of this fascinating class of organic compounds.     

亚甲基氰自由基(HCCN)与一氧化氮(NO)反应势能面的理论研究

在CCSD(T)/6-311G(d,p)//B3LYP/6-311G(d,p)+ZPE水平上对反应HCCN+NO的二重态反应势能面进行了计算,得到了4种产物:P1(HCN+NCO),P2(OH+NCCN),P₃[HCN+(CNO)]和P4(HCN+CNO).其中产物P1为主要产物,P2为次要产物,P₃和P4很难得到.在G2(B3LYP/MP2/CC)水平,对产物P1和P2的反应通道的单点能量进行了校正.     

Spectroscopy of Dibenzorubicene: Experimental Data for a Search in Interstellar Spectra

We report on the characterization of dibenzo[cde,opq]rubicene (C₃₀H₁₄). The molecule was studied in solution at room temperature with absorption spectroscopy in the visible (vis) and ultraviolet (UV) wavelength ranges, and with emission spectroscopy. The infrared (IR), visible, ultraviolet, and vacuum ultraviolet (VUV) absorption spectra of a thin film were measured also at room temperature. In addition, the UV/vis absorption spectrum was measured at cryogenic temperatures using the matrix isolation spectroscopy technique. The interpretation of spectra was supported by theoretical calculations based on semiempirical and ab initio models, as well as on density functional theory. Finally, the results of the laboratory study were compared with interstellar spectra.     

羟基取代的多环芳烃与一氧化氮的反应研究

内源性的一氧化氮(NO)是多种生理过程中必不可少的信使分子,它在神经系统递质传导、神经发育、脑血流调节以及免疫调节等过程中具有十分重要的作用。为揭示NO生理功能的化学本质,NO成为化学家近年来研究的重要课题之一。     

Theoretical studies on the structure of M+BF−4 ion pairs M = Li+, NH+4: the role of electrostatics and electron correlation

A systematic investigation of the M⁺BF₄ ⁻ (M = Li or NH₄) ion-pair conformers has been carried out using an electrostatic docking model based on the molecular electrostatic potential topography of the free anion. This method provides a guideline for the subsequent ab initio molecular orbital calculations at the Hartree-Fock (HF) and second-order M?ller-Plesset perturbation theory (MP2) levels. It has been demonstrated that the model presented here yields more than 75% of the HF interaction energy when Li⁺ is the cation involved and more than 90% for the case of NH₄ ⁺. Inclusion of MP2 correlation in the HF-optimized geometries leads to stationary point geometries with different numbers of imaginary frequencies and in some places where the energies of two adjacent conformers are very close, the energy rank order is altered. The HF lowest-energy minima for the Li⁺BF₄ ⁻ and NH₄ ⁺BF₄ ⁻ show a bidentate and tridentate coordinating cation, respectively, whereas at the MP2 level, this ordering is reversed. Received: 9 September 1997 / Accepted: 5 November 1997     

基态碱土金属原子Ba（^1S）与ClO2分子反应的从头计算研究

对基态碱金属原子Ｂａ（１Ｓ）与ＣｌＯ２分子的反应进行了量子化学从头计算，讨论并比较了二重态势能面上的两条不同的反应产物通道，结果表明，通道（１）－即生成氯化物的通道在能量上有利，反应的主要产物是ＢａＣｌ和Ｏ２，这与实验结果一致，此外，对少数低激发态产物的生成进行了讨论。     

Reaction path following by quadratic steepest descent

An efficient steepest descent algorithm for the integration of minimum energy paths, based on local quadratic approximations of the potential energy surface, is presented. The algorithm incorporates a selection procedure for the points at which the second derivatives of the energy are calculated fully or partially, thus minimizing the computational effort while maintaining high accuracy. This makes the method especially well suited for application in variational transition state theory calculations with tunnelling corrections, which have very high accuracy requirements. The performance of the algorithm is illustrated by ab initio calculations for four chemical reactions of differing complexity. The overall computational cost is less than for, or comparable to that of, first- or second-order algorithms published previously. Received: 28 July 1998 / Accepted: 10 August 1998 / Published online: 28 October 1998     

Reaction Mechanism and Product Branching Ratios of OH+C2H3F Reaction: A Theoretical Study

Ab initio CCSD(T)/CBS//B3LYP/6-311G(d, p) calculations of the potential energy surface for possible dissociation channels of HOC\begin{document}$_2$\end{document}H\begin{document}$_3$\end{document}F, as well as Rice-Ramsperger-Kassel-Marcus (RRKM) calculations of rate constants, were carried out, in order to predict statistical product branching ratios in dissociation of HOC\begin{document}$_2$\end{document}H\begin{document}$_3$\end{document}F at various internal energies. The most favorable reaction pathway leading to the major CH\begin{document}$_2$\end{document}CHO+HF products is as the following: OH+C\begin{document}$_2$\end{document}H\begin{document}$_3$\end{document}F\begin{document}$\rightarrow$\end{document}i2\begin{document}$\rightarrow$\end{document}TS14\begin{document}$\rightarrow$\end{document}i6\begin{document}$\rightarrow$\end{document}TS9\begin{document}$\rightarrow$\end{document}i3\begin{document}$\rightarrow$\end{document}TS3\begin{document}$\rightarrow$\end{document}CH\begin{document}$_2$\end{document}CHO+HF, where the rate-determining step is HF elimination from the CO bridging position via TS11, lying above the reactants by 3.8 kcal/mol. The CH\begin{document}$_2$\end{document}O+CH\begin{document}$_2$\end{document}F products can be formed by F atom migration from C\begin{document}$_\beta$\end{document} to C\begin{document}$_\alpha$\end{document} position via TS14, then H migration from O to C\begin{document}$_\alpha$\end{document} position via TS16, and C-C breaking to form the products via TS5, which is 1.8 kcal/mol lower in energy than the reactants, and 4.0 kcal/mol lower than TS11.     

Unexpected Michael Additions on the Way to 2.3,8.9‐Dibenzanthanthrenes with Interesting Structural Properties

The synthesis and properties of a new polycyclic aromatic hydrocarbon containing eight annulated rings and based on the anthanthrene core is described. An unexpected, nucleophile‐dependent Michael addition to a dibenzanthanthrene‐1,7‐dione is found, giving a product with three triisopropylsilylacetylene units and a remarkable solid‐state structure (as determined by X‐ray crystallography).     

Investigation of the Benzene–Naphthalene and Naphthalene–Naphthalene Potential Energy Surfaces: DFT/CCSD(T) Correction Scheme

The potential energy surfaces of the naphthalene dimer and benzene–naphthalene complexes are investigated using the recently developed DFT/CCSD(T) correction scheme [J. Chem. Phys. 2008 , 128, 114 102]. One and three minima are located on the PES of the benzene–naphthalene and the naphthalene dimer complexes, respectively, all of which are of the parallel‐displaced type. The stabilities of benzene–naphthalene and the naphthalene dimer are ?4.2 and ?6.2 kcal mol^?1, respectively. Unlike the benzene dimer, where the T‐shaped complex is the global minimum, the lowest‐energy T‐shaped structure is about 0.2 and 1.6 kcal mol^?1 above the global minimum on the benzene–naphthalene and the naphthalene dimer potential energy surfaces, respectively.     

Binding Matter with Antimatter: The Covalent Positron Bond

We report sufficient theoretical evidence of the energy stability of the e⁺?H₂^2? molecule, formed by two H^? anions and one positron. Analysis of the electronic and positronic densities of the latter compound undoubtedly points out the formation of a positronic covalent bond between the otherwise repelling hydride anions. The lower limit for the bonding energy of the e⁺?H₂^2? molecule is 74 kJ mol^?1 (0.77 eV), accounting for the zero‐point vibrational correction. The formation of a non electronic covalent bond is fundamentally distinct from positron attachment to stable molecules, as the latter process is characterized by a positron affinity, analogous to the electron affinity.     

Polycyclic aromatic hydrocarbons (PAHs) in surface sediments of Monastir Bay (Tunisia,Central Mediterranean): distribution,origin and seasonal variations

To assess PAH contamination state of Monastir Bay, surface sediments were collected and analysed for 17 selected parent PAHs by GC/MS. Sediments were sampled from five sites in wet and dry seasons. Total PAH concentrations were in the range of 25.6 to 576.8?ng/g d.wt in winter and 44.9–395.8?ng/g d.wt in summer. Comparison of results with Sediment Quality Guidelines suggested no eco-toxicological risk for benthic organisms. The use of molecular indices has shown that PAHs in surface sediments originate mainly from pyrolytic sources. Total PAH concentrations in surface sediments showed no significant correlations with organic matter content and pelite fraction. The analysis of spatial and seasonal variations of PAHs in surface sediments has demonstrated that final distribution of PAHs in surface sediments is mainly governed by hydrological conditions.     

An Additivity Scheme for Aromaticity: The Heteroatom Case

The nucleus-independent chemical shift (NICS)-XY-Scan is a simple and easy tool for the quantitative measurement of the aromaticity of polycyclic aromatic hydrocarbons and identification of the existence of local and global ring currents. We recently introduced an additivity scheme that uses the NICS-XY-Scans of smaller building blocks to predict the aromatic profiles of larger polycyclic aromatic hydrocarbon systems. We now report on an expansion of the methodology to include systems of varying aromatic natures containing the heteroatoms B, N, O, and S. The additivity approach allows for rapid and resource-efficient generation of NICS-XY-Scans of large, complex systems. Moreover, it reveals that the magnetic criterion of aromaticity behaves in an additive manner, and that the ring currents of multi-ring systems appear to be mostly localized within subunits of up to three rings.     

A Conformational Study on Silacyclohexane. Comparison of ab initio (HF,MP2), DFT,and Molecular Mechanics Calculations. Conformational Energy Surface of Silacyclohexane

The structures and relative energies for the basic conformations of silacyclohexane 1 have been calculated using HF, RI‐MP2, RI‐DFT and MM3 methods. All methods predict the chair form to be the dominant conformation and all of them predict structures which are in good agreement with experimental data. The conformational energy surface of 1 has been calculated using MM3. It is found that there are two symmetric lowest energy pathways for the chair‐to‐chair inversion. Each of them consists of two sofa‐like transition states, two twist forms with C₁ symmetry (twist‐C₁), two boat forms with Si in a gunnel position (C₁ symmetry), and one twist form with C₂ symmetry (twist‐C₂). All methods calculate the relative energy to increase in the order chair < twist‐C₂ < twist‐C₁ < boat. At the MP2 level of theory and using TZVP and TZVPP (Si atoms) basis sets the relative energies are calculated to be 3.76, 4.80, and 5.47 kcal mol^–1 for the twist‐C₂, twist‐C₁, and boat conformations, respectively. The energy barrier from the chair to the twisted conformations of 1 is found to be 6.6 and 5.7 kcal mol^–1 from MM3 and RI‐DFT calculations, respectively. The boat form with Si at the prow (C_s symmetry) does not correspond to a local minimum nor a saddle point on the MM3 energy surface, whereas a RI‐DFT optimization under C_s symmetry constraint resulted in a local minimum. In both cases its energy is above that of the chair‐to‐twist‐C₁ transition state, however, and it is clearly not a part of the chair‐to‐chair inversion.     

F+NCO反应的机理和动力学

利用Ｇ２（ＭＰ２）理论研究Ｆ（＾２Ｐ）与ＮＣＯ（Ｘ＾２ＩＩ）在三重激发态（ａ＾３Ａ＂）势能面上的反应机理揭示了生成ＮＦ）Ｘ＾３Σ＾－）的两种反应途径,即顺式和反式加成－消除,其中顺式反应途径的势垒较低（２０．０ｋｊ．ｍｏｌ＾－１）,。动力学计算显示,在室温 上,Ｆ与ＮＣＯ反应于三重态势能面上只能较缓慢地生成ＮＦ（Ｘ＾３Σ＾－）自由基。预测ＦＣ（Ｎ）Ｏ是另一可能的反应产物     

The Application of Active Biomonitoring with the Use of Mosses to Identify Polycyclic Aromatic Hydrocarbons in an Atmospheric Aerosol

The use of biological indicators of environmental quality is an alternative method of monitoring ecosystem pollution. Various groups of contaminants, including organic ones, can be measured in environmental samples. Polycyclic aromatic hydrocarbons (PAHs) have not yet been determined by the moss bag technique. This technique uses several moss species simultaneously in urban areas to select the best biomonitoring of these compounds, which are dangerous to humans and the environment. In this research, a gas chromatography coupled with mass spectrometry was used for the determination of selected PAHs in three species of mosses: Pleurozium schreberi, Sphagnum fallax and Dicranum polysetum (active biomonitoring) and for comparison using an air filter reference method for atmospheric aerosol monitoring. The chlorophyll fluorescence of photosystem II (PSII) was also measured to assess changes in moss viability during the study. As a result of the study, the selective accumulation of selected PAHs by mosses was found, with Pleurozium schreberi being the best bioindicator—9 out of 13 PAHs compounds were determined in this species. The photosynthetic yield of photosystem (II) decreased by 81% during the exposure time. The relationship between PAHs concentrations in mosses and the total suspended particles (TSP) on the filter indicated the possibility of using this bioindicator to trace PAHs in urban areas and to apply the moss bag technique as a method supporting classical instrumental air monitoring.     

A combined experimental and theoretical study on the complexation of Li<Superscript>+</Superscript> with valinomycin

Abstract  From extraction experiments in the two-phase water–nitrobenzene system and γ-activity measurements, the stability constant of the valinomycin–lithium complex in nitrobenzene saturated with water was determined. Further, the structure of the resulting complex was indicated by means of the density functional level of theory (DFT) calculations. Graphical abstract        

Chemisorption of HSi(OH)3 on silica surfaces: an ab initio periodic study

The study of the grafting of trialkoxysilane R′Si(OR)₃ molecules on amorphous silica has been undertaken at the Hartree–Fock level using a biperiodic model for the surface. Different types of slab cut out from the model system Edingtonite (a tetragonal silica structure with five SiO₂ groups per unit cell) have been used to simulate isolated and interacting silanol sites at the amorphous silica surface, while only the simple case of HSi(OH)₃ has been considered for the interacting molecule. In a first step, for each type of surface the geometrical parameters have been optimised and the surface formation energy determined. The geometrical structure of the grafted molecule is compared with that of the isolated one. The geometrical strains of the surfaces with either isolated or interacting silanols are also compared before and after the grafting reactions. The calculated values of the chemisorption energies show that the grafting process is favored on isolated silanols only if correlation effects are included.