X₂Y₂ Isomers: Tuning Structure and Relative Stability through Electronegativity Differences (X = H, Li, Na, F, Cl, Br, I; Y = O, S, Se, Te)

We have studied the XYYX and X₂YY isomers of the X₂Y₂ species (X = H, Li, Na, F, Cl, Br, I; Y = O, S, Se, Te) using density functional theory at the ZORA-BP86/QZ4P level. Our computations show that, over the entire range of our model systems, the XYYX isomers are more stable than the X₂YY forms except for X = F and Y = S and Te, for which the F₂SS and F₂TeTe isomers are slightly more stable. Our results also point out that the Y–Y bond length can be tuned quite generally through the X–Y electronegativity difference. The mechanism behind this electronic tuning is the population or depopulation of the π* in the YY fragment

(4 + 2) and (2 + 2) Cycloadditions of Benzyne to C₆₀ and Zig-Zag Single-Walled Carbon Nanotubes: The Effect of the Curvature

Addition of benzyne to carbon nanostructures can proceed via (4 + 2) (1,4-addition) or (2 + 2) (1,2-addition) cycloadditions depending on the species under consideration. In this work, we analyze by means of density functional theory (DFT) calculations the reaction mechanisms for the (4 + 2) and (2 + 2) cycloadditions of benzyne to nanostructures of different curvature, namely, C₆₀ and a series of zigzag single-walled carbon nanotubes. Our DFT calculations reveal that, except for the concerted (4 + 2) cycloaddition of benzyne to zigzag single-walled carbon nanotubes, all cycloadditions studied are stepwise processes with the initial formation of a biradical singly bonded intermediate. From this intermediate, the rotation of the benzyne moiety determines the course of the reaction. The Gibbs energy profiles lead to the following conclusions: (i) except for the 1,4-addition of benzyne to a six-membered ring of C₆₀, all 1,2- and 1,4-additions studied are exothermic processes; (ii) for C₆₀ the (2 + 2) benzyne cycloaddition is the most favored reaction pathway; (iii) for zigzag single-walled carbon nanotubes, the (4 + 2) benzyne cycloaddition is preferred over the (2 + 2) reaction pathway; and (iv) there is a gradual decrease in the exothermicity of the reaction and an increase of energy barriers as the diameter of the nanostructure of carbon is increased. By making use of the activation strain model, it is found that the deformation of the initial reactants in the rate-determining transition state is the key factor determining the chemoselectivity of the cycloadditions with benzyne

Theoretical Study of the Structure and Bonding in ThC₂ and UC₂

The electronic structure and various molecular properties of the actinide (An) dicarbides ThC₂ and UC₂ were investigated by relativistic quantum chemical calculations. We probe five possible geometrical arrangements: two triangular structures including an acetylide (C₂) moiety, as well as the linear AnCC, CAnC, and bent CAnC geometries. Our calculations at various levels of theory indicate that the triangular species are energetically more favorable, while the latter three arrangements proved to be higher-energy structures. Our SO-CASPT2 calculations give the ground-state molecular geometry for both ThC₂ and UC₂ as the symmetric (<i>C</i>_2<i>v</i>) triangular structure. The similar and, also very close in energy, asymmetric (<i>C</i>_<i>s</i>) triangular geometry belongs to a different electronic state. DFT and single-determinant ab initio methods failed to distinguish between these two similar electronic states demonstrating the power of multiconfiguration ab initio methods to deal with such subtle and delicate problems. We report detailed data on the electronic structure and bonding properties of the most relevant structures

Media Distribution in Heterogeneous Environments using IP-Multicast

This document discusses problems and solutions around distribution of media in heterogeneous environments when using IP-multicast.Godkänd; 1998; 20080505 (ysko

Neutral Six-Coordinate and Cationic Five-Coordinate Silicon(IV) Complexes with Two Bidentate Monoanionic <i>N</i>,<i>S</i>‑Pyridine-2-thiolato(−) Ligands

A series of neutral six-coordinate silicon­(IV) complexes (<b>4</b>–<b>11</b>) with two bidentate monoanionic <i>N</i>,<i>S</i>-pyridine-2-thiolato ligands and two monodentate ligands R¹ and R² was synthesized (<b>4</b>, R¹ = R² = Cl; <b>5</b>, R¹ = Ph, R² = Cl; <b>6</b>, R¹ = Ph, R² = F; <b>7</b>, R¹ = Ph, R² = Br; <b>8</b>, R¹ = Ph, R² = N₃; <b>9</b>, R¹ = Ph, R² = NCO; <b>10</b>, R¹ = Ph, R² = NCS; <b>11</b>, R¹ = Me, R² = Cl). In addition, the related ionic compound <b>12</b> was synthesized, which contains a cationic five-coordinate silicon­(IV) complex with two bidentate monoanionic <i>N</i>,<i>S</i>-pyridine-2-thiolato ligands and one phenyl group (counterion: I^–). Compounds <b>4</b>–<b>12</b> were characterized by elemental analyses, NMR spectroscopic studies in the solid state and in solution, and crystal structure analyses (except <b>7</b>). These structural investigations were performed with a special emphasis on the sophisticated stereochemistry of these compounds. These experimental investigations were complemented by computational studies, including bonding analyses based on relativistic density functional theory

Neutral Six-Coordinate and Cationic Five-Coordinate Silicon(IV) Complexes with Two Bidentate Monoanionic <i>N</i>,<i>S</i>‑Pyridine-2-thiolato(−) Ligands

A series of neutral six-coordinate silicon­(IV) complexes (<b>4</b>–<b>11</b>) with two bidentate monoanionic <i>N</i>,<i>S</i>-pyridine-2-thiolato ligands and two monodentate ligands R¹ and R² was synthesized (<b>4</b>, R¹ = R² = Cl; <b>5</b>, R¹ = Ph, R² = Cl; <b>6</b>, R¹ = Ph, R² = F; <b>7</b>, R¹ = Ph, R² = Br; <b>8</b>, R¹ = Ph, R² = N₃; <b>9</b>, R¹ = Ph, R² = NCO; <b>10</b>, R¹ = Ph, R² = NCS; <b>11</b>, R¹ = Me, R² = Cl). In addition, the related ionic compound <b>12</b> was synthesized, which contains a cationic five-coordinate silicon­(IV) complex with two bidentate monoanionic <i>N</i>,<i>S</i>-pyridine-2-thiolato ligands and one phenyl group (counterion: I^–). Compounds <b>4</b>–<b>12</b> were characterized by elemental analyses, NMR spectroscopic studies in the solid state and in solution, and crystal structure analyses (except <b>7</b>). These structural investigations were performed with a special emphasis on the sophisticated stereochemistry of these compounds. These experimental investigations were complemented by computational studies, including bonding analyses based on relativistic density functional theory

Anion Recognition by Organometallic Calixarenes: Analysis from Relativistic DFT Calculations

The physical nature of the noncovalent interactions involved in anion recognition was investigated in the context of metalated calix[4]­arene hosts, employing Kohn–Sham molecular orbital (KS-MO) theory, in conjunction with a canonical energy decomposition analysis, at the dispersion-corrected DFT level of theory. Computed data evidence that the most stable host–guest bonding occurs in ruthenium complexed hosts, followed by technetium and molybdenum metalated macrocyclic receptors. Furthermore, the guest’s steric fit in the host scaffold is a selective and crucial criterion to the anion recognition. Our analyses reveal that coordinated charged metals provide a larger electrostatic stabilization to anion recognition, shifting the calixarenes cavity toward an electron deficient acidic character. This study contributes to the design and development of new organometallic macrocyclic hosts with increased anion recognition specificity

Tuning Heterocalixarenes to Improve Their Anion Recognition: A Computational Approach

We have explored and analyzed the physical factors through which noncovalent interactions in anion sensing based on calixarene-type hosts can be tuned, using dispersion-corrected DFT and Kohn–Sham molecular orbital (KS-MO) theory in conjunction with a canonical energy decomposition analysis (EDA). We find that the host–guest interaction can be enhanced through the introduction of strongly electron-withdrawing groups at particular positions of the arene and triazine units in the host molecule as well as by coordination of a metal complex to the arene and triazine rings. Our analyses reveal that the enhanced anion affinity is caused by increasing the electrostatic potential in the heterocalixarene cavities. This insight can be employed to further tune and improve their selectivity for chloride ions

Activation-Strain Analysis Reveals Unexpected Origin of Fast Reactivity in Heteroaromatic Azadiene Inverse-Electron-Demand Diels–Alder Cycloadditions

Heteroaromatic azadienes, especially 1,2,4,5-tetrazines, are extremely reactive partners with alkenes in inverse-electron-demand Diels–Alder reactions. Azadiene cycloaddition reactions are used to construct heterocycles in synthesis and are popular as bioorthogonal reactions. The origin of fast azadiene cycloaddition reactivity is classically attributed to the inverse frontier molecular orbital (FMO) interaction between the azadiene LUMO and alkene HOMO. Here, we use a combination of ab initio, density functional theory, and activation-strain model calculations to analyze physical interactions in heteroaromatic azadiene–alkene cycloaddition transition states. We find that FMO interactions do not control reactivity because, while the inverse FMO interaction becomes more stabilizing, there is a decrease in the forward FMO interaction that is offsetting. Rather, fast cycloadditions are due to a decrease in closed-shell Pauli repulsion between cycloaddition partners. The kinetic–thermodynamic relationship found for these inverse-electron-demand cycloadditions is also due to the trend in closed-shell repulsion in the cycloadducts. Cycloaddition regioselectivity, however, is the result of differences in occupied–unoccupied orbital interactions due to orbital overlap. These results provide a new predictive model and correct physical basis for heteroaromatic azadiene reactivity and regioselectivity with alkene dieneophiles

Normal-to-Abnormal Rearrangement and NHC Activation in Three-Coordinate Iron(II) Carbene Complexes

The ‘normal’ three-coordinate iron–NHC complex [(IPr)­Fe­(N′′)₂] (N″ = N­(SiMe₃)₂) rearranges to its abnormal NHC analogue [(<i>a</i>IPr)­Fe­(N″)₂] (<b>6</b>) on heating, providing a rare abnormal iron–<i>a</i>NHC complex, and the first such three-coordinate complex. The <i>tert</i>-butyl-substituted complex [(I^<i>t</i>Bu)­Fe­(N″)₂] (<b>4</b>) undergoes a thermal decomposition that has not previously been observed in iron–NHC chemistry, resulting in the bis­(imidazole) complex [(^<i>t</i>BuIm)₂Fe­(N″)₂] (<b>7</b>). A mechanism that involves consecutive C–H and C–N activation is proposed to account for the formation of <b>7</b>