Cucurbituril - Molecule of the Month March 2006 [Archived version]

This is the Molecule of the Month entry for March 2006 about cucurbituril. It is a pdf archive version of the HTML webpage

Dioxin - Molecule of the Month September 2005 [Archived version]

This is the Molecule of the Month entry for September 2005 about dioxin. It is a pdf archive version of the HTML webpage

Pentacene - Molecule of the Month November 2006 [Archived version]

This is the Molecule of the Month entry for November 2006 about pentacene. It is a pdf archive version of the HTML webpage

Antiaromatic Character of 16 π Electron Octaethylporphyrins: Magnetically Induced Ring Currents from DFT-GIMIC Calculations

The magnetically induced current density susceptibility, also called current density, has been calculated for a recently synthesized octaethylporphyrin (OEP) zinc­(II) dication with formally 16 π electrons. Numerical integration of the current density passing selected chemical bonds yields the current pathway around the porphyrinoid ring and the strength of the ring current. The current strengths show that the OEP-Zn­(II) dication is strongly antiaromatic, as also concluded experimentally. The calculation of the ring current pathway shows that all 24 π electrons participate in the transport of the ring current because the current splits into inner and outer branches of practically equal strengths at the four pyrrolic rings. The corresponding neutral octaethylporphyrinoid without Zn and inner hydrogens is found to be antiaromatic, sustaining a paratropic ring current along the inner pathway with 16 π electrons. The neutral OEP-Zn­(II) molecule with formally 18 π electrons is found to be almost as aromatic as free-base porphyrin. However, also in this case, all 26 π electrons contribute to the ring current, as for free-base porphyrin. A comparison of calculated and measured ¹H NMR chemical shifts is presented. The current strength susceptibility under experimental conditions has been estimated by assuming a linear relation between experimental shielding constants and calculated current strengths

Effect of Fluorine Substitution on the Aromaticity of Polycyclic Hydrocarbons

The effect of fluorine substitution on the aromaticity of polycyclic hydrocarbons (PAH) is investigated. Magnetically induced current densities, current pathways, and current strengths, which can be used to assess molecular aromaticity, are calculated using the gauge-including magnetically induced current method (GIMIC). The degree of aromaticity of the individual rings is compared to those obtained using calculated nucleus-independent chemical shifts at the ring centers (NICS(0) and NICS(0)_<i>zz</i>). Calculations of explicitly integrated current strengths for selected bonds show that the aromatic character of the investigated polycyclic hydrocarbons is weakened upon fluorination. In contrast, the NICS(0) values for the fluorinated benzenes increase noteworthy upon fluorination, predicting a strong strengthening of the aromatic character of the arene rings. The integrated current strengths also yield explicit current pathways for the studied molecules. The current pathways of the investigated linear polyacenes, pyrene, anthanthrene, coronene, ovalene, and phenanthro-ovalene are not significantly affected by fluorination. NISC(0) and NICS(0)_<i>zz</i> calculations provide contradictory degrees of aromaticity of the fused individual ring. Obtained NICS values do not correlate with the current strengths circling around the individual rings

Does Synergism in Microscopic Polarity Correlate with Extrema in Macroscopic Properties for Aqueous Mixtures of Dipolar Aprotic Solvents?

Aqueous mixtures of dipolar aprotic solvents (acetonitrile, γ-valerolactone, γ-butyrolactone, tetrahydrofuran, 1,4-dioxane, acetone, pyridine, <i>N</i>-methyl-2-pyrrolidone, <i>N</i>,<i>N</i>-dimethylformamide, <i>N</i>,<i>N</i>-dimethylacetamide, and dimethyl sulfoxide) show synergism in microscopic polarity and extrema in macroscopic viscosity (η) and molar excess enthalpy (<i>H</i>^E) in water-rich compositions that correlate with solvent functional group electrostatic basicity (β₂^H). Microscopic polarities of aqueous solvent mixtures were estimated by measuring the spectral shift (λ_max) of 4-nitroaniline with UV–vis spectroscopy at 25 °C. Dynamic viscosities (η) and densities were measured for eight aqueous dipolar aprotic mixtures over the full range of compositions at (25 to 45) °C. The λ_max, η, and <i>H</i>^E values of the aqueous mixtures showed a linear trend with increasing electrostatic basicity of the solvent functional groups that is attributed to the size and strength of the hydration shell of water. Density functional theory (DFT) calculations were performed for 1:3 complexes (solvent: (H₂O)₃) and it was found that aqueous mixtures with high basicity have high binding energies and short hydrogen bonding distances implying that the size and strength of the hydration shell of water is proportional to functional group basicity. Consideration of functional group basicity of dipolar aprotic solvents allows one to relate synergism in microscopic polarity to extrema in macroscopic properties for a wide range of aqueous dipolar aprotic solvent mixtures

Anion Complexes with Tetrazine-Based Ligands: Formation of Strong Anion−π Interactions in Solution and in the Solid State

Ligands <b>L1</b> and <b>L2</b>, consisting of a tetrazine ring decorated with two morpholine pendants of different lengths, show peculiar anion-binding behaviors. In several cases, even the neutral ligands, in addition to their protonated HL⁺ and H₂L²⁺ (L = <b>L1</b> and <b>L2</b>) forms, bind anions such as F^–, NO₃^–, PF₆^–, ClO₄^–, and SO₄^2– to form stable complexes in water. The crystal structures of H₂<b>L1</b>(PF₆)₂·2H₂O, H₂<b>L1</b>(ClO₄)₂·2H₂O, H₂<b>L2</b>(NO₃)₂, H₂<b>L2</b>(PF₆)₂·H₂O, and H₂<b>L2</b>(ClO₄)₂·H₂O show that anion−π interactions are pivotal for the formation of these complexes, although other weak forces may contribute to their stability. Complex stability constants were determined by means of potentiometric titration in aqueous solution at 298.1 K, while dissection of the free-energy change of association (Δ<i>G</i>°) into its enthalpic (Δ<i>H</i>°) and entropic (TΔ<i>S</i>°) components was accomplished by means of isothermal titration calorimetry measurements. Stability constants are poorly regulated by anion–ligand charge–charge attraction. Thermodynamic data show that the formation of complexes with neutral ligands, which are principally stabilized by anion−π interactions, is enthalpically favorable (−Δ<i>G</i>°, 11.1–17.5 kJ/mol; Δ<i>H</i>°, −2.3 to −0.5 kJ/mol; <i>T</i>Δ<i>S</i>°, 9.0–17.0 kJ/mol), while for charged ligands, enthalpy changes are mostly unfavorable. Complexation reactions are invariably promoted by large and favorable entropic contributions. The importance of desolvation phenomena manifested by such thermodynamic data was confirmed by the hydrodynamic results obtained by means of diffusion NMR spectroscopy. In the case of <b>L2</b>, complexation equilibria were also studied in a 80:20 (v/v) water/ethanol mixture. In this mixed solvent of lower dielectric constant than water, the stability of anion complexes decreases, relative to water. Solvation effects, mostly involving the ligand, are thought to be responsible for this peculiar behavior