Combining halogen bonds and hydrogen bonds in the modular assembly of heteromeric infinite 1-D chains

Hydrogen bonds and halogen bonds operate in concert in the directed assembly of infinite 1-D chains in binary co-crystals of iodine iso-nicotinamide (2 : 2), 1 and tetrafluorodiiodobenzene iso-nicotinamide (1 : 2) 2.     

Constructing, deconstructing, and reconstructing ternary supermolecules

A hypothesis-driven protocol comprising precise and predictable molecular recognition events based upon an electrostatic view of competing hydrogen-bond interactions is proposed and subsequently employed in the construction of ternary co-crystals.     

Investigation of the microstructure of metallocene-catalyzed norbornene–ethylene copolymers using NMR spectroscopy

Norbornene–ethylene copolymers were prepared using the metallocene catalyst ethylene bis (indenyl) zirconium dichloride with MAO, and their microstructure was characterized with ¹H-NMR and ¹³C-NMR methods. From a Cosy ¹H-NMR spectrum it was found that all norbornene units are enchained in the exo-configuration. The sequence distribution of norbornene units was investigated using ¹³C-¹H correlations, hmqc for one-bond correlations, and hmbc for two- or three-bond correlations. It was shown that norbornene diads were formed at a high norbornene content (45 mol %). When further increasing the norbornene incorporation (66 mol %) a number of new signals were obtained. A Cosy ¹H-NMR spectrum revealed a new crosspeak which, according to the corresponding ¹³C-NMR shifts (hmqc), correlated well with a terminal unit of a trimer of norbornene. This means that at very high norbornene contents, norbornene triads can be formed. Because the formation of isotactic norbornene triads is very difficult to understand from a sterical point of view, an epimerization process causing stereoirregularities in the norbornene triad is proposed. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1633–1638, 1998     

THE EFFECT OF THE ANTIPSORIATIC DRUG METABOLITE ETRETIN (Ro 10-1670) ON UVB IRRADIATION INDUCED CHANGES IN THE METABOLISM OF ARACHIDONIC ACID IN HUMAN KERATINOCYTES IN CULTURE

[¹⁴C]Arachidonic acid was avidly incorporated into human keratinocytes in culture and following exposure to UVB irradiation of 9 mJ/cm² (erythemally effective, EE) substantial amounts of ¹⁴C-radiolabel were released from the cells. The release of radiolabel was accompanied by a decrease in the labelling of phosphatidylethanolamine whereas the labelling of triacylglycerols and cholesteryl esters was increased. Keratinocytes produced significant amounts of prostaglandin E₂ (PGE₂) and following UVB irradiation of 9 mJ/cm² (EE) the formation of prostaglandin E₂ was increased.
Etretin (Ro 10-1670), the active metabolite of the antipsoriatic drug etretinate (Ro 10-9359), affected significantly neither the total release of radiolabel induced by UVB nor the formation of prostaglandin E₂. However, in the presence of etretin the UVB irradiation induced transfer of [^l4C]arachidonic acid into triacylglycerols and cholesteryl esters was not increased as much as in the corresponding experiments without etretin. On the basis of the present study it appears that etretin does not interfere with the release of arachidonic acid in amounts which could be related to the therapeutic effects of the combination of retinoids with UVB irradiation (Re-UVB) in the treatment of psoriasis.     

Charge-Assisted Hydrogen Bonds and Halogen-Halogen Interactions in Organic Salts: Benzylammonium Benzoates and Pentafluorobenzoates

The enhanced influence of charge-assisted hydrogen bonds upon the resulting crystal structure in the presence of competing noncovalent interactions has been examined using single-crystal X-ray diffraction. The crystal structures of six organic salts are presented: 4-chlorobenzylammonium 2,4-dichlorobenzoate, 3-chlorobenzylammonium 2,4-dichlorobenzoate, 4-methylbenzylammonium pentafluorobenzoate, 4-chlorobenzylammonium pentafluorobenzoate, 3-methylbenzylammonium benzoate, and 4-chlorobenzylammonium benzoate monohydrate. All structures are dominated by charge-assisted hydrogen bonds that generate either one-dimensional ribbons or two-dimensional sheets. The structural influence of weaker intermolecular interactions is limited as there are very few short, halogen-halogen, -, or distances in this series of compounds.     

Excess thermodynamic properties of thin water films confined between hydrophobized gold surfaces

Surface forces between gold surfaces were measured in pure water at temperatures in the range of 10-40 °C using an atomic force microscope (AFM). The surfaces were hydrophobized by self-assembly of alkanethiols (C(n)SH) with n=2 and 16 in ethanol solutions. The data were used to determine the changes in excess free energies (ΔG(f)) of the thin water films per unit area by using the Derjaguin approximation [1]. The free energy data were then used to determine the changes in excess film entropy (ΔS(f)) and the excess film enthalpy (ΔH(f)) per unit area. The results show that both ΔS(f) and ΔH(f) decrease with decreasing film thickness, suggesting that the macroscopic hydrophobic interaction involves building some kind of structures in the intervening thin films of water. It was found that |ΔH(f)|>|TΔS(f)|, which is a necessary condition for an attractive force to appear when the enthalpy and entropy changes are both negative. That macroscopic hydrophobic interaction is enthalpically driven is contrary to the hydrophobic interactions at molecular scale. The results obtained in the present work are used to discuss possible origins for the long-range attractions observed between hydrophobic surfaces.     

Multiresidue analysis of 95 pesticides at low nanogram/liter levels in surface waters using online preconcentration and high performance liquid chromatography/tandem mass spectrometry

A new multiresidue method for the analysis of 95 pesticides and breakdown products with a wide range of physiochemical properties in surface and ground water using online solid-phase extraction coupled with HPLC/MS/MS is presented. Using an injection volume of only 500 microL filtered water, an LOD in the 1-10 ng/L range was achieved for all but one substance, and recoveries were, with a few exceptions, in the 70-120% range. Large differences were found in the adsorption of pesticides on nine filter materials tested for filtration of the water samples. Filters of regenerated cellulose were chosen due to good recoveries and easy handling. Results from the proposed method were compared with results from previously used methods for 31 pesticides in 99 surface water samples, collected in 2008, with good agreement.     

Formation of DMSO and DMF radicals with minute amounts of base

So far overlooked DMSO and DMF form long-lived radicals in the presence of small amounts of bases, DMF radicals being less stable than DMSO radicals. In solvent mixtures, the presence of DMSO prolonged the lifetime of DMF radicals. The occurrence of radicals may explain previously reported unexpected outcomes of reactions performed in these solvents. The commonly accepted inertness of these solvents towards minor quantities of alkali seems not to be warranted.