Search results for " Force"
showing 10 items of 2250 documents
Behaviour of modified and unmodified polybutadienes in the semidilute regime
1997
Abstract The association behaviour of polybutadiene modified with 4-phenyl-l,2,4-triazoline-3,5-dione was studied in semidilute solution through static (SLS) and dynamic light scattering (DLS). The results were compared with the unmodified polybutadiene, whose behavior in semidilute solution was also studied as function of the molecular weight. At higher concentrations a small-angle excess scattering in SLS and a slow mode motion in DLS were observed both for modified and unmodified polybutadienes suggesting the presence of large clusters. Increasing the polymer concentration, the mass fraction of the slow motion increases up to values near 1, indicating a proximity to the gel point. For po…
Density functional theory study of the mechanism of the proline-catalyzed intermolecular aldol reaction
2002
Transition structures associated with the C-C bond-formation step of the proline-catalyzed intermolecular aldol reaction between acetone and isobutyraldehyde have been studies using density functional theory methods at the B3LYP/6-31G** computational level. A continuum model has been selected to represent solvent effects. For this step, which is the stereocontrolling and rate-determining step, four reactive channels corresponding to the syn and anti arrangement of the active methylene of the enamine relative to the carboxylic acid group of l-proline and the re and si attack modes to both faces of the aldehyde carbonyl group have been analyzed. The B3LYP/6-31G** energies are in good agreemen…
Halogen bonded supramolecular complexes and networks
2008
In spite of some controversy of the true nature of the interaction between polarized halogen atoms and neutral or charged Lewis bases, termed “halogen bonding”, as a primary interaction, it is a very useful new tool/way to construct supramolecular complexes and networks. This is especially true in solid state supramolecular chemistry where utilization of weak intermolecular interactions such as halogen bonding opens up new insights to materials design and supramolecular synthesis.
Properties and interactions – melting point of tribromobenzene isomers
2021
The melting points of tribromobenzene isomers are correlated with the number, nature and distribution of intermolecular interactions in their structures.
17O NMR study of isomeric monochloro- and monohydroxy-benzaldehydes and chlorinated salicylaldehydes
1991
The 17O NMR chemical shifts of isomeric monochloro- and monohydroxy-benzaldehydes and chlorinated salicylaldehydes were measured at 40°C for 0.25 M CDCI3 solutions. The 17O NMR chemical shift of the carbonyl oxygen of the compounds studied varies from 506 to 573 ppm measured from external D2O. The observed variation is probably mainly due to the intramolecular hydrogen bonding between the adjacent carbonyl and hydroxyl groups. The 17O NMR chemical shift range of the hydroxyl oxygen is from 80 to 98 ppm.
Calix[4]arenes with resorcinol units incorporated in 2,6-position
1990
Abstract Calix[4]arenes containing one or two resorcinol units incorporated via their 2,6-positions were prepared by fragment condensation. Due to the cyclic array of intramolecular hydrogen bonds these molecules assume the cone-conformation.
Different structural destinations: comparing reactions of [CuBr2(3-Brpy)2] crystals with HBr and HCl gas
2011
Reaction of green crystalline solid trans-[CuBr2(3-Brpy)2] 1 (3-Brpy = 3-bromopyridine) with HBr (aq) vapour yields brown crystalline salt (3-BrpyH)2[CuBr4] 2 with quantitative conversion. Notably 2 adopts a different crystal structure to the three mutually isostructural compounds (3-XpyH)2[CuCl4] (X = Cl, Br) and (3-BrpyH)2[CuBr2Cl2] which result from reaction with HCl. Crystalline product 2 has been characterised by X-ray powder diffraction and its conversion back to 1 at 370–400 K has been followed in situ by synchrotron X-ray powder diffraction. Crystalline 1 and 2 are further notable for the presence of intermolecular C–Br⋯Br–Cu halogen bonds and (only in the case of 2) N–H⋯Br–Cu hydro…
“Tail–Tail Dimerization” of Ferrocene Amino Acid Derivatives
2010
Acid anhydrides of N-protected 1'-aminoferrocene-1-carboxylic acid (Fca) have been prepared and spectroscopically characterized (protection group Boc, Fmoc, Ac; 4a―4c). The structure of the Boc-derivative 4a has been determined by single-crystal X-ray crystallography. An intramolecular N― H···O hydrogen bond involving the carbamate units results in a ring structure containing the two ferrocene units, the anhydride moiety, and the hydrogen bond. In the crystal, the individual molecules are connected by intermolecular N-H···O hydrogen bonds of the carbamate unit. Experimental and theoretical studies suggest that the ring motif is also a dominant species in solution. Electronic communication a…
Diferrocenyl tosyl hydrazone with an ultrastrong NHFe hydrogen bond as double click switch.
2014
The intramolecular NH⋯Fe hydrogen bond in diferrocenyl hydrazone 2 raises the barrier for intramolecular electron transfer in its mixed-valent cation 2+ and is only disrupted by double oxidation to 22+.
Recognition of N-Alkyl- and N-Aryl-Acetamides by N-Alkyl Ammonium Resorcinarene Chlorides
2014
N-alkyl ammonium resorcinarene chlorides are stabilized by an intricate array of intra- and intermolecular hydrogen bonds that leads to cavitand-like structures. Depending on the upper-rim substituents, self-inclusion was observed in solution and in the solid state. The self-inclusion can be disrupted at higher temperatures, whereas in the presence of small guests the self-included dimers spontaneously reorganize to 1:1 host-guest complexes. These host compounds show an interesting ability to bind a series of N-alkyl acetamide guests through intermolecular hydrogen bonds involving the carbonyl oxygen (C=O) atoms and the amide (NH) groups of the guests, the chloride anions (Cl(-)) and ammoni…