Modulated DSC as a tool for polyethylene structure characterization

The Modulated Differential Scanning Calorimeter (MDSC) technique, using TA Q1000 instrument, has been applied as a tool to study the reversible and non-reversible heat flow characteristics of a wide range of polyethylenes. It was found that the heat flow characteristic is dependent upon the heating rates and modulation period used in the test. By using a set of standard test conditions, MDSC was found to be useful in studying the effect of previous thermal processing conditions, additive effects, and also the density, MI, type of comonomer, and molecular architecture.     

Geometries and molecular properties of phosphaethyne cations

Franck-Condon analyses were carried out on vibrational intensity distributions of the first two photoelectron bands of phosphaethyne (HCP). The CH and CP bond lengths of HCP⁺ were found to be respectively 1.073 Å and 1.600 A in the X²π state and 1.077 Å and 1.572 Å in the A²Σ⁺ state. The calculated structural parameters were compared with those from experimental work and molecular orbital calculations. In addition, force constants for the first two lowest energy ionic states were determined from observed vibrational frequencies. Bonding properties of the ions are discussed in the light of the molecular parameters obtained.     

Structural selection by microsolvation: conformational locking of tryptamine

The conformational space of tryptamine has been thoroughly investigated using rotationally resolved laser-induced fluorescence spectroscopy. Six conformers could be identified on the basis of the inertial parameters of several deuterated isotopomers. Upon attaching a single water molecule, the conformational space collapses into a single conformer. For the hydrogen-bonded water cluster, this conformer is identified unambiguously as tryptamine A. In the complex, the water molecule acts as proton donor with respect to the amino group. An additional interaction with one of the aromatic C-H bonds selectively stabilizes the observed conformer more than all other conformers. Ab initio calculations confirm much larger energy differences between the conformers of the water complex than between those of the monomers.     

Synthetic Studies on Vitamin D3 Analogues: The Synthesis of 8-P-(Hydroxymethyl)-Phenyl-Des-A,B-Cholest-8( 9) +8( 14)-ENE

The Grignard reagent derived from p-bromobenzyltetrahydropyranyl ether on reaction with the des-A, B-cholestan-8-one yielded 8-p-(terahydropyran-2′-yloxymethyl)-phenyl-des-A, B-cholestan-8-ol which on treatment with H²SO⁴ to give 8-p-(hydroxymethyl)-phenyl-des-A, B-cholest-8(9)+8(14)-ene.     

Interfacial glass transition profiles in ultrathin, spin cast polymer films

Interfacial glass transition temperature (T(g)) profiles in spin cast, ultrathin films of polystyrene and derivatives were investigated using shear-modulated scanning force microscopy. The transitions were measured as a function of film thickness (delta), molecular weight, and crosslinking density. The T(g)(delta) profiles were nonmonotonic and exhibited two regimes: (a) a sublayer extending about 10 nm from the substrate, with T(g) values lowered up to approximately 10 degrees C below the bulk value, and (b) an intermediate regime extending over 200 nm beyond the sublayer, with T(g) values exceeding the bulk value by up to 10 degrees C. Increasing the molecular weight was found to shift the T(g)(delta) profiles further from the substrate interface, on the order of 10 nm/kDa. Crosslinking the precast films elevated the absolute T(g) values, but had no effect on the spatial length scale of the T(g)(delta) profiles. These results are explained in the context of film preparation history and its influence on molecular mobility. Specifically, the observed rheological anisotropy is interpreted based on the combined effects of shear-induced structuring and thermally activated interdiffusion.     

Ligand atom partial charges assignment for complementary electrostatic potentials

Summary The design of molecules to fit into the active site of receptors is a rapidly developing area of pharmacology and medicinal chemistry. A good ligand needs a suitable geometry and also appropriate electrostatic properties. The electrostatic properties of the ligand should complement those of the receptor. We present a method for the assignment of atom-centred point charges for a ligand, based on the electrostatic potential of the receptor. These point charges are chosen to give the best possible complementarity to the receptor electrostatic potential over the van der Waals surface of the ligand. We demonstrate that point charges can be chosen to give good electrostatic complementarity, and suggest that a molecule with similar electrostatic properties should bind well to the receptor.     

Convergent Synthesis of Naphthylisoquinoline Alkaloids: Total Synthesis of (+)-O-Methylancistrocline

A highly convergent synthesis of the methyl ether derivative 2a of the naphthylisoquinoline alkaloid ancistrocline (2) is described. The key step involves a stereoselective biaryl coupling between the chiral oxazoline 3 and the Grignard reagent 4 derived from the optically active tetrahydroisoquinoline 8. The atropisomeric mixture was then converted to the separable acetamides 11 and 12, which were obtained in a ratio of 16:84 and an overall yield of 32% for the three steps. The major atropisomer 12 was then converted into O-methylancistrocline (2a), which was identical to a semisynthetic sample derived from the related alkaloid ancistrocladinine (14).     

Reactions of C-phosphorylated acetamidines with dialkylchlorophosphates

The reactions of the C-phosphorylated acetamidines sodium derivatives with dialkylchlorophosphates were studied. They occur selectively to afford N ¹,N ¹-dialkyl-N ²-benzoylbis(dialkoxyphosphoryl)-acetamidines. Based on the CH-acid properties of C-phosphorylated acetamidines, we developed a convenient method for the synthesis of new diphosphorylated amidines.