S. Brückner

The structure of γ-polypivalolactone: a combined analysis of single-crystal electron diffraction data and powder X-ray diffraction profiles with the Rietveld method

Abstract The crystal structure of the γ-phase of polypivalolactone (γ-PPVL) has been determined from electron diffraction data and tested with the Rietveld method against X-ray powder diffraction profiles. The electron diffraction study is based on single-crystal patterns obtained from melt-crystallized solution-cast thin films, while the Rietveld analysis was performed on data from melt-crystallized samples containing comparable amounts of α- and γ-phase PPVL. The crystal class is orthorhombic (space group P2 1 2 1 2 1 ), and two molecules with the same chirality and opposite directions pass through the unit cell having lattice dimensions a = 8.23, b = 11.28, c( chain repeat ) = 6.02 A . Both h k 0 and tilted-specimen electron diffraction data were used in the refinement, carried out in the kinematic approximation. The analysis based on these data and on powder profiles coherently indicates that the refined structural model has a main-chain conformation intermediate between the one found in the α-phase and the minimum-energy conformation, while differing significantly from both.

Steric and polar effects in free-radical reactions. - An unusual type of azocoupling by free radical decomposition of diazonium salts

Abstract A new free-radical azocoupling has been accomplished by decomposition of diazonium salts with monoelectronic reducing agents in the presence of 4-methyl-3-penten-2-one. Steric and polar factors are responsible of the unusual behaviour.

Synthesis and structure of the new mesoionic 1-thiasydnonimine system

Abstract Synthesis of 5-acylimino-3-methyl-1,2,3-thiadiazole 4 is described and its molecular structure is confirmed by NMR and X-Ray diffraction analysis.

Structure of α-polypivalolactone: a refinement based on the Rietveld method

Abstract A comparative study is carried out on the structure of the α form of polypivalolactone obtained (i) from the analysis of oriented-fibre X-ray diffraction diagrams, (ii) from conformational energy calculations and (iii) by refining the first two models through best fitting on the powder X-ray diffraction profile according to the Rietveld method. A single refined model is obtained that shows very good agreement both with experimental fibre intensities and with the powder profile. This result is discussed in terms of the accuracy of structural parameters derived from the Rietveld method compared with the much more numerous determinations based on X-ray diffraction from oriented samples.