Colloquium
Department of Physics, NCU
VARICELLA: a variable cell direct space algorithm for structure determination from powder diffraction data
Speaker
Dr. A Rapallo
Istituto per lo Studio delle Macromolecole (ISMAC) Consiglio Nazionale delle Ricerche (CNR), Milano, Italy
Date 2013.11.12(Tue)
Time 14:00
Place S4-625
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ABSTRACT: A direct space method for structure determination from powder diffraction data is proposed. Employing a hybrid Monte Carlo algorithm for generating the random conformations of a flexible molecular model, and sampling in a modified multicanonical statistical ensemble, it allows for variable cell parameters during an iterative search process. The acceptance-rejection criterion involves both a disagreement factor between the calculated and the experimental diffraction profiles and a modified crystal energy so that the space of tentative solutions can be widely explored while maintaining some physical meaningfulness of the proposals. The method is tested on a case study, a crystal of 3-ethyl 2,3-exo-disyndiotactic norbornene heptamer recently solved by single crystal X-ray diffraction techniques, and used to solve a real life problem, namely the structure of a 2,3-exo-disyndiotactic dicyclopentadiene tetramer. In this latter case an uncertainty about the molecular mass of the crystallizing molecules was present, the experiments detecting oligomers ranging from the trimers to the hexamers in the powder samples. In these conditions the standard methods cannot be used at all to solve the problem, while VARICELLA allowed for both find the mass of the crystallizing species and solve the crystal structure problem. To our knowledge no previous application of powder X-ray diffraction techniques allowed to solve such kind of ambiguity in the molecular mass of the crystallizing species.