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The ECA kindly invites you to the 27th ECA Lunch webinar:

 

 

Abstract

For nanocrystalline organic compounds, crystal structure determination is often challenging. If the X-ray powder diffractogram (XRPD) contains less than about 20-30 sharp peaks, indexing frequently fails, and the lattice parameters remain unknown. Consequently, all classical methods for structure determination from powder data cannot be applied.

If indexing fails, only few methods remain:

  1. Recrystallisation attempts trying to obtain better powder data
  2. A search for an isostructural compound with a known crystal structure
  3. Crystal structure prediction by global lattice-energy minimisations
  4. Structure solution by a global fit to the powder pattern without prior knowledge of lattice parameters and space group (“FIDEL Global Fit”)
  5. Structure determination by a fit to the pair-distribution function (“Global PDF fit”)
  6. Electron diffraction

This presentation focusses on the FIDEL Global fit and on the Global PDF fit.

The FIDEL Global Fit [1] starts from a large number (106 -108) of random structures, having random lattice parameters, molecular position and orientation in different space groups. All structures are fitted to the experimental X-ray powder pattern with the program FIDEL (“Fit with deviating lattice parameters”), which uses cross-correlation functions for comparing simulated and experimental powder patterns as long as the lattice parameters do not match. The best resulting structures are subjected to a Rietveld refinement using TOPAS [2]. The presentation explains the method, gives examples and shows the limitations of the method.

The Global PDF fit [3] functions in a similar fashion as the FIDEL Global Fit, but the fit is performed to the PDF instead of the powder pattern itself. Again, the cross-correlation function is used for comparing simulated and experimental PDFs [4,5]. Finally, a PDF fit with TOPAS is performed. The presentation explains the method, gives examples and shows the limitations of the method.

Amorphous organic compounds: If the powder pattern contain no peaks at all, the PDF can still be used to investigate, whether an amorphous powder is fully amorphous (i.e. consists of a random arrangement of molecules), or if it exhibits a preferred local structure (i.e. a  preferred arrangement of neighbouring molecules), and if this local structure is similar to the structure of a crystalline form.

References:

[1] S. Habermehl, C. Schlesinger, M. U. Schmidt: “Structure determination from unindexed powder data from scratch by a global optimization approach using pattern comparison based on cross-correlation functions”. Acta Cryst. 2022, B78, 195–213. doi.org/10.1107/S2052520622001500.

[2] TOPAS Academic, see A.A Coelho: “TOPAS and TOPAS-Academic: an optimization program integrating computer algebra and crystallographic objects written in C++” . J. Appl. Cryst. 2018, 51, 210–218.doi.org/10.1107/S1600576718000183.

[3] C. Schlesinger, S. Habermehl, D. Prill: “Structure determination of organic compounds by a fit to the pair distribution function from scratch without prior indexing”. J. Appl. Cryst. 2021, 54, 776–786.doi.org/10.1107/S1600576721002569.

[4] S. Habermehl, C. Schlesinger, D. Prill: “Comparison and evaluation of pair distribution functions, using a similarity measure based on cross-correlation functions”. J. Appl. Cryst. 2021, 54, 612–623.doi.org/10.1107/S1600576721001722.

[5] S. Habermehl, C. Schlesinger, M.U. Schmidt, D. Prill: “Vergleich von Atompaar-Verteilungsfunktionen mittels Kreuzkorrelationsfunktionen (Comparison of atomic pair distribution functions using cross-correlation functions)”. German Patent Application 2022, DE 10 2020 004 292 A1.depatisnet.dpma.de/DepatisNet/depatisnet?action=pdf&docid=DE102020004292A1&xxxfull=1

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Intro + ~45min + ~15min Q&A
When: Thursday, 14th March 2024 at 13:00 CET (i.e. 1:00 p.m. in Paris/Berlin/Vienna…).
Where: Online :::::::link to registration::::::: 

 

You have the chance to receive an info-mail when a new webinar is going to take place. Click here to register for our newsletter.

For inquiries please send a message to eca-webinar_at_ecanews.org.

“””””””””””””””””””””””””””””””””””””””””””””””””””””””””

About the ECA Lunch webinars:

The ECA lunch webinars with an educational or scientific focus were invented following the strong wish to strengthen and vitalize the European crystallographic community. They are a monthly webinar series by the European Crystallographic Association (ECA) and involve all GIGs/SIGs.

The webinars are organized in an online setting such as Zoom (45min + 15min Q&A) and take place every first or second (depending on the schedule of other ECA activities) Thursday of a month at lunchtime (1:00 PM CET). Webinars will be announced via social media, mailing list, and here.

The webinars are organized by a steering committee of GIG01-members:
Breternitz, Joachim
Gaidamaka, Anna
Germann, Luzia
Hans, Philipp
Mazzeo, Paolo P.
Morana, Marta
Pisacic, Mateja
Prencipe, Michele
 

 

 
Advisory board:
Heinemann, Udo
van der Lee, Arie

 

The ECA Lunchtime Webinars are on a break over the summer and will restart again soon!

Click here to find details and recordings of past webinars

 

You have the chance to receive an info-mail when a new webinar is going to take place. Click here to register for our newsletter.

For inquiries please send a message to eca-webinar_at_ecanews.org.

“””””””””””””””””””””””””””””””””””””””””””””””””””””””””

About the ECA Lunch webinars:

The ECA lunch webinars with an educational or scientific focus were invented following the strong wish to strengthen and vitalize the European crystallographic community. They are a monthly webinar series by the European Crystallographic Association (ECA) and involve all GIGs/SIGs.

The webinars are organized in an online setting such as Zoom (45min + 15min Q&A) and take place every first or second (depending on the schedule of other ECA activities) Thursday of a month at lunchtime (1:00 PM CET). Webinars will be announced via social media, mailing list, and here.

The webinars are organized by a steering committee of GIG01-members:
Breternitz, Joachim
Gaidamaka, Anna
Germann, Luzia
Hans, Philipp
Mazzeo, Paolo P.
Morana, Marta
Pisacic, Mateja
Prencipe, Michele
 

 

 
Advisory board:
Heinemann, Udo
van der Lee, Arie