Crystal and molecular structures of 2-

Area of Science:

• Crystallography and Materials Science
• Chemical Structure Determination
• Powder Diffraction Analysis

Background:

• Accurate determination of crystal and molecular structures is fundamental to understanding chemical properties and reactivity.
• Previous structural analyses of complex organic molecules often face challenges with data quality and refinement.
• The development and application of advanced diffraction techniques are crucial for overcoming these challenges.

Purpose of the Study:

• To elucidate the crystal and molecular structures of 2-[1-(2-aminoethyl)-2-imidazolidinylidene]-2-nitroacetonitrile (I) and 2,6-diamino-5-hydroxy-3-nitro-4H-pyrazolo[1,5-a]pyrimidin-7-one monohydrate (II).
• To evaluate and compare the effectiveness of various powder diffraction methods (X-ray, synchrotron, neutron) and computational approaches for structure determination.
• To validate the reliability and reproducibility of different powder diffraction software and equipment.

Main Methods:

• Crystal and molecular structure determination using X-ray, synchrotron, and neutron powder diffraction data.
• Initial structure solution employing Guinier photographs, grid search, and a priori information from NMR and mass spectra.
• Advanced structure solution using high-resolution synchrotron data with direct methods (SHELXS96, SIRPOW.92, POWSIM) and Patterson search methods (DIRDIF96, PATSEE).
• Structure solution including hydrogen atoms using neutron diffraction data and grid search procedures.

Main Results:

• The crystal and molecular structures of compounds (I) and (II) were successfully determined.
• High-resolution synchrotron data proved essential for confirming initial results and resolving structural ambiguities, particularly texture effects in Guinier patterns.
• Both direct methods and Patterson search methods, when applied to synchrotron data, yielded reliable solutions without preliminary models.
• Neutron diffraction data enabled the accurate placement of hydrogen atoms, leading to complete molecular models.

Conclusions:

• The study demonstrates the successful application of multiple powder diffraction techniques for complex organic molecule structure determination.
• Synchrotron radiation facilities offer significant advantages for obtaining high-quality, texture-free data crucial for accurate structure refinement.
• The results underscore the high reproducibility and reliability of various powder diffraction software and equipment, with synchrotron data showing a preference.
• This work validates the robustness of modern crystallographic methods in solving challenging structures.