Modelling the crystal structure of the 2-hydronitronylnitroxide radical (HNN): observed and computer-generated...

Area of Science:

• Crystallography
• Solid-state chemistry
• Computational materials science

Background:

• Polymorphism significantly impacts material properties.
• Predicting crystal structures from first principles remains a challenge.
• Understanding the packing of organic radicals is crucial for their applications.

Purpose of the Study:

• To analyze the crystal structures of 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-oxide (HNN) polymorphs.
• To generate and evaluate unobserved polymorphic crystal structures using computational methods.
• To assess the stability of known HNN polymorphs against predicted structures.

Main Methods:

• Analysis of existing crystal structures using packing energy criteria.
• Ab initio crystal structure prediction using a polymorph predictor package.
• Application of three distinct force fields, including one without Coulomb terms.
• Comparison of simulated powder X-ray diffraction patterns.

Main Results:

• The polymorph predictor generated numerous crystal structures within a narrow energy range, highlighting prediction challenges.
• The relative importance of structural motifs like hydrogen-bonded dimers and chains was discussed.
• The experimentally determined alpha polymorph of HNN was consistently ranked as one of the most stable predicted structures, irrespective of the force field used.

Conclusions:

• Ab initio crystal structure prediction remains a complex and evolving field.
• The stability of the alpha polymorph of HNN is robust and predictable.
• Computational approaches, even simplified ones, can provide valuable insights into crystalline solid-state structures.