The shortest chalcogen...halogen contacts in molecular crystals

Area of Science:

• Solid-state chemistry
• Crystallography
• Supramolecular chemistry

Background:

• Molecular crystals are held together by various intermolecular forces.
• Chalcogen (Ch) and halogen (X) interactions are known to contribute to crystal cohesion.
• Understanding these interactions is crucial for predicting and validating crystal structures.

Purpose of the Study:

• To investigate the prevalence and significance of chalcogen-halogen (Ch...X) contacts as cohesion forces in molecular crystals.
• To determine the conditions under which Ch...X interactions act as primary cohesive forces.
• To utilize contact analysis for validating crystal structural models.

Main Methods:

• Analysis of crystal structures deposited in the Cambridge Structural Database.
• Calculation of the parameter δ (interatomic distance minus sum of van der Waals radii) for Ch...X contacts.
• Identification of structures with shortest Ch...X contacts and analysis of their relative significance.

Main Results:

• Chalcogen...halogen, halogen...halogen, and chalcogen...chalcogen interactions can compete as cohesion forces.
• Shortest Ch...X contacts (δ < 0) were found in only 0.86% of 30,766 relevant structures.
• In less than 1% of structures, Ch...X forces are the primary cohesive forces.
• Four 'loose crystals' with no contacts shorter than van der Waals radii were identified among 263 structures with shortest Ch...X contacts.

Conclusions:

• Ch...X interactions are generally weak and rarely the dominant cohesive force in molecular crystals.
• The δ criterion is effective for differentiating bonding and non-bonding interactions.
• This approach aids in the validation of crystal structural models.