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1,1-Dichloroethane: a molecular crystal structure without van der Waals contacts?
Maciej Bujak1, Marcin Podsiadło, Andrzej Katrusiak
1Institute of Chemistry, University of Opole, Oleska 48, 45-052 Opole, Poland.
The Journal of Physical Chemistry. B
|January 10, 2008
Summary
Researchers studied the crystal structure of 1,1-dichloroethane under varying pressure and temperature conditions. They found that high pressure significantly reduces intermolecular distances, influencing molecular packing and melting points.
Area of Science:
- Solid-state chemistry
- Crystallography
- Molecular physics
Background:
- Understanding the solid-state structures of halogenated hydrocarbons is crucial for predicting their physical properties.
- 1,1-dichloroethane (CH3CHCl2) exhibits unique intermolecular interactions influenced by its molecular structure.
Purpose of the Study:
- To determine the crystal structure of 1,1-dichloroethane under various pressure and temperature conditions.
- To investigate the effect of pressure on intermolecular distances and molecular packing.
- To rationalize the difference in melting points between 1,1- and 1,2-dichloroethane.
Main Methods:
- Isochoric and isobaric freezing techniques were employed.
- X-ray crystallography was used to determine the crystal structures.
- Structures were analyzed at ambient and high pressures (0.1 MPa to 1.51 GPa) and varying temperatures (100 K to 295 K).
Main Results:
- The orthorhombic structure (space group Pnma) with fully ordered, staggered molecules was observed.
- Ambient pressure structures showed intermolecular distances exceeding van der Waals radii sums.
- High pressure (approx. 1.5 GPa) compressed Cl...Cl and Cl...H contacts to distances comparable to van der Waals radii sums.
Conclusions:
- Molecular packing efficiency significantly influences the melting points of dichloroethane isomers.
- Intermolecular interactions and van der Waals radii are dependent on atomic positions within the molecular structure.
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