Towards an atomistic understanding of polymorphism in molecular solids

Area of Science:

• Solid-state chemistry
• Computational chemistry
• Crystal engineering

Background:

• Polymorphism in molecular solids is a challenge in crystal engineering.
• Accurate lattice energies are calculable, but linking them to specific forces is difficult.
• Existing methods struggle to explain polymorph stability ordering based on molecular interactions.

Purpose of the Study:

• To develop and apply a Quantum Chemical Topology (QCT) protocol for analyzing intra- and intermolecular interactions in molecular solids.
• To compare the three known polymorphs of succinic acid, including the novel gamma form.
• To provide a quantitative and chemically intuitive description of interactions influencing polymorph stability.

Main Methods:

• Application of Quantum Chemical Topology (QCT) to molecular solids.
• Rigorous partitioning of total energy into atomic contributions.
• Utilizing the Relative Energy Gradient (REG) method to rank energy terms (steric, electrostatic, exchange).
• Analysis of succinic acid polymorphs.

Main Results:

• QCT provides a quantitative description of intra- and intermolecular interactions.
• The Relative Energy Gradient (REG) method successfully ranks the importance of energy terms.
• Succinic acid conformation is dictated by electrostatic forces; H-bond dimerization involves electrostatics and sterics.
• An atomistic energy balance governs the contraction of molecular clusters in the solid state.

Conclusions:

• The developed QCT protocol offers novel insights into the forces governing molecular solid polymorphism.
• The REG method aids in understanding the energetic contributions to chemical processes.
• This approach is general and applicable to a wide range of chemical systems for studying polymorphism.