page_title: Molecular solid
section_title: Coulombic interactions
hierarchical_section_title: Molecular solid / Composition and structure / Coulombic interactions
caption_attribution_description: English: This image shows the intermolecular interactions and packing in three dimensions of TTF-TCNQ as a solid.
context_page_description: A molecular solid is a solid consisting of discrete molecules. The cohesive forces that bind the molecules together are van der Waals forces, dipole-dipole interactions, quadrupole interactions, π-π interactions, hydrogen bonding, halogen bonding, London dispersion forces, and in some molecular solids, coulombic interactions. Van der Waals, dipole interactions, quadrupole interactions, π-π interactions, hydrogen bonding, and halogen bonding are typically much weaker than the forces holding together other solids: metallic, ionic, and network solids. Intermolecular interactions, typically do not involve delocalized electrons, unlike metallic and certain covalent bonds. Exceptions are charge-transfer complexes such as the tetrathiafulvane-tetracyanoquinodimethane, a radical ion salt. These differences in the strength of force and electronic characteristics from other types of solids give rise to the unique mechanical, electronic, and thermal properties of molecular solids.
  For instance, molecular solids such as coronene have low conductivity making them poor electrical conductors.
context_section_description: Coulombic interactions are manifested in some molecular solids. A well-studied example is the radical ion salt TTF-TCNQ with a conductivity of 5 x 10² Ω⁻¹ cm⁻¹, much closer to copper (ρ = 6 x 10⁵ Ω⁻¹ cm⁻¹) than many molecular solids (e.g. coronene, ρ = 1 x 10⁻¹² to 1 x 10⁻¹⁸ Ω⁻¹ cm⁻¹). The coulombic interaction in TTF-TCNQ stems from the large partial negative charge (δ = -0.59) on the cyano- moiety on TCNQ  at room temperature. For reference, a completely charged molecule δ = ±1. This partial negative charge leads to a strong interaction with the thio- moiety of the TTF. The strong interaction leads to favorable alignment of these functional groups adjacent to each other in the solid state. While π-π interactions cause the TTF and TCNQ to stack in separate columns.
caption: The partial ionic bonding between the TTF and TCNQ molecules partially guides the organization of the crystal structure. The van der Waals interactions of the core for TTF and TCNQ guide adjacent stacked columns.[31] (a) A lewis dot structure and ball and stick model of  TTF and TCNQ. The partial ionic bond is between the cyano- and thio- motifs. (b) Demonstration of how van der Waals and partial ionic bonding guide the crystal lattice structure.
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