page_title: Molecular solid
hierarchical_section_title: Molecular solid
caption_attribution_description: English: Shows the packing of molecules and visual appearance of carbon dioxide and caffeine.
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: 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 (2-127 kJ mol⁻¹) are typically much weaker than the forces holding together other solids: metallic (metallic bonding, 400-500 kJ mol⁻¹), ionic (Coulomb’s forces, 700-900 kJ mol⁻¹), and network solids (covalent bonds, 150-900 kJ mol⁻¹). Intermolecular interactions, typically do not involve delocalized electrons, unlike metallic and certain covalent bonds. Exceptions are charge-transfer complexes such as the tetrathiafulvane-tetracyanoquinodimethane (TTF-TCNQ), a radical ion salt. These differences in the strength of force (i.e. covalent vs. van der Waals) and electronic characteristics (i.e. delocalized electrons) 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 (ρ = 1 x 10⁻¹² to 1 x 10⁻¹⁸ Ω⁻¹ cm⁻¹) making them poor electrical conductors. As mentioned there are exceptions such as TTF-TCNQ (ρ = 5 x 10² Ω⁻¹ cm⁻¹), but it still substantially less than the conductivity of copper (ρ = 6 x 10⁵ Ω⁻¹ cm⁻¹). Molecular solids tend to have lower fracture toughness (sucrose, KIc = 0.08 MPa m1/2) than metal (iron, KIc = 50 MPa m1/2), ionic (sodium chloride, KIc = 0.5 MPa m1/2), and covalent solids (diamond, KIc = 5 MPa m1/2). Molecular solids have low melting (Tₘ) and boiling (Tb) points compared to metal (iron), ionic (sodium chloride), and covalent solids (diamond). Examples of molecular solids with low melting and boiling temperatures include argon, water, naphthalene, nicotine, and caffeine (see table below). The constituents of molecular solids range in size from condensed monatomic gases to small molecules (i.e. naphthalene and water) to large molecules with tens of atoms (i.e. fullerene with 60 carbon atoms).
caption: Models of the packing of molecules in two molecular solids, carbon dioxide or Dry ice (a),[1] and caffeine (c).[2] The gray, red, and purple balls represent carbon, oxygen, and nitrogen, respectively. Images of carbon dioxide (b) and caffeine (d) in the solid state at room temperature and atmosphere. The gaseous phase of the dry ice in image (b) is visible because the molecular solid is subliming.
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