User:Cryst eng 340/sandbox
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A molecular solid is a solid consisting of discrete atoms and/or molecules held together by a collection of intermolecular interactions.[3][4][5][6][7][8][9] Examples of intermolecular interactions include 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.[3][4][5][6][7][8][9][10][11][12] Van der Waals, dipole interactions, quadrupole interactions, π-π interactions, hydrogen bonding, and halogen bonding (2-127 kJ mol-1)[12] are typically much weaker than the forces holding together other solids: metallic (metallic bonding, 400-500 kJ mol-1),[4] ionic (Coulomb’s forces, 700-900 kJ mol-1),[4] and network solids (covalent bonds, 150-900 kJ mol-1).[4][12] Intermolecular interactions, typically do not involve delocalized electrons, unlike metallic and certain covalent bonds. Exceptions are charge-transfer complexes such as the tetrathiofulvane-tetracyanoquinodimethane (TTF-TCNQ), a radical ion salt.[5] 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.[3][4][5][8]
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For instance, molecular solids such as coronene have low conductivity (ρ = 1 x 10-12 to 1 x 10-18 Ω-1 cm-1)[13] making them poor electrical conductors.[4][5] As mentioned there are exceptions such as TTF-TCNQ (ρ = 5 x 102 Ω-1 cm-1),[5] but it still substantially less than the conductivity of copper (ρ = 6 x 105 Ω-1 cm-1).[8] Molecular solids tend to have lower fracture toughness (sucrose, KIc = 0.08 MPa m1/2)[14] than metal (iron, KIc = 50 MPa m1/2),[14] ionic (sodium chloride, KIc = 0.5 MPa m1/2),[14] and covalent solids (diamond, KIc = 5 MPa m1/2).[15] Molecular solids have low melting (Tm) and boiling (Tb) points compared to metal (iron), ionic (sodium chloride), and covalent solids (diamond).[4][5][8][16] Examples of molecular solids with low melting and boiling temperatures include argon, water, naphthalene, nicotine, and caffeine (see table below).[16][17] The constituents of molecular solids range in size from condensed monoatomic gases[18] to small molecules (i.e. naphthalene and water)[19][20] to large molecules with tens of atoms (i.e. fullerene with 60 carbon atoms).[21]
Type of Solid | Material | Tm (°C) | Tb (°C) |
---|---|---|---|
Metallic | Iron | 1,538[16] | 2,861[16] |
Ionic | Sodium chloride | 801[16] | 1,465[16] |
Covalent | Diamond | 4,440[16] | - |
Molecular | Argon | -189.3[16] | -185.9[16] |
Molecular | Water | 0[16] | 100[16] |
Molecular | Naphthalene | 80.1[16] | 217.9[16] |
Molecular | Nicotine | -79[16] | 491[16] |
Molecular | Caffeine | 235.6[16] | 519.9[17] |