Molecular solid
![](https://upload.wikimedia.org/wikipedia/commons/thumb/d/d1/Carbon_dioxide_and_caffeine.jpg/220px-Carbon_dioxide_and_caffeine.jpg)
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,
Molecular solids are poor electrical conductors,[4][5] although some, such as TTF-TCNQ are semiconductors (
Type of Solid | Material | Tm (°C) | Tb (°C) |
---|---|---|---|
Metallic | Iron | 1,538[13] | 2,861[13] |
Ionic | Sodium chloride | 801[13] | 1,465[13] |
Covalent | Diamond | 4,440[13] | - |
Molecular | Argon | -189.3[13] | -185.9[13] |
Molecular | Water | 0[13] | 100[13] |
Molecular | Naphthalene | 80.1[13] | 217.9[13] |
Molecular | Nicotine | -79[13] | 491[13] |
Molecular | Caffeine | 235.6[13] | 519.9[14] |
Composition and structure
[edit]Molecular solids may consist of single atoms, diatomic, and/or polyatomic molecules.[1][2][3][4][5][6][7] The intermolecular interactions between the constituents dictate how the crystal lattice of the material is structured.[19][20][21] All atoms and molecules can partake in van der Waals and London dispersion forces (sterics). It is the lack or presence of other intermolecular interactions based on the atom or molecule that affords materials unique properties.[19]
Van der Waals forces
[edit]![](https://upload.wikimedia.org/wikipedia/commons/thumb/1/1c/Iodine_van_der_Waals.jpg/220px-Iodine_van_der_Waals.jpg)
Argon, is a noble gas that has a full octet, no charge, and is nonpolar.[3][4][7][8] These characteristics make it unfavorable for argon to partake in metallic, covalent, and ionic bonds as well as most intermolecular interactions.[3][4][7][8] It can though partake in van der Waals and London dispersion forces.[3][4] These weak self-interactions are isotropic and result in the long-range ordering of the atoms into face centered cubic packing when cooled below -189.3.[13] Similarly iodine, a linear diatomic molecule has a net dipole of zero and can only partake in van der Waals interactions that are fairly isotropic.[3][4][7][8] This results in the bipyramidal symmetry.
Dipole-dipole and quadrupole interactions
[edit]![](https://upload.wikimedia.org/wikipedia/commons/thumb/a/a6/Acetone_dipole-dipole_bonding.jpg/220px-Acetone_dipole-dipole_bonding.jpg)
For acetone dipole-dipole interactions are a major driving force behind the structure of its crystal lattice. The negative dipole is caused by oxygen. Oxygen is more electronegative than carbon and hydrogen,[13] causing a partial negative (
![](https://upload.wikimedia.org/wikipedia/commons/thumb/5/5d/Naphthalene_quadropole_bonding.jpg/220px-Naphthalene_quadropole_bonding.jpg)
A quadrupole, like a dipole, is a permanent pole but the electric field of the molecule is not linear as in acetone, but in two dimensions.[25] Examples of molecular solids with quadrupoles are octafluoronaphthalene and naphthalene.[17][25] Naphthalene consists of two joined conjugated rings. The electronegativity of the atoms of this ring system and conjugation cause a ring current resulting in a quadrupole. For naphthalene, this quadrupole manifests in a
Hydrogen and halogen bonding
[edit]![](https://upload.wikimedia.org/wikipedia/commons/thumb/0/00/Acetic_acid_hydrogen_bonding.jpg/220px-Acetic_acid_hydrogen_bonding.jpg)
A hydrogen bond is a specific dipole where a hydrogen atom has a partial positive charge (
![](https://upload.wikimedia.org/wikipedia/commons/thumb/6/65/Bromine_dioxane_halogen_bonding.jpg/220px-Bromine_dioxane_halogen_bonding.jpg)
A halogen bond is when an electronegative halide participates in a noncovalent interaction with a less electronegative atom on an adjacent molecule.[10][28] Examples of molecular solids that halogen bond are hexachlorobenzene[11][29] and a cocrystal of bromine 1,4-dioxane.[27] For the second example, the
Coulombic interactions
[edit]![](https://upload.wikimedia.org/wikipedia/commons/thumb/4/4a/TFF-TCNQ_charge_transfer_complex.jpg/220px-TFF-TCNQ_charge_transfer_complex.jpg)
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
Allotropes
[edit]One form of an element may be a molecular solid, but another form of that same element may not be a molecular solid.[3][4][5] For example, solid phosphorus can crystallize as different allotropes called "white", "red", and "black" phosphorus. White phosphorus forms molecular crystals composed of tetrahedral P4 molecules.[32] Heating at ambient pressure to 250 °C or exposing to sunlight converts white phosphorus to red phosphorus where the P4 tetrahedra are no longer isolated, but connected by covalent bonds into polymer-like chains.[33] Heating white phosphorus under high (GPa) pressures converts it to black phosphorus which has a layered, graphite-like structure.[34][35]
The structural transitions in phosphorus are reversible: upon releasing high pressure, black phosphorus gradually converts into the red phosphorus, and by vaporizing red phosphorus at 490 °C in an inert atmosphere and condensing the vapor, covalent red phosphorus can be transformed into the molecular solid, white phosphorus.[36]
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White, red, violet, and black phosphorus samples | Structure unit of white phosphorus |
Structures of red | violet | and black phosphorus |
Similarly, yellow arsenic is a molecular solid composed of As4 units.[37] Some forms of sulfur and selenium are composed of S8 (or Se8) units and are molecular solids at ambient conditions, but converted into covalent allotropes having atomic chains extending throughout the crystal.[38][39]
Properties
[edit]Since molecular solids are held together by relatively weak forces they tend to have low melting and boiling points, low mechanical strength, low electrical conductivity, and poor thermal conductivity.[3][4][5][6][7][8]it will Also, depending on the structure of the molecule the intermolecular forces may have directionality leading to anisotropy of certain properties.[4][5][8]
Melting and boiling points
[edit]The characteristic melting point of metals and ionic solids is ~ 1000 °C and greater, while molecular solids typically melt closer to 300 °C (see table), thus many corresponding substances are either liquid (ice) or gaseous (oxygen) at room temperature.[4][6][7][8][40] This is due to the elements involved, the molecules they form, and the weak intermolecular interactions of the molecules.
Formula | Tm °C |
---|---|
H2 | −259.1 |
F2 | −219.6 |
O2 | −218.8 |
N2 | −210.0 |
CH4 | −182.4 |
C2H6 | −181.8 |
C3H8 | −165.0 |
C4H10 | −138.3 |
C5H12 | −129.8 |
Cl2 | −101.6 |
C6H14 | −95.3 |
HBr | −86.8 |
HF | −80.0 |
NH3 | −80.0 |
HI | −50.8 |
C10H22 | −29.7 |
HCl | −27.3 |
Br2 | −7.2 |
H2O | 0.0 |
C6H6 | 5.5 |
I2 | 113.7 |
S8 | 119.0 |
C6Cl6 | 220.0 |
- See also
higher alkanes
Allotropes of phosphorus are useful to further demonstrate this structure-property relationship. White phosphorus, a molecular solid, has a relatively low density of 1.82 g/cm3 and melting point of 44.1 °C; it is a soft material which can be cut with a knife. When it is converted to the covalent red phosphorus, the density goes to 2.2–2.4 g/cm3 and melting point to 590 °C, and when white phosphorus is transformed into the (also covalent) black phosphorus, the density becomes 2.69–3.8 g/cm3 and melting temperature ~200 °C. Both red and black phosphorus forms are significantly harder than white phosphorus.[43]
Mechanical properties
[edit]Molecular solids can be either ductile or brittle, or a combination depending on the crystal face stressed.[5][11] Both ductile and brittle solids undergo elastic deformation till they reach the yield stress.[8][11] Once the yield stress is reached ductile solids undergo a period of plastic deformation, and eventually fracture. Brittle solids fracture promptly after passing the yield stress.[8][11] Due to the asymmetric structure of most molecules or the presence of functional groups capable of forming specific interaction (e.g. hydrogen bonds, halogen bonds), many molecular solids have directional intermolecular forces.[11] This phenomenon can lead to anisotropic mechanical properties. Typically, a molecular solid is ductile when it has isotropic intermolecular interactions. This allows for dislocation between layers of the crystal much like metals.[5][8][11] For example, plastic crystals are soft, they resemble waxes and are easily deformed.
One example of a ductile molecular solid, that can be bent 180°, is hexachlorobenzene (HCB).[11][29] In this example the
Electrical properties
[edit]Molecular solids are generally insulators.[5][18] This large band gap (compared to germanium at 0.7 eV)[8] is due to the weak intermolecular interactions, which result in low charge carrier mobility. Some molecular solids exhibit electrical conductivity, such as TTF-TCNQ with
Thermal properties
[edit]The thermal properties of molecular solids (as, for instance, specific heat capacity, thermal expansion, and thermal conductance to name a few) are determined by the intra- and intermolecular vibrations of the atoms and molecules of the molecular solid.[3][5][6][7][8] In molecular solids the transitions of an electron contribute negligibly to thermal properties compared to the vibrational contribution.[5][8]
See also
[edit]References
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