[4][5][8][13] Examples of molecular solids with low melting and boiling temperatures include argon, water, naphthalene, nicotine, and caffeine (see table below).
[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.
[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] 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.
The dipole-dipole and other intermolecular interactions align to minimize energy in the solid state and determine the crystal lattice structure.
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.
Octafluoronaphthalene follows this path of organization to build bulk material except the δ- and δ+ are on the exterior and interior of the ring system, respectively.
A well-studied example is the radical ion salt TTF-TCNQ with a conductivity of 5 x 102 Ω−1 cm−1,[5] much closer to copper (ρ = 6 x 105 Ω−1 cm−1)[8] than many molecular solids.
[31][18][30] 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.
The strong interaction leads to favorable alignment of these functional groups adjacent to each other in the solid state.
[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.
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.
[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.
[11][29] MTN is flexible due to its strong hydrogen bonding and π-π interactions creating a rigid set of dimers that dislocate along the alignment of their terminal methyls.
[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 ρ = 5 x 102 Ω−1 cm−1 but in such cases orbital overlap is evident in the crystal structure.
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
.
Van der Waals and London dispersion forces guide iodine to condense into a solid at room temperature.
[
22
]
(a) A lewis dot structure of iodine and an analogous structure as a spacefill model. Purple balls represent iodine atoms. (b) Demonstration of how van der Waals and London dispersion forces guide the organization of the crystal lattice from 1D to 3D (bulk material).
The dipole-dipole interactions between the acetone molecules partially guide the organization of the crystal lattice structure.
[
23
]
(a) A dipole-dipole interaction between acetone molecules stacked on top of one another. (b) A dipole-dipole interaction between acetone molecules in front and bock of each other in the same plane. (c) A dipole-dipole interaction between acetone molecules flipped in direction, but adjacent to each other in the same plane. (d) Demonstration of how quadrupole-quadrupole interactions are involved in the crystal lattice structure.
The quadrupole-quadrupole interactions between the naphthalene molecules partially guide the organization of the crystal lattice structure.
[
24
]
(a) A lewis dot structure artificially colored to provide a qualitative map of where the partial charges exist for the quadrupole. A 3D representation of naphthalene molecules and quadrupole. (b) A 3D representation of the quadrupole from two naphthalene molecules interacting. (c) A dipole-dipole interaction between acetone molecules flipped in direction, but adjacent to each other in the same plane. (c) Demonstration of how quadrupole-quadrupole interactions are involved in the crystal lattice structure.
The hydrogen bonding between the acetic acid molecules partially guides the organization of the crystal lattice structure.
[
26
]
(a) A lewis dot structure with the partial charges and hydrogen bond denoted with blue dashed line. A ball and stick model of acetic acid with hydrogen bond denoted with blue dashed line. (b) Four acetic acid molecules in zig-zag hydrogen bonding in 1D. (c) Demonstration of how hydrogen bonding are involved in the crystal lattice structure.
The halogen bonding between the bromine and 1,4-dioxane molecules partially guides the organization of the crystal lattice structure.
[
27
]
(a) A lewis dot structure and ball and stick model of bromine and 1,4-dioxane. The halogen bond is between the bromine and 1,4-dioxane. (b) Demonstration of how halogen bonding can guide the crystal lattice structure.
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.
[
30
]
(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.
