分子晶体Molecular solid

分子内部是强的共价键，分子之间是弱的范德华力（约 1–40 kJ/mol），熔化或升华时只需克服分子间作用力，不破坏共价键，因此熔点低。组成和结构相似的分子，相对分子质量越大、色散力越强，熔沸点越高（如 F₂ < Cl₂ < Br₂ < I₂）。含氢键的分子晶体熔点反常偏高，如冰。只受色散力作用的球形分子（如 CH₄、稀有气体）常采用密堆积。

Molecules are held together internally by strong covalent bonds but to each other by weak van der Waals forces (about 1–40 kJ/mol); melting or subliming overcomes only the latter, so melting points are low. Among similar molecules, larger molar mass means stronger dispersion forces and higher melting and boiling points (F₂ < Cl₂ < Br₂ < I₂). Hydrogen-bonded molecular crystals such as ice melt anomalously high. Roughly spherical molecules bound only by dispersion, such as CH₄ and the noble gases, often adopt close packing.

干冰（CO₂）为面心立方，−78.5 °C 升华；碘为正交晶系层状排列，加热易升华；冰 Ih 中每个水分子以四面体方式与 4 个水分子形成氢键，结构疏松，密度小于液态水；C₆₀ 富勒烯晶体以面心立方堆积。大多数药物、染料、有机半导体都是分子晶体，同一分子的不同堆积方式（多晶型）可能有明显不同的溶解度、颜色或导电性。

Dry ice (CO₂) is face-centred cubic and sublimes at −78.5 °C; iodine forms an orthorhombic layered structure that sublimes easily; in ice Ih each water molecule hydrogen-bonds tetrahedrally to four others, an open structure less dense than liquid water; C₆₀ fullerene packs face-centred cubic. Most drugs, dyes and organic semiconductors are molecular crystals, and different packings (polymorphs) of the same molecule can differ markedly in solubility, colour or conductivity.

1955 年佩珀科恩使用“晶体工程”一词，1989 年德西拉朱将其发展为利用氢键、卤键、π-π 堆积等分子间作用设计晶体结构的学科。药物共晶、多晶型控制是制药工业的重要课题：例如利托那韦在上市后出现更稳定、溶解度更低的晶型，曾导致产品召回。

Pepinsky used the phrase crystal engineering in 1955, and Desiraju developed it in 1989 into the design of crystal structures through hydrogen bonds, halogen bonds, π-stacking and other intermolecular interactions. Co-crystals and polymorph control are major issues in pharmaceuticals: after ritonavir was launched, a more stable, less soluble polymorph appeared and forced a product recall.