c. Molecular packing

Hydrogen bonds X--H$\cdots$Y are 2.38 to 3.20 Å with the proviso that the H$\cdots$Y distance should generally be 2.2 Å or less. The X--H$\cdots$Y angle is usually 150-180$^{\circ}$. Symmetrical O$\cdots$H$\cdots$O hydrogen bonds are very strong and are approximately 2.38 Å long (O$\cdots$O distance).

Van der Waals contacts may be represented by considering van der Waals radii for each atom. These are listed by Pauling in `The Nature of the Chemical Bond'. H 1.2 Å, C 1.7, N 1.5, O 1.4, P 1.9, S 1.85, F 1.35, Cl 1.8, Br 1.95, I 2.15. Half-thickness of aromatic molecule is 1.85 Å. Radius of methyl group CH₃ 2.0 Å.

Metal coordination . Metal ions tend to gather negatively charged groups, e.g. the oxygen atoms of carboxylate ions, around them. Distances are shorter than van der Waals distances. The values depend on the number of surrounding oxygen atoms. Li⁺$\cdots$O 2.0 Na⁺$\cdots$O 2.4 K⁺$\cdots$O 2.8-2.9 NH⁺₄$\cdots$O 3.0 Mg²⁺$\cdots$O 2.1 Ca²⁺$\cdots$O 2.4-2.5 Zn²⁺$\cdots$O 2.0-2.2 Mn²⁺$\cdots$O 2.2 Å.

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