8 Relationship Between Atomic Contact and Intermolecular …
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Fig. 8.8 Crystal structure of
hexamine
Table 8.5 Intermolecular interaction energies between adjacent hexamine molecules in crystal.
Energy in kcal mol −1
E B97D
E HF
E disp
Hexamine
−4.64
0.31
−4.95
8.7 Intermolecular Interactions of Hexamine
The arrangement of hexamine molecules in the crystal is shown in Fig. 8.8. There
is no short atom–atom contact between adjacent molecules in the hexamine crystal.
However, there exists significant attraction between adjacent molecules. The adjacent molecule pair indicated by arrow in Fig. 8.8 has the strongest interaction. The
intermolecular interaction energy (E B97D ) calculated for this pair is −4.64 kcal/mol
as shown in Table 8.5. The calculated E disp shows that the dispersion interactions are
again responsible for the strong attraction.
8.8 Why Strong Attraction Exists Between Molecules
Without Short Atom–Atom Contact?
Analysis of intermolecular interactions in the crystals of organic molecules shows
that the strong attraction caused by the dispersion interactions exists between adjacent molecules in crystals, even if there is no short atom–atom contact. Therefore,
the question arises why strong attraction owing to the dispersion interactions exists