19: Noncovalent Interactions in Crystal Structures: Quantifying Cooperativity in Hydrogen and Halogen Bonds

The cooperativity effects for hydrogen and halogen bonded systems are analyzed; examples of clusters where this phenomenon occurs are presented – the systems analyzed theoretically are compared with examples of the corresponding arrangements occurring in crystal structures. This chapter relates to early studies on this phenomenon and it presents different meanings of the term cooperativity. Distinct indices and measures of the cooperativity are presented in order to quantify this phenomenon.

Different classifications of the cooperativity are presented; for example, σ- cooperativity and π-cooperativity are described. For the latter case one can distinguish between intermolecular and intramolecular π-cooperativity. It is justified that for numerous kinds of interactions, the hydrogen and halogen bonds, as well as other interactions classified as σ-hole bonds, the same indices describing the cooperativity may be applied. It is shown that in crystal structures where cooperativity effects are mostly related to the symmetry relations, the situation is very complex since various phenomena related to the term cooperativity may act simultaneously.