Intermolecular Bonds in Crystals

Up to now we have studied the properties of intermolecular bonds as if they were independent entities, that is, without taking into account the effect that nearby bonds can induce in the bond of interest. Crystals are the result of a complex network of intermolecular bonds. Thus it is appropriate to discuss here some important specific issues that derive from the fact that intermolecular bonds form complex connected networks. 

It has been demonstrated that the properties of multiple, interconnected intermolecular bonds are different from those of isolated bonds. This effect is called cooperativity and its origin is the polarization that the formation of an intermolecular bond induces in the electron density of the interacting molecules. When a molecule makes more than one bond, the second one is made with the polarized molecule. The relevance of the effect is clearly shown in Table 1.2.4 for the O-H O bond in water aggregates. But such a polarization effect is expected to be present in all kinds of intermolecular bonds, and will be proportional to the electronic polarizability of the molecule. Experimentally, the existence of polarization effects can be demonstrated by comparing the formation energy of an isolated water dimer (-5.44 kcal mol-1 [45], that is, 2.72 kcal mol-1 per water molecule) with the formation energy per water molecule in ice at 0 K (-11.3 kcal mol-1 [46]). 

One can also use the motifs as conceptual tools to understand the structure of aggregates and crystals. This is done by substituting the concept of bond by the concept of motif as the central concept in our analysis. This is the idea behind the concept of supramolecular synthons [51], defined as energetically robust motifs that serve as the basis for the crystal structure by propagation of their structure. A 

Here we have reviewed the nature of the intermolecular interactions and analyzed when they become bonds. Bonds are the dominant attractive interactions not all attractive interactions are bonds, but to be a bond they must be attractive. We have analyzed their types according to the dominant term in the intermolecular energy, and found qualitative expressions that allow us to make a qualitative estimate of their values. 

Building on these considerations, we have paid special attention to the hydrogen bond and its classification, also analyzing the special issues associated with these bonds in solids, namely, range of energy, identification, and cooperativity. 

---