Intermolecular interaction cooperativity origins

It is concluded that the cooperative effect observed is of long-range nature and therefore of elastic rather than of electronic origin. Recently, the additional suggestion has been made [138] that, due to intermolecular interactions in the crystal environment of [Fe(ptz)g](BF4)2, domains of iron(II) complexes interconvert together. The observed kinetics would then correspond to a first- or higher-order phase transition rather than to the kinetics which are characteristic for the conversion of isolated molecules. 

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]). 

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