Non-additivity of intermolecular interactions

Interaction energy represents the non-additivity of the totai energy 

The total energy of interacting molecules is not an additive quantity, i.e. does not represent the sum of the energies of the isolated molecules. The reason for this non-additivity is the interaction energy. 

Let see, whether the interaction energy itself has some additive properties. First of all the interaction energy requires the declaration of which fragments of the total system we treat as (interacting) molecules (see beginning of this chapter). The only real system is the total system, not these fragments. The fragments or subsystems can be chosen in many ways (Fig. 13.10). 

If the theory is exact, the total system can be described at any such choice (cf. p. 492). 

A theory has to be invariant with respect to any choice of subsystems in the system under consideration. Such a choice (however in many cases apparently evident) represents an arbitrary operation, similar to the choice ol coordinate system. 

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Certainly, the discrete SWB model is rather crude. It does not take into account the long-range nature of Coulomb interaction, although it is of great significance for aqueous systems. Moreover, this model does not consider molecules polarizability and other effects related to the non-additivity of intermolecular interaction. Therefore it is quite surprising to observe a close correlation between the theoretical predictions based on the SWB model and contemporary quantum-chemical calculations. Some of the coincidences are listed below. 

ABSTRACT Ground state properties of several conceivable hydrogen bonded trimers composed of HCN and HP molecules have been evaluated at the SCP level. The most stable ones of these trimeric complexes have subsequently been reinvestigated with electron correlation methods (ACPP). We provide a survey of stabilization energies, dipole moments, selected harmonic vibrational frequencies and corresponding infrared intensities. We also discuss various aspects of the non-additivity of intermolecular interaction taking place in these clusters. 

The non-additivity of the intermolecular interaction results mainly from the non-additivity of the induction contribution. 

In the absence of moisture cation 25 is stable over a period of weeks, 5, however, decomposes slowly at room temperature to non-identified produets [14-16]. Both cations 5 and 25 show distinct behaviour towards nucleophilic solvents. While their Si-NMR and C-NMR resonances are not influeneed by arene solvents, the addition of the higher nucleophilic acetonitril as cosolvent leads to the formation of silylated nitrilium cations 26 and 27 (Scheme 8). They are characterized by their Si and C chemical shifts (see Figs. 6 and 7) [14, 16]. The silicon resonances are high field shifted by ca 51-56 ppm and similarely, the signals of the vinylic carbon atoms are shifted by 20 ppm to higher field in the NMR spectra of the nitrilium ions. This clearly indicates the breakdown of the intramolecular stabilization of the positively charged silicon center on the expense of intermolecular interactions between the silyl cations and acetonitril. 

With the exception of PC, amorphous, non-oriented polymers did not produce measurable amounts of broken segments when subjected to tension. As has been shown in previous paragraphs, large axial stresses capable of chain scission in amorphous polymers can only be transmitted into the chain by friction of slipping chains requiring strong intermolecular interactions. In addition, macroscopic fracture occurs before a widespread chain overloading and scission occurs, which is opposite to the behavior of semicrystalline polymers. 

In the development of the set of intermolecular potentials for the nitramine crystals Sorescu, Rice, and Thompson [112-115] have considered as the starting point the general principles of atom-atom potentials, proven to be successful in modeling a large number of organic crystals [120,123]. Particularly, it was assumed that intermolecular interactions can be separated into dispersive-repulsive interactions of van der Waals and electrostatic interactions. An additional simplification has been made by assuming that the intermolecular interactions depend only on the interatomic distances and that the same type of van der Waals potential parameters can be used for the same type of atoms, independent of their valence state. The non-electric interactions between molecules have been represented by Buckingham exp-6 functions,... 

The intermolecular interactions are usually assumed to be pair-additive functions such as the Lennard-Jones 12-6 or 9-6 potentials or the Buckingham expontential-6 type of potentials and are parameterized using methods similar to those described in the previous paragraph to reproduce the crystallographic structure and the lattice energy. For the case of liquid systems the parameterization of non-bonded interactions can be done to reproduce the liquid densities and the heats of vaporization. 

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