Intermolecular Forces

Abstract

No fundamental difference exists between cohesion forces and chemical bonds. They chiefly originate from coulombic interactions between charged particles. Repulsion forces are only important when the distance between the atoms falls-below the sum of the van der Waals radii. Van der Waals forces are cohesive attractions between molecules that are already active at long interdistances. They result from interactions between permanent, induced or temporary electric dipoles. The last are called “dispersion forces”. Specific interactions are cohesion forces that are only effective when so called specific sites of both molecules come into contact. In fact, specific interactions, as for instance hydrogen bonds, are short-range site-bounded cohesion forces that considerably weaken a given chemical bond of one of the partners. In the A-H...B hydrogen bond, the interdistance between the proton and the nearest nucleus of B is much shorter than the sum of the van der Waals radii, the distance A-H is larger than in the unperturbed molecule and the cohesion energy is intermediate between that of pure dispersion forces in the liquid state and the energy of normal chemical bonds. Hydrogen bonds appear as an intermediate step of the transfer of a proton from AH to B. In this transfer a new chemical bond BH+ is formed. H-bonds already share two characteristics of the chemical bonds: the stoichiometry and the directionality. In contrast their lifetime is very short. These characteristics are also those of the so-called n-σ EDA bonds. The energy of a hydrogen bond is governed by the difference in proton affinity of B and the anion A- A quantitative expression is proposed.