INTERMOLECULAR FORCES AND INTERACTION ENERGY 



233 



Equilibrium Distances between Molecules 



It is at present impossible to determine the equilibrium distances be- 

 tween interacting molecules in solution. However, sufficient data from 

 diffraction studies in solids are available to estimate distances of separation 

 of molecules held together by only van der Waals' forces. One may de- 

 fine van der Waals' forces as including all the forces responsible for the 

 constant a in van der Waals' equation. These would comprise dipole-dipole, 

 dipole-induced dipole, and dispersion interactions; when the molecules do 

 not possess permanent dipoles, only dispersion forces are involved. Van 

 der Waals' radii of atoms or molecules are half the distance separating two 

 identical atoms or molecules at equilibrium in the solid state. Table 6-8 

 presents both van der Waals' and covalent radii for important atoms. Since 



Table 6-8 



COVALEXT AXD VAN DER WaALS' RaDII OF AtOMS " 



" The van der Waals' radius for the carbon atom has been estimated on the basis 

 that van der Waals' radii are approximately 0.8 A greater than the single bond co- 

 valent radii for most atoms. The values for the van der Waals' radii are to be taken 

 as approximate and appreciable variation may occur in different combinations of 

 the atoms. 



van der Waals' radii have been used to determine the interaction distance of 

 molecules forming complexes in solution, it is necessary to enquire into 

 the reliability of such a procedure. In the first place, the reasonable as- 

 sumption has been made that van der Waals' radii are additive when dif- 

 ferent atoms interact; however, the accuracy of this has never been experi- 

 mentally assessed. Secondly, there is little assurance that the internuclear 

 distances occurring when two molecules interact in solution are the 

 same as in a compact lattice structure; it might be anticipated that the 



