IXTERMOLECULAR FORCES AND INTERACTION ENERGY 211 



the quantity V ft • // might be said to be the effective moment and replaces 

 //(, in the older formulations. The value of g is generally unknown and may be 

 less or greater than unity. For water it is around 2.65 (Oster and Kirkwood, 

 1943) but the high value is in part due to hydrogen bonding. The effective 

 dipole moment of a molecule in aqueous solution may thus be different 

 from the usually given value. Such deviation may be of some significance 

 in the interaction of small molecules with proteins and important in future 

 refinements of the treatment of intermolecular forces. 



Dipole-Dipole Interactions 



The potential energy of two dipoles depends not only on the distance 

 between them but on their relative orientations. The interactions of both 

 fixed dipoles and freely rotating dipoles must be considered. If two dipoles 

 are fixed relative to one another as in Fig. 6-4(8) and the distance, d, 



(a) 



+ - 



- i - + 



(b) 



d3 



Fig. 6-4. Interaction of two dipoles. 



