88 JOSEPH O. HIRSCHFELDER 



In order to determine the dispersion energy, the first step is to ascertain the 

 principal charge transfer transitions which take place in the molecules. The 

 energy associated with these transitions can be experimentally determined 

 from a study of the index of refraction of light passing through the pure sub- 

 stance as a function of frequency. Crude quantum mechanical arguments can 

 then establish the nature of the transitions and a rough description of the wave- 

 functions in the excited and in the ground states. The free-electron model has 

 been very successful for locating the monopoles in conjugate double bond 

 molecules. Pauling's theory of color should be useful for other kinds of biologi- 

 cal molecules. The dispersion energy between large molecules leads to highly 

 directional forces depending on the location of the monopoles. Thus conjugate 

 double bond molecules are found to have a maximum energy of interaction 

 when their axes are neither parallel nor perpendicular. 



Finally, we come to the much disputed question as to whether resonance 

 forces play an important role in biological specificities. Let us suppose that 

 molecules A and B are identical but molecule A is in an excited state and mole- 

 cule B is in the ground state. Then, if the optical selection rules do not forbid, 

 molecule A can emit a virtual photon which is then captured by B. The two 

 molecules can interact at surprisingly long distances. Concurrently the two 

 molecules either are attracted or else repelled by forces which are much stronger 

 than would occur if the two molecules were not identical. Some physicists and 

 biologists have jumped to the conclusion that such forces must be important in 

 explaining specificity. However, a little further consideration would seem to 

 make this explanation seem unlikely. As the two molecules approach each 

 other the usual electrostatic and dispersion forces become much stronger than 

 the resonance forces. 



Thus we have outlined the types of problems which will be encountered in 

 a study of the intermolecular forces between biologically important molecules 

 (Haugh and Hirschfelder, 1955; Hirschfelder, Curtiss and Bird, 1954). The 

 problems can be clearly stated but a great deal of work remains to work out 

 the detailed applications to specific examples in order to establish our under- 

 standing on a quantitative basis. 



References 



Haugh, E. F. and J. 0. Hirschfelder. 1955. J. Chem. Phys. 23: 1778. 



Hirschfelder, J. O., C. F. Curtiss and R. B. Bird. 1954. Molecular Theory of Gases 



and Liquids. John Wiley & Sons, Inc. Chapt. 13. 

 London, F. 1942. J. Chem. Phys. 46: 305. 



Dr. Sidney Bernhard (National Medical Research Institute) : In light of 

 the discussion of special roles of resonance, it occurs to me that the vaporiza- 

 tion of cyclohexane is not very different from the vaporization of benzene and 



