INTERMOLECULAR FORCES 89 



an additional fact that occurs to me, particularly from work of the type of 

 Dr. Pressman's, is that if one compares the bindings of the molecules which 

 have the benzene ring stuck on one end and then one also measures the binding 

 of a molecule similar in other respects but lacking the benzene rings at one end, 

 then the difference in free energy of bindings turns out to be of the order of the 

 enthalpy of vaporization of benzene and, hence, cyclohexane. In light of such 

 argument how much difference is there in our biological system from the or- 

 dinary liquids and the gas phase, and how much does this resonance phenomena 

 really contribute? 



Professor Hirschfelder : I wonder whether the fact that you speak of is 

 due to the cancellation between half of these resonant states being attractive 

 and half being repulsive. This would show up if one studied the viscosity and 

 found that the viscosity differed greatly in the two cases. 



Chairman Pauling: This discussion is about the question of how strong the 

 interaction due to dispersion forces is between conjugated systems. I think, 

 myself, that the people who have made calculations about this probably have 

 been led to overemphasize this importance in some cases. 



As has just been pointed out, the van der Waals forces, the polarizability 

 and van der Waals interaction of benzene do not really seem to be abnormally 

 great as compared with cyclohexane. But on the other hand, we know that 

 dyes which have long conjugated systems, such as carotenoids, do interact 

 more strongly with one another than ordinary colorless, unconjugated mole- 

 cules do. 



Dr. Pressman: There is evidence that van der Waals forces and polarizabil- 

 ity are important in the cases of interaction of haptens with specific antibodies, 

 for example, in those cases where a naphthalene ring is accommodated, such 

 as in the case of /3-naphthoate reacting with anti-/>ara-azobenzoate antibodies, 

 or the alpha naphthoate ion with anti-or///o-azobenzoate antibodies. In these 

 cases, there actually does seem to be an enhanced attraction of the naphthalene 

 over what one would expect for a non-aromatic grouping. We have attributed 

 that to the polarizability. 



Professor Pitzer: I would, just as an aside, like to associate myself with 

 those who would regard the conjugation as real but not so important. I think 

 that even when you go to decalins and naphthalene, say, the difference is not 

 very large; it is measurable and significant, but not a major effect. Indeed, we 

 all write on paper with graphite and here you have something of microscopic 

 dimensions conjugated and, even so, the forces between the planes are not so 

 great but that you can tear the crystal apart. Of course, you can tear parafi&n 

 crystals apart too, pretty easily, but I do not think this is a major thing. 



I do think there is a very interesting question here and I would like enlighten- 

 ment as much as trying to contribute anything. That is, to what extent do 

 electronically excited states appear in what might be described as the general 

 run of biological interacting systems? There is no doubt that in photosynthesis 



