MOLECULAR SHAPES 153 



respect to the single bond between double bonds as well as the double bonds 

 themselves. Also the trans orientation with respect to the largest, most bulky 

 groups is usually more stable than the cis. 



Chairman Pauling: I think that I agree with Professor Pitzer that this as- 

 pect of molecular structure, the restriction of rotation around single bonds, is 

 the one that is most overlooked, especially by people who make use of molecu- 

 lar models in the correlation of experimental results. I can emphasize another 

 point that he made that is an interesting commentary on the role of quantum 

 mechanical calculations in this field of chemistry. Eighteen years after Pitzer 

 and Kemp discovered this phenomenon of restricted rotation around single 

 bonds there still is no valid theory, at any rate, there is no theory that Professor 

 Pitzer and I are willing to stand up for. Almost all of our knowledge about mole- 

 cules is chemical in origin. It has been obtained empirically by chemical experi- 

 ment and by generalizations, that the theories that exist are essentially induc- 

 tive theories that have been obtained by chemists by analysis of chemical 

 information. 



Dr. Sidney Bernhard: I would like to ask Dr. Pitzer whether, with refer- 

 ence to enzymes and specific sites in general, when a molecule is twisted in order 

 to fit a specific site, one empirically makes some guess as to how much these dis- 

 tortion effects are going to work against you or for you? 



Professor Pitzer: Yes, I think that in most cases we have the component 

 information such that if you distort a molecule into other than its natural lowest 

 energy conformation, you can calculate within possibly about 20 per cent the 

 strain energy that would be required. 



I have been amazed at the way these numbers can be transferred. The 

 number that started out in normal butane is good to within 20 per cent in cal- 

 culating the energy difference between cis and trans decahydronaphthalene. 

 Thus as long as you have the energy values for good model substances I am 

 sure they can be applied to other substances with quite high accuracy. 



Dr. G. B. M. Sutherland (University of Michigan): I made an excursion 

 into this field a long time ago, looking at the structure of hydrogen peroxide 

 and hydrazine where it seemed that there was, from the spectroscopic data, 

 very restricted rotation; in fact, only one form. I cannot remember the height 

 of the barrier that we calculated. It seems to me, as a generalization, that all 

 of the first row ones would be around about one to three kilocalories, but this, 

 perhaps, was not true in the case of hydrogen peroxide and hydrazine. 



Professor Pitzer: I am glad you brought that up. I will have to stretch 

 my range a little. Dr. Giguere in Quebec has been working on hydrogen perox- 

 ide by the same general methods and he finds a barrier somewhat higher than 



