154 K. S. PITZER 



that in ethane. I think that it is four and a half kilocalories. In other words, it 

 is not grossly different, but it is higher than the figure I gave. 



I do not believe there is really good value for hydrazine, but I have no doubt 

 that it will turn out to be about the same. 



The principle that you and Penney brought out in your paper about unshared 

 pairs of electrons as compared to the bonding ones is, I think, quite valid, and 

 that is the reason why this barrier is a bit higher. On the other hand, your cal- 

 culations may have put it a little on the high side; I have forgotten just what 

 your numbers were. 



Chairman Pauling: I think that with respect to proteins it may be of inter- 

 est to mention a paper that I wrote two or three years ago about sulfur-sulfur 

 single bonds in which it was shown that the dihedral angle determined by a se- 

 quence of three bonds is observed to be usually around 100° and that there is a 

 theoretical reason for expecting 90° for this dihedral angle. Just as in hydrogen 

 peroxide, the unshared electron pair on each of the two oxygen atoms or sulfur 

 atoms in pi orbitals should set themselves at right angles to one another. 



This disulfide configuration is really significant to the problem of protein 

 structure. 



Professor Hirschfelder : I would like to ask Dr. Pitzer whether there are 

 any good review articles or sources of data such as he has been talking about? 



Professor Pitzer: Not as good as one would like. I summarized the hydro- 

 carbon data in the Discussion of the Faraday Society in 1951 and the hydrocar- 

 bons had been fairly well worked out at that time. 



Mizushima has a small book on internal rotation which summarizes cjuite well 

 the work on conformational energy differences, which is his principal subject, 

 but gives relatively little information about the potential barriers between 

 these potential minima. Also there is the chapter by Dauben and myself in the 

 book "Steric Effects in Organic Chemistry" edited by M. Newman. 



