Molecular Shapes, Especially Orientation 



around Single Bonds 



K. S. Pitzer 



University of California, Berkeley, Calif. 



MY REMARKS ARE OF THE SAME SORT as a number of the other papers in 

 referring to small and model substances that are only indirectly ap- 

 plicable to the sort of problems we are really interested in. I know 

 relatively little about the big molecules. On the other hand, I suspect that it 

 was felt that a continuous dose of fundamental principles would be just a little 

 bit too much so I am scheduled today to be diluted by the more biological pa- 

 pers. 



I believe the subject of internal rotation about single bonds must be kept 

 under consideration when structures are postulated for the sort of substances 

 that we are all interested in here. I was particularly reassured in this regard by 

 the Chairman's remark yesterday that a tenth of a kilocalorie was about as large 

 a distortion in certain other types of angular orientation as has been actually 

 observed. Since the potential energy change restricting rotation or varying with 

 angle in ethane is about 30 times a tenth of a kilocalorie, this gives me a factor 

 of safety of about 30, in the significance of what I am talking about. 



Another general remark I might make is that with all of their advantages 

 these scale model sets that we buy and use are nevertheless misleading in that 

 they take no account of barriers to internal rotation about single bonds. If 

 you try to judge them by just looking at the sizes of the groups attached you 

 will almost certainly come to the wrong answer. Instead one must maintain a 

 parallel set of information about potential effects associated with torsion about 

 single bonds and apply these pieces of information as supplementary and ex- 

 traneous conditions to the angular orientation which the models may take. 



I do not blame the model makers in the slightest in this respect. If I knew 

 how to produce models that would take this into account, I can assure you I 

 would have gone ahead and done something about it. I do not have any idea 

 as to how this can be done feasibly, but one of the things to remember is that 

 the models exaggerate the rigidity of angles — for example, the tetrahedral 

 angles around the four single bonds of carbon. In other words, they are not 

 flexible enough at this point and they are much too flexible with respect to the 

 twisting orientation about single bonds. 



I am not going to try to say anything about what you might call first prin- 



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