146 K. S. PITZER 



If we substitute, say, an additional carbon atom at one end of the molecule, 

 the other end still has a methyl group on it and has three-fold symmetry and 

 the potential barrier gets a little higher but it doesn't change much. If you ex- 

 tend the carbon chain to four and sight down the middle carbon-carbon bond 

 in normal butane, you see that the presence of the two terminal methyls intro- 

 duces new complexity to the potential curve. The three potential minima are 

 no longer identical but rather correspond to rotational isomers. When the 

 methyl at one end is in the same plane and on the opposite side as the one at 

 the other end, the conformation is called trans. Then there are two forms which 

 are optically related to one another as D and L forms with a 60 degree angle 

 between methyl groups. 



I think the organic chemists have decided that these rotational isomers are 

 going to be called conformations. There has been considerable confusion in the 

 terminology, but I am perfectly willing to go along with this. The potential 

 energy curve still retains the three minima but two of them are at a different 

 energy than the others. We know this energy difference to be a little less than 

 one kilocalorie, maybe eight-tenths of a kilocalorie. There are two potential 

 maxima about the same height as in ethane, 3 kilocalories or a little more, and 

 one which is considerably higher but I do not think we know very well how high 

 it is. This position corresponds to the methyl groups being lined up with one 

 another (cis) but I am sure there is some additional strain there and I do not 

 think any of our present methods of investigation give any good measure of it. 



Fig. 1 is a model of normal butane. There is the trans-conformation and skew 

 or gauche conformation. The eight-tenths of a kilocalorie of strain presumably 

 arises from the repulsion between the hydrogen atoms of the methyl groups. 



Fig. 2 is the Raman spectrum of normal butane at two different temperatures. 

 At the lower temperature one of the doublet of bands near 800 cm~^ is much 



Fig. 1 



