148 K. S. PITZER 



very much whether we have oxygen or another CH2 group. We take one of our 

 end-on diagrams in which we now look axiaUy along the single bond. On the 

 far end we have the double bond coming down to the oxygen or CH2 and the 

 single one up to a hydrogen. Then, on the near end we still have our triangular 

 methyl group. There is quite convincing evidence that the stable orientation is 

 staggered with respect to the single bond at the far end and opposed with re- 

 spect to the double bond. 



The values of the potential barrier, that is, the maximum range of potential, 

 are about 2 kilocalories for the propylene with the CH2 and about 1 for acetal- 

 dehyde with the oxygen. 



If other atoms or radicals are substituted for the hydrogen and the oxygen 

 or CHo the barrier will still be about the same unless the new atoms are very 

 large or have other special properties. 



On the other hand, in nitromethane or toluene the situation is a bit different. 

 We still have the methyl group, but now we have something that is not only flat 

 but precisely symmetrical at the other end of the bond. Both of these cases are 

 the same by symmetry and the potential barrier is known to be small. As far 

 as the older methods were concerned, it was within experimental error zero 

 but this only meant less than a few tenths of a kilocalorie. 



My colleague, Professor Gwinn, in Berkeley recently studied nitromethane 

 with the micro-wave method. His answer for this molecule was 6 small calories 

 and not just 6, but 6.00 small calories plus or minus a few hundredths. This is 

 smaller than the level of significance of a tenth of a kilocalorie which we were 

 mentioning by at least one order of magnitude. Thus, for our purposes, we can 

 say that in these cases the potential barrier is simply zero. 



Another case that gives truly free rotation, although probably of trivial in- 

 terest, is the acetylene case. Dimethyl acetylene, or its derivatives, where the 

 rotation can occur across a three-bond length linkage in a linear structure has 

 zero potential barrier. 



I might remark at this point that in my experience I have never found a case 

 where the solid failed to accommodate the low energy conformation of a mole- 

 cule. It is a wonderfully complex subject but apparently there is always some 

 crystal structure, some arrangement which satisfactorily accommodates the 

 low energy form of the molecule without enough difference in crystal energy to 



