MOLECULAR SHAPES 



147 



Fig. 2 



more intense while at the higher temperature they are nearly equal. The intense 

 band at the lower temperature corresponds to the trans conformation and the 

 other one to the skew conformation. In the spectrum of the solid the bands 

 corresponding to the skew conformation disappear completely. 



This is a type which appears in principle in many molecules. The methyl 

 group is purely a sample that does not involve any new types of atoms; if you 

 put two chlorines on ethane you get the same type of curve. The energy differ- 

 ence between minima is about 1 .3 kilocalories for dichlorethane in the gas state. 



In the case of dichlorethane there is a large solvent effect on that energy 

 term. It goes from practically zero in the liquid to 1.3 or 1.4 kilocalories in the 

 gas, whereas, for normal butane, there is practically no difference. 



Since we are not primarily interested in molecules that have only carbon 

 atoms in them, let us ask what happens when you put in other atoms. In general, 

 the same qualitative picture holds but the potential barriers are usually some- 

 what diminished. For example, in methyl alcohol where you have an oxygen 

 at one end and carbon at the other end of the single bond, the barrier is down 

 to about one kilocalories. This is quite accurately known now, but I do not re- 

 member the precise figure. With an NH2 , the value is intermediate. 



In dimethyl ether, where you put methyl groups on both bonds of oxygen, 

 the interference between the methyl groups is appreciable and the potential 

 barrier goes back up to about 3 kilocalories. Thus one has potential energy ef- 

 fects associated with single bond rotation with first row elements in the one to 

 three kilocalorie range with the atoms other than carbon giving a somewhat 

 lower figure. As you go down the periodic table the barriers tend to become 

 even smaller. 



Now let us turn to some molecules with double bonds. It does not matter 



