THE VISUAL PIGMENTS 



multiplicity of isomers. Free rotation about the single bonds is, of 

 course, still possible and formula for the hexatrienes such as 



can be regarded as equivalent, for the lability of transition from one 

 form to another precludes the possibility of obtaining a separate 

 specimen of any one. By contrast, since rotation about a double 

 bond is not allowed, several non-labile, and hence separable cis-trans 

 isomers, are possible. Conventional formula for these — eight in the 

 present instance — are given below. 



Theoretically possible cis-trans isomers of the hexatrienes 

 R— (CH=CH)3— R' 



all cis 



Theoretically, a polyene of the formula R— (CH=CH)„— R', 

 containing n double bonds, can exist in 2" different cis-trans confor- 

 mations. When the molecule is symmetrical, i.e. when R is the same 

 as R', the number of isomers is less, namely 2"~^ + 2^^ where 

 p = nil if n is even and {n + l)/2 if n is odd. 



STERIC HINDRANCE 



However, some theoretically possible forms involve the folding- 

 back of the molecule upon itself to such an extent as to cause mutual 

 obstruction by different parts. Obstruction is more likely in highly 



