ISOMERISM AND THE VISUAL PIGMENTS 



viz. : (1) in the visible (400-500 m^), where the most intense absorp- 

 tion band is located, (2) in the near ultraviolet (c. 340 m//), the region 

 of 'cis peaks' and (3) in the far ultraviolet (c. 280 m/i). 



These absorption characteristics can be interpreted in terms of 

 molecular structure (lewis and calvin, 1939; pauling, 1939, 

 ZECHMEiSTER, 1944). The chain of alternate single and double 

 valency bonds, 



^^^^^i^^^^.i)^^^^^'^^^^^^^^^ (1) 



can resonate between a large number of ionic forms such as : 



(2) 



The conventional formula (1) represents the condition of a normal, 

 unexcited molecule : the formulae (2), a few of the possible resonance 

 structures for an excited one. Because of the multiplicity of ionic 

 forms, however, no single form is adequate to describe an excited 

 molecule. As described in Chapter 4 (p. 101), the electrons which are 

 *left over,' after allocating a single covalent bond throughout the 

 conjugated chain, mutually interact to form so-called 7r-orbitals. 



The oscillating electric vector of Hght causes the Ti-electrons to 

 congregate backwards and forwards along the conjugated chain, first 

 at one end and then at the other. When the light is of the proper 

 frequency to correspond with the rhythm of these oscillations it is 

 strongly absorbed (the fundamental band in the visible). The inten- 

 sity of the band is proportional to the square of the regularly alter- 

 nating dipole moment of the molecule, and hence to the square of the 

 length of the conjugated chain. 



The first overtone band in the near ultraviolet (m-peak at c. 

 340 mfx) arises from an oscillation of 77-electrons towards the middle 

 of the conjugated chain and from the middle towards the two ends. 

 The dipoles of these two half oscillations are opposed and hence the 

 absorption band is of very low intensity in 'straight' or diiX-trans 

 isomers. In a cis isomer, however, the molecule is bent and hence 



133 



