56 RELATIONSHIPS BETWEEN COLOUR AND CONSTITUTION VII 



7. Carbonyl groups (ketone, aldehyde or carboxyl groups) conjugated with 

 the system of conjugated double bonds have a pronounced effect on the ab- 

 sorption spectrum, and produce bathochromic displacements, the magnitude 

 of which varies from case to case. If the introduction of the carbonyl group, 

 involves the opening of a ring, the effects of the two changes are, of course 

 superimposed. Introduction of a second conjugated carbonyl group has a smaller 

 effect on the position of the absorption maxima than the introduction of the 

 first carbonyl group. 



Examples: 



/3-Apo-2-carotenal. The system of 9 conjugated ethylenic bonds should exhibit an 

 absorption maximum at about 480-485 m^u. The observed longest wavelength 

 absorption maximum lies at 525 m/i and the difference of 40-45 m// is to be 

 ascribed to the carbonyl group. 



Capsanthin. The system of 10 conjugated ethylenic bonds should exhibit an ab- 

 sorption maximum at 500-505 m/^. The longest wavelength absorption 

 maximum actually observed lies at 542 m/i and the difference is to be ascribed 

 to the carbonyl group. 



8. Cis-trans configuration also has a definite, though small, influence on 

 the position of the absorption maxima. Thus, the longest wavelength maximum 

 of a compound containing one c/s-ethylenic bond, is displaced by ca. 3-4 m/z 

 towards shorter wavelengths as compared with the isomer with complete 

 ^mws-configuration. 



Examples: 



Stable (/rans-) bixin longest wavelength maximum in CSg at 526.5 m^u. 



Labile (cf5-) bixin longest wavelength maximum in CSg at 523.5 m^a. 



Stable {trans-) crocetin .... longest wavelength maximum in CSj at 463 vci/x. 



Labile {cis-) crocetin longest wavelength maximum in CSg at 458 m^. 



After dealing with the effect of different atomic groups on the position of 

 the absorption maxima, the influence of the solvent must be briefly discussed. 

 Experiments have shown that the wavelength locations and fine structure of 

 the absorption maxima are considerably dependent on the solvent. The solvent 

 also has some influence on the absorption coefficients. The interaction between 

 polar solvent such as alcohol and carotenoids with carboxyl groups, (e.g. 

 capsanthin) is further described below. 



The table below shows that the absorption maxima of most carotenoids 

 are displaced by about 30-40 m^ towards shorter wavelengths in hexane and 

 alcohol as compared with carbon disulphide. In chloroform, the hypsochromic 

 displacement amounts to about 24 m//. The differences in the positions of the 

 maxima in hexane and carbon disulphide increase with the wavelengths at 

 which the carotenoids absorb. 



In carotenoids containing carbonyl groups (e.g. capsanthin, /5-apo-2- 

 References p. 59. 



