THEORY OF CHLOROPHYLL SPECTRUM 



1795 



two states, with the angular momenta of 1 and 9, respectively. The second 

 state lies below the first one; its excitation gives rise to the red band, while 

 transition to the state with angular momentum 1 must be responsible for 

 the blue- violet band. 



Each of the two main levels is split in two, when the T)^^^ symmetry of 

 porphin is destroyed by substitutions, or partial hydrogenation of the 

 double bonds. 



Table 37C.I 

 Calculated and Observed Energy Levels of Porphin and Tetrahydroporphin 



(after Platt 1950) 



Estimated level in cm."' 

 (center of gravity, singlet + triplet) 



Polariza- 



tion Free electron model LCAO model Observed singlet 



(relative transition (band 



to Uncorr. Corr. L^ncorr. Corr. peak in ether) 



" 613 ni/ii in porphin in dioxane, cf. fig. 21.9; 621 mn in protochlorophyll in ether, 

 cf. fig. 21.8. 



^ "Soret band," 430 ni/i in porphin in dioxane, rf. fig. 21.9; same wave length in 

 protochlorophyll in ether, cf. fig. 21.8. 



" 772 lii/j. in baeteriochlorophyll in ether, 771 m^i in methanol (Weigl 1952). 



■^ 575 niju in baeteriochlorophyll in ether, 609 ni/u in methanol (\Veigl 1952): in 

 Piatt's table a value of 613 ni/u was used. 



'" A weak band at 513 niyu, noted by Weigl in impure baeteriochlorophyll prepara- 

 tions, belongs to spirilloxanthin (Holt and .Jacolis 1954-). 



■'' A "satellite" of the 360 m/j. band, cf. section 2, below. 



" "Soret band" of baeteriochlorophyll; the different value (417 m^) in Piatt's 

 original table was based on French's curve (fig. 21.6) that did not extend below 400 m/i. 



The spectrum of dihydroporphin (chlorin) which has only one short 

 two-fold axis of symmetry, cf. fig. 37C.1, has not yet been analyzed; but 

 we can expect it to resemble that of tetrahydroporphin more than that 

 of porphin, because the circular symmetry of the conjugated system is 

 destroyed by the hydrogenation of a single pyrrole ring. It can therefore 

 be expected that the absorption bands of dihydroporphin derivatives — 

 similarly to those of tetrahydroporphin derivatives — will be split into 

 components polarized in the direction of the symmetry axis of the con- 



