SOLVENT EFFECT 643 



tion on a wave length scale. In the comparison of the shifts of bands in 

 different parts of the spectrum, on the other hand, the use of wave lengths 

 may be quite misleading; for example, a shift by 10 m/x at 440 m^ is 

 equivalent, on the energy scale, to a shift by 22.5 m^ at 660 mju. 



The effect of solvents on absorption maxima of chlorophyll other than 

 the main red peak has not been studied systematically, but it is known that 

 in general all of them experience shifts toward longer waves with increasing 

 refractivity of the solvent (see e. g., Egle 1939). Exact measurements of 

 this shift may prove useful in the interpretation of the spectrum, since ab- 

 sorption bands that lead to the same electronic upper state can be expected, 

 according to page 636, to show the same shift. Krasnovsky et al. (1949) 

 found bands II and III to be shifted, in pyridine, by 30-35 m^x (to 643 

 and 622 m^), while other bands were shifted by 8-15 m/x only. 



The solvent effect is not restricted to hand shifts, but also involves 

 changes in the width and shape of the bands and (perhaps as a conse- 

 quence of these changes) alterations in the absolute and relative intensities 

 of the band maxima. In this case, too, bands leading to the same excited 

 electronic level must show a similarity of behavior (cf. Pruckner 1940). 

 In the case of chlorophyll, the ratio between the intensities of the blue- 

 violet and the red peak is quite different in different solvents. While 

 Table 21. IB showed a value of 1.3 for ether solutions of chlorophyll a, the 

 ratio drops to 1 in methanol (Mackinney 1940, Albers 1941, Harris and 

 Zscheile 1943) and rises to approximately 1.5 in dioxane (Harris and 

 Zscheile 1943) and perhaps also in benzene (according to Hausser's meas- 

 urements, cf. Fischer and Stern 1940; not confirmed by Table 21. VIII). 



Figure 21.26 shows the absorption curves of chlorophylls a and h in a 

 variety of solvents, according to Harris and Zscheile. 



Table 21. VIII 



Relative Intensity of Chlorophyll Absorption Peaks in Different Solvents 



(after Harris and Zscheile 1943) 



Blue max. /red max. 

 Solvent Chi. a Chi. b 



Methanol 1.00 2.85 



2-Ethvl-l-hexanol 1 .00 — 



2-Methyl-l-propanol 1 . 02 — 



1-Butanol 1 . 03 — 



Methyl oleate 1.25 — 



Acetone 1.26 2.95 



Olive oil 1.29 — 



Isopropyl ether 1 29 



Carbon tetrachloride 1 . 32 2 . 54 



Benzene 1 . 33 2 . 45 



Ethyl ether 1.33 2.98 



Cyclohexane 1 . 36 — 



1-n-Dioxane 1 .46 — 



