636 



ABSORPTION SPECTRA OF PIGMENTS IN VITRO 



CHAP. 21 



London's theory of molecular attraction predicts that solvents with a 

 high refractive index (i. e., strong polarizability) should exercise strong at- 

 traction on solute molecules, thereby causing considerable deformation of 

 their electronic systems and considerable shifts of their energy levels. 

 The direction of the resulting displacement of the absorption bands depends 

 on the comparative polarizability of the solute molecules in the ground 

 state and in the excited state. Figure 21.23 shows that the excitation 



NUCLEAR DISTANCE 



Fig. 21.23. Influence of solvation energy on energy of excitation (if 

 B* > S,hp > hv.oi). S + hi' = S* + hv^o\ -5. d + AS = hv,o\ - hv. 



energy of a bound (e. g., solvated) molecule, /ij^soi.j is related to that of the 

 free molecule, hv, by the equation : 



(21.4) 



hv,o\. = hp + S - S* = hv + AS 



where AS is the difference between solvation energies of the normal and the 

 excited molecule. If A*S < 0, i. e., if the excited molecule is attracted 

 by the solvent more strongly than the normal one, hv^oi. is smaller than hv 

 and the absorption band is shifted toward the red (as postulated by 

 Kundt). That A.S should be negative is plausible, since excited molecules 

 usually have a looser electronic structure and are therefore more easily 

 polarizable than the normal ones. 



Figure 21.23 shows that equation (21.4) is only correct if the equilibrium distances 

 r and ro are identical. This will not generally be the case (r * is likely to be somewhat larger 

 than ro). The e.xact equation for the "red shift" according to figure 21.23 is: 



