826 



LIGHT AND LIFE 



The optical rotatory strength of an electron transition is a scalar 

 product of two integrals, one of them that for the transition moment 

 integral already given and the other a corresponding integral in 

 which M^^y^^ (sum of the magnetic dipole vectors along the axes 

 x,y,z) replaces £,..„... If there is a plane of symmetry or a center of 

 inversion in the molecule, then the rotatory strength w^ill be zero. 

 If the molecule does possess optical rotatory power there is a possi- 

 bility—because two different integrals are involved, only one of 

 which is related to the intensity of the transition — that weak bands 

 may contribute more strongly to optical rotation than strong bands. 

 In point of fact, n ^ it* bands, which are inherently w^eak, usually 

 contribute strongly to optical rotation. 



H 



\ 



/ 



H 



____:p— -y 



/ I 

 / I 



^ 1 



y^ / 





a 



Fig. 2. Coordinate axes for the (a) fornialdeliydc, (b) pyridine, 

 (c) pyrimidine, and the (d) purine molecules. 



The characteristics of the n -» it* transitions are summarized by 

 Kasha as follows: (1) they are absent in hydrocarbon analogues of 

 the molecule in question, e.g., the 2900 A band of formaldehyde 

 is absent in ethylene; (2) they disappear in acid media, although 

 this observation may be obscured by the considerable shift of tt -^tt* 

 bands under the same conditions; (3) in polar solvents they show 

 a relatively large blue shift, whereas the tt -> tt* transitions shift 

 toward the red end of the spectrum; (1) the blue shift is enhanced 

 if an electron-donating group (such as — NHo, — N/?2. —OH, or — SH) 

 has by conjugating with the molecule increased the electron density 

 of its TT-orbitals; (5) they have a very low intensity of absorption 

 (molar absorption coefficients of 300 to 2000 in comparison with 

 TT -» TT* coefficients of 10,000 to 100,000) ; (6) the n -^ tt* singlet- 

 singlet transitions arise only from the absorption of light polarized out 

 of plane, whereas tt -^ tt* singlet-singlet transitions are polarized in 



