MICHAEL KASHA 



51 



Z7 29 31 33 35 



\A/AVENUMBtRS x 10"^ 



Fig. 12. Blue shift of fust ;; -^ w* siiiglet-singlet absorption band of pyridazine 

 after Brealey and Kasha (3). Solutions in hexane, with added ethanol, from curve 

 1, percent ethanol to cur\e (>. 3.2 percent ethanol by volume. 



D. Conjugative-Siihstituent Blue-Shift 



As mentioned earlier, Burawoy (4, 4a) had used the effect of elec- 

 tron-donating substituents such as — OH, — SH, — NHo, — NiRo. to 

 classify electronic band types in molecides. A theoretical treatment 

 of these effects was supplied by Nagakura (29), for the case of re- 

 placement of a methyl group of acetone by the substituents — CI, 

 — OC2H5, — N(CH:,)., using an LCAO-Molecular Orbital calcula- 

 tion. In the n — > tt* transition in carbonyl compounds, electronic 

 charge is, in effect, transferred from the oxygen to the carbon of 

 the carbonyl group. Nagakura (29) showed that substituent groups 

 which donate charge to the carbonyl carbon raise the energy of the 

 antibonding TT-orbital, causing a blue-shift of the n — > tt* transition 

 in carbonyl compoimds, proportional to the electron-donating ability 

 of the substituent group. Goodman and Shull (10a) made a general 

 prediction of the blue-shift effect of (purely conjugative) electron- 

 donating substituents on n -» tt* transitions. However, their dis- 

 cussion considers the effect on corresponding state energies, and oc- 

 casionally predicts a zero-shift effect (cf. the 5-methylpyrimidine case, 

 Goodman and Harrell, 9a) . 



