MICHAEL KASHA 



33 



sorption curves for /3-cyclocitral in hexane and ethanol, showing a 

 blue-shift of the weak long wavelength band (n -^ tt*) and a red- 

 shift of the strong second absorption band (tt -^ tt*) , both being 

 singlet-singlet transitions. 



W4VENUMBERS , CM 

 50,000 40,000 30,000 



3000 

 WAVELENGTH, 4 



Fig. 1. Absorption spectra of /3-cyclocitral in (a) hexane and 

 (b) ethanol, after Burawoy (4, 4a). 



Burawoy did not seem to consider quinones, which also could be 

 included among compounds showing 'R'-bands. No N-heterocycHc 

 molecules were studied by him. On the other hand, he included the 

 visible absorption band (a doublet -^ doublet transition) of triaryl- 

 methyl radical (discussion given below) , as well as what may be a 

 singlet -^ triplet (vr-electron) absorption band in trimethylethylene 

 (A 2340, € = 0.06) , among the 'R' bands. 



It is instructive to classify the types of "lone-pair" orbitals in 

 polyatomic molecules, and to examine the origin of Burawoy's error. 

 This will be done in the subsequent section. 



B. Types of "Lone Pair" Orbitals in Molecules 



In writing classical structures of hetero-molecules the "lone-pair" 

 {double t-libre) electrons are frequently omitted. Nevertheless, it is 

 these electrons which usually give rise to the lowest singlet excited 

 states of the molecules considered. Moreover, these excited states 

 frequently determine the principal optical and photochemical proper- 

 ties of most hetero-molecules. 



