50 LIGHT AND LIFE 



implications. Exploring these, a set ol empirical criteria for charac- 

 terizing n -^ TT* transitions was develoj^ed by Kasha (17) , which 

 correlated most ot the 'R'-band cases of Burawoy (4, 4a) with the 

 Mulliken-McMurry interpretation (27, 22) . These characterizing cri- 

 teria were later extended further by Sidman (34). 



The characteristics oi n —> tt* transitions may be usefully enum- 

 erated as follows: 



A. Absence in Hydrocarbon Analogs 



An obvious useful check on the identity of a band as an ji -^ tt* 

 band may sometimes be made by a comparison of the spectrum of a 

 hydrocarbon analog of a molecule: formaldehyde may be compared 

 with ethylene: pyridine or the diazines may be compared with ben- 

 zene; phthalazine and quinoxaline may be compared with naphtha- 

 lene; etc. To observe a long wavelength // -^ tt* band a necessary 

 but not sufficient condition is that an electron lone-pair should occur 

 on a heteroatom which is Tr-conjugated to other atoms. 



B. Disappearance in Acid Media 



If only one lone-pair is available in the molecule, then in the cation 

 or protonated molecule the n -^ tt* bands may shift far out of range 

 of study and effectively disappear. However, the tt ^> tt* transitions 

 also may shift considerably, obscuring the region of observation. A 

 variation on the use of acidic solvents is to study the spectra of the 

 methhalide salts (12). If two or more heteroatoms are present in the 

 molecule, effectively only one lone-pair is bound by a proton in 

 general, so that although considerable changes in the spectrum may 

 result — sometimes clearly showing the removal of an n -^ tt* band 

 — other 71 -^ TT* transitions may still be present. 



C. Bine SJiift in Polar Solvefiis 



In comparing bands of different orbital promotion type in hy- 

 droxylic solvents like ethanol and water, versus hydrocarbon solvents, 

 a relatively large blue shift (to higher frecjuency) is observed for 

 n — > TT* transitions, whereas a relatively small tt ^ tt* red-shift (to 

 lower frequencies) is observed. McConnell (21) has tabulated a large 

 collection of such shifts, demonstrating their utility in classification of 

 transitions by orbital type. Brealey and Kasha (.S) demonstrated 

 that a major cause of the ;/ -^ tt* blue shift is the formation of hydro- 

 gen-bonded solvates at the heteroatom lone-pair of the molecule. Fig. 

 12 illustrates tlie sensitivity of the phenomenon for the case of pyri- 

 dazine in hexane-ethanol solvents. 



