148 



LIGHT AND LIFE 



Fig. 3. Suggested stnuture of thionicotinamide DPNH. R. lilmse. 



effect of hydrogen-bonding on the n -^ n* transition is shown in Fig. 

 6. The excitation ol one of the 7i (non-bonding, lone pair) electrons 

 to the antibonding n orbital requires a certain amount of energy, 

 hv. If the 71 electrons participate in hydrogen-bonding, as shown by 

 the equation at the bottom of the figure, the energy (/ij/^) required 

 to promote the electron to the same excited state will be greater than 

 hp, and will give an absorption maximum at shorter wavelengths. 

 Brealey and Kasha (6) have convincingly demonstrated that in 

 hydroxylic solvents the blue shift of n -^ tt* transitions is due pri- 

 marily to hydrogen-bonding of the non-bonding electrons by the 

 hydroxylic solvent, and in these cases specific hydrogen-bonding is 

 more important than any general effect \\hich depends on the 

 dielectric constant. 



N-CH^CH^OH 



Fig. 1. Siriutmc ol llie rcdiuc'<l elhvlcne oxide analogue of DI'N. R. rihose. 



