38 



LIGHT AND LIFE 



were of n —> n* type. This suggestion was quickly confirmed in 

 subsequent studies by Halverson and Hirt (12) on the diazines and by 

 Sponer and Rush (36) on the vapor spectra of picolines, and by 

 Stephenson (38) on solution spectra of pyridine and various deriva- 

 tives. In the last few years many additional researches on n — ^ tt* 

 transitions in N-heterocyclics have been published. Sidman (34) has 

 recently reviewed the general topic of 7i -^ tt* transitions, stressing 

 vibrational and electronic symmetry assignments. Goodman and 

 Harrell (9a) have recently given a useful semi-empirical calctilation 

 of energy levels for this case. 



Before a summary is made of the principal characteristics of n — > n* 

 transitions in N-heterocyclics, some of the features of the molecular 

 orbitals of these molecules will be analyzed. 



The 77-molecular orbitals and the (mainly) non-bonding lone-pair 

 or n-orbital of pyridine are shown schematically in Fig. 4. There are 

 three bonding 77-orbitals, and three antibonding tt or 77'*-orbitals. 

 The orbitals are arranged in order of decreasing binding energy, left 

 to right. The lowest orbital has only the characteristic node in the 

 plane of the molecule (solid contovu- outside means (-|-) part of 

 the orbital wavefiniction above; dashed contour inside means ( — ) 

 part of the orbital wavefunction below the plane of the molecule) . In 

 typical quantum mechanical fashion, with increasing energy of the 

 TT-orbitals, there is an increase in the number of nodes (perpendicular 

 to the molecular plane) . The orbitals 7r2 and 773 have the same energy 



Sn/ 



^{sp2,N) 



lig. 1. Schematic contour diagiams of the Tr-orbitals and the 

 /(-orbital of the pyritiinc molecule. 



