16 



LIGHT AND LIFE 



7r*-onti bonding 



TT -ontibonding 



jL lone pair 

 ^ non-bonding 

 uj (pyridine only) 



UJ 



TT-bonding 



TT-bonding 



PYRIDINE (BENZENE) 



ORBITALS CONFIGURATIONS 



• • 



• • • • 



• • 



CT-orbitols 

 pyridine 34 electrons 

 benzene 36 electrons 



n-TT ir-TT 



GROUND EXCITED EXCITED 



(SINGLET) /2 SINGLETSX /4 SINGLETS\ 

 I AND AND I 



Vz TRIPLETS/ V4 TRIPLETS/ 





Fig. 2. Molecular Orbitals and Configurations in Pyridine (Benzene). The cross- 

 hatching is the part of the molecular orbital formed from atomic orbitals with 

 negative signs. The orbitals, originally doubly degenerate in benzene, do not 

 have the same energies in pyridine. The tt and v* orbitals have a nodal plane 

 coincident with the plane of the molecule. (1 he author wishes to thank Professor 

 M. Kasha for pointing out an error originally present in this figure.) 



a fully allowed electric dipole transition, such as the transition cor- 

 responding to the "D-lines" of the sodium atom (/ = .976) (3) . 

 Most of the intensity contribution in the formaldehyde n-ir* spectrum 

 comes from the unsynmietrical distortion of the electronic charge 

 clouds by unsymmetrical molecular vibrations (27a) . Interestingly 

 enough, there is about a 3 per cent contribution from a magnetic 

 dipole transition (31) . The singlet-triplet h-tt* transitions in these 

 molecules are much weaker in absorption and consequently the life- 

 time of the upper triplet state is much longer than that of the upper 

 singlet state. In the formaldehyde ti-tt* transition, which has a 

 relatively large singlet-triplet transition probability compared with 

 most molecules containing only first-row atoms, the transition is only 



