14 



LIGHT AND LIFE 



Excited configurations arise when one or more electrons are pro- 

 moted to orbitals of higher energy. A single electronic configuration 

 often results in more than one electronic state. For example, if the 

 configuration contains, besides the filled orbitals, two half-filled non- 

 degenerate orbitals (one electron each) , singlet and triplet states 

 result depending upon whether the electron spins are respectively 

 anti-parallel or parallel. Certain configurations in highly symmetrical 

 molecules give rise to more than two states of various multiplicities. 

 The various states derived from a single configuration, primarily be- 

 cause of different electron-electron repulsion contributions, may have 

 energies differing by as much as two electron volts (1 e.v. =: 23 

 kcal/mole) . These concepts are partially illustrated in Fig. 1, where 

 the formaldehyde low-lying orbitals and configurations are schemati- 

 cally represented. 



Non-bondins orbitals. In molecules there are often electrons, 



FORMALDEHYDE 



ORBITALS 



CONFIGURATIONS 



cr -antibonding 



Hg-CEKX) 



7r*-antibonding 



^ non-bonding 



cc lone pair 

 u 



LU 



TT-bonding 



H 



H 



>^ 



M,-^ 



• • 



a-bonding 



H^-mm 



Inner Orbitals 

 (10 electrons) 



H 



— z 



n-TT" 

 GROUND EXCITED 

 (SINGLET) /SINGLETN 

 AND 1 



Vtriplet/ 



lig. I. Molecular Orbitals and Configurations in Formaldehyde. 1 he cross-hatch- 

 ing is the ]);ni ol ihc molecular orbital formed from atomic orbitals with nega- 

 tive signs. 



