C. U'/fSf'. ROlilNSON 13 



between electronic orbital and electronic spin or molecular vibra- 

 tional motion which are known to cause the appearance of certain 

 weak l)ut important molecular spectra in the visible and near ultra- 

 violet regions. Thus, one must go beyond the initial approximation 

 if interesting molecular i)henomena are to be explained. 



The solutions to the electronic wave equation give not only the 

 electronic energy levels but also, and of equal importance, the elec- 

 tronic charge distribution in the molecular state. When a molecule 

 absorbs or emits light and undergoes an electronic transition from 

 one state to another, it is important to realize that not only the 

 energy but also the electronic charge distribution in the molecule 

 changes. Since the chemical properties depend not only upon the 

 energy but also upon the charge density at various sites, essentially 

 a new chemical species is formed by the electronic excitation. With 

 rare exceptions excited states are more chemically reactive than ground 

 states of molecules. 



The equilibrium nuclear geometry, being dependent upon the 

 charge density in a rather sensitive way, is also expected to differ 

 in the various electronic states of the molecule. While the Franck- 

 Condon principle states that the most probable electronic transition 

 is one in which the nuclear positions do not change, it often happens 

 that much of this excess nuclear energy (vibrational) in the final 

 electronic state is rapidly transferred to the environment before the 

 molecule undergoes a chemical reaction or a transition to a new elec- 

 tronic state. Hence the final state of an electronic transition may be 

 a molecule having different geometry and different chemical proper- 

 ties from those of the initial state. Because of the large amount of 

 empirical data available (13, 14, 26, 29) for simple molecules in 

 addition to the existence of more refined molecular quantum mechani- 

 cal calculations (23, 34) , it is possible to predict fairly accurately the 

 nature of these changes for many simple molecules (22, 38) . 



It is convenient as part of the initial approximation, but not 

 rigorously correct, to place the molecular electrons into what are 

 known as molecular orbitals (analogous to atomic orbitals) , and to 

 consider transitions between molecular electronic states to be the 

 process of electrons "jumping" from one orbital to another. Some 

 orbitals may be highly localized in the neighborhood of one or two 

 atoms in the molecule while others may be delocalized over a large 

 number of atoms. The ground electronic orbital configuration is 

 that where all the electrons of the molecule have filled the orbitals 

 of lowest energy without violation of the Pauli exclusion principle. 



