174 RADIATION BIOLOGY 



structure. By combining these possible forms in various proportions, a 

 combination may be found which produces the lowest energy state. The 

 energy level of this state is generally less than that of any of the indi- 

 vidual canonical forms, a result known as "resonance stabilization" 

 (Whohind, 1911) and due to "exchange energy" (Hcitler, 1945). Other 

 combinations, with varying proportions of the possible canonical forms, 

 give rise to higher energy levels to whi('h the molecule may be excited on 

 the absorption of radiation. 



Resonance among possible structures in the excited state can lower the 

 energy of the excited state and thus reduce the energy difference between 

 it and the ground state, thereby increasing the wave length of the absorp- 

 tion associated with the electronic transition. The intensity of absorp- 

 tion will be greatest for transitions between states involving resonating 

 structures which have appreciable dipole moments. 



The molecular-orbital method, which has been the more succes.sful in 

 regard to the correlation of calculated with ob.served spectra, has been 

 based on the assumption of molecular orbitals for the valence electrons of 

 the atoms involved in the chemical bonds. In this development the 

 molecular orbitals are usually made up formally of linear combinations of 

 appropriately chosen atomic orbitals — LCAO method' (Mulliken and 

 Reike, 1941; Chirgwin and Coulson, 1950; Lennard-Jones, 1949; Matsen, 

 1950; Dewar, 1950; Piatt, 1950; Longuet-Higgins d al, 1950). The con- 

 struction of the molecular orbitals may involve only the atomic orbitals 

 of two atoms, as is usually the case with single bonds and with isolated 

 double bonds, in which case they are referred to as "localized orbitals." 

 or, as in the case of conjugated chains of double bonds, the molecular 

 orbitals may involve contributions from the tt orbitals of all the atoms 

 involved in the chain. In this latter case, the orbital is said to be "unlo- 

 calized," and electrons in such orbitals are considered to migrate freely 

 along the chain (Fig. 5-7). 



Varying the combinations of atomic orbitals will produce molecular 

 orbitals of various energy levels. The electrons available for bonding 

 (all in the outer atomic shell) are then disposed in successively higher 

 energy levels, two to a molecular orbital with spins opposed, until all 

 electrons are accounted for. Absorption of radiation may then cause 

 an excitation of an electron from the highest filled molecular orbital to the 

 lowest unfilled orbital. The energy difference between orbital levels 



' Refinements of this procedure involve the ii.se of "antisyiiuiietrized inoiccuhir 

 orbitals" to reduee the apparent contribution of configurations including multiply 

 ionized atom.s ((loepixTt-Mayer and Skiar, 1938; Roothaan, 1951) and recognition of 

 configurational interaction (Jacobs, 1919; Craig, 1950). .\notiier theoretical ajiproach 

 which has had considerable success is the "free-electron model" in which all the ir 

 electrons are considered to be able to migrate freely throughout the molecule, along the 

 atomic bonds, in a jiotential field that is constant, or, in some instances, sinusoidally 

 varying (Bayliss, 1918; Piatt, 1949; Kulm, 1949; Simpson, 1949). 



