C. U'lLSE ROBINSON 15 



termed non-bonding electrons (.H3) , which take a very small part in 

 the actual bonding together ol the nnclei. These electrons are in or- 

 bitals wliich concentrate little electron density between the nuclei. 

 Examples are the lone pair electrons on a nitrogen atom in the or- 

 ganic azines or those on oxygen in a carbonyl group. Such electrons 

 are characterized by the relatively small amount of ionization energy 

 necessary to remove them completely from the molecular system, and 

 by the insensitivity to the removal of such an electron of the molecular 

 geometry and vibrational constants of the resulting positive ion. Be- 

 cause of their loose coupling to the rest of the molecule, non-bonding 

 electrons, when present, are very often involved in the lowest energy 

 electronic transition. 



Bonding and antibondiug ir-orbitals. Molecular electrons which 

 are similar to atomic p-orbitals except that they extend over at least 

 two atoms are called Tr-electrons. The electron density has a node 

 along the line joining the nuclei over which the Tr-orbital extends. 

 A TT-orbital may be approximately constructed by adding together or 

 by substracting atomic p-orbitals on the atoms involved in the 7r-bond. 

 In the first case electron charge density is "piled up" between the 

 nuclei and the orbital is bonding, while in the second case a node 

 of electronic charge is formed between the nuclei in a plane perpendi- 

 cular to the internuclear axis.^ This latter type of orbital is termed 

 antibondiug, since the resulting electrostatic repulsion causes the bond 

 to become weaker. An electron added to such an orbital results in an 

 increased bond length and a much lower vibrational frequency as- 

 sociated with that part of the molecule. 



77-77* Transitions. Transitions, termed n-ir*, caused by electrons 

 promoted from a non-bonding orbital (77) to a TT-antibonding orbital 

 (tt*) , are important in molecules containing either the azine or car- 

 bonyl groups (33) . These transitions, which occur in the near ultra- 

 violet or visible portions of the spectrum, are very often the lowest 

 energy transitions in such molecules. They are relatively weak as 

 molecular transitions go. The molecular orbitals important for the 

 discussion of the lowest lying 77-77* electronic transitions in formalde- 

 hyde were given previously in Fig. 1. Those for pyridine are shown 

 in Fig. 2. The 77-77* singlet-singlet transition in formaldehyde is 

 2.4 X 10"'' (oscillator strength, / = 2.4 X 10-^) ^ as intense (9) as 



*In the case of conjugated ring hydrocarbons this axis need not be a chemical 

 bond axis, but may cross the ring. See, for example. Fig. 2 and ref. 17. 



= / (dimensionless) = 4.32 X 10-^ e dv, where the molar extinction coefficient 

 e = \/cd logio (lOO/T); c in moles/liter, d in cm, v in cm-\ T — % transmis- 

 sion. The integration is over the absorption band. 



