BENT LEY GLASS 821 



orbitals. Corresponding to the s and p orbitals of atoms are the so- 

 called cr and tt orbitals of moleciUes, with the exception that these 

 molecular electrons extend over at least two atoms. The o" orbitals 

 possess a cylindrical symmetry, with high electron density along the 

 line joining the atomic nuclei, tt orbitals have a symmetry such that 

 the phase (or sign) of the electron is opposite on opposite sides of 

 the line joining the atomic nuclei. In the TT-bonding orbital the 

 electron charge density is concentrated between the nuclei. In the 

 TT-antibonding, or tt*, orbital, the electron charge density forms an 

 additional node "between the nuclei in a plane perpendicular to the 

 internuclear axis," so that electrostatic repulsion weakens the bond 

 (Fig. 1) . [There are two types of both cr and n orbitals, but we 

 will be concerned chiefly with the latter.] 



o" "n- Ti(2p„) TT* cr 



Fig. 1. Classification of molecular orbitals (schematic contour 

 diagrams, formaldehyde skeleton). 



The transitions most significant in the spectroscopic study of the 

 excitation of molecules by ultraviolet or visible light are the n ^ tt* 

 (non-bonding to Tr-antibonding) , either singlet-singlet or singlet- 

 triplet, and the tt —^ tt* (TT-bonding to 7r-antibonding) . The former 

 are important in azine or carbonyl groups of molecules. The singlet- 

 triplet n -^ TT* transition is characteristically very much weaker in 

 absorption than a singlet-singlet 7i -^ tt*, a relation indicating a 

 much longer lifetime of the triplet excited state than of the com- 

 parable excited singlet state, tt -^ tt* transitions occur typically in 

 (conjugated) hydrocarbons such as ethylene, benzene, and pyridine. 

 As the molecule increases in size the singlet-singlet transition, which 

 occurs at 260 m/^ in benzene and pyridine, shifts to longer wave- 

 lengths (e.g., in carotene) . 



The geometry of a molecule ordinarily changes when it becomes ex- 

 cited since the configuration of the bonding electrons is changed. 

 Thus, the C-C bond in benzene lengthens with the promotion of an 



