G. WfLSE ROlilNSON 17 



1/200 as intense (8) as the singlet-singlet, which itselt is of relatively 

 low intensity. This lact illustrates the extreme weakness ot such 

 interconibination bands. The litetinic ol the excited triplet state of 

 formaldehyde is about .002 seconds. 



The n-TT* singlet-singlet transition probability in pyridine has an 

 oscillator strength of about .008 (36) , larger by a factor of about 

 ten than the u-n* transition in formaldehyde. Unlike formaldehyde, 

 the major part of this intensity seems not to be caused by antisym- 

 metric vibrations.*^ The two singlet-singlet n-ir* transitions in pyridine 

 should be particularly noted (Fig. 2) . Electric dipole transitions 

 between mutually perpendicular 2p-orbitals on the same atom can 

 be shown to have zero transition probability while 2s-2p transitions 

 are extremely intense. Any .y-character in the non-bonding orbital 

 will therefore show up as an increased intensity in the n-ir* transition. 

 In the molecular orbital approximation only combinations of atomic 

 orbitals having the proper local molecular symmetry properties can 

 be mixed to form the molecular orbitals between which electrons 

 undergo transitions. Because of this symmetry restriction, the most 

 loosely bound non-bonding orbital in aldehydes and ketones, being 

 derived from a p-orbital on oxygen, antisymmetric with respect to 

 the xz plane (Fig. 1) , cannot gain local s-character, symmetric with 

 respect to the xz plane, from oxygen or from the adjacent carbon 

 atom. In l^etter approximations where the configurations are allowed 

 to mix, the excited states of keto groups may show the effect of 

 additional ^-character in u-tt* transitions. 



TT-TT* Transitions. In conjugated hydrocarbons of which ethylene, 

 benzene, and pyridine are prototypes, important transitions are caused 

 by an electron jump between a tt and a tt* orbital. The lowest energy 

 singlet-singlet tt-tt* transitions occur at about 260 mjx in benzene and 

 in pyridine and at a slightly shorter wavelength in ethylene. They 

 are progressively shifted to longer wavelength as the size of the mole- 

 cule increases (e.g., in carotene) . The transition in benzene is sym- 

 metry-forbidden but, like the n-vr* transition in formaldehyde, is made 

 weakly allowed (/ = .0014) through vibrational-electronic interac- 

 tion (24) . The lowest tt-tt* transition in pyridine is allowed by 

 symmetry but is intrinsically weak (/ r= .04) (36), since the electronic 

 structure of benzene, where the transition is forbidden, is approxi- 

 mately retained. It is notable that the spectra of many substituted 

 benzenes can be described in terms of relatively small perturbations on 

 the parent molecule (II, 25) . Fig. 2 shows schematically the molecii- 



"But one must I)c careful in assigning (0-0) bands! 



