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Quantum Mechanical Basis of Molecular Spectra /27 : 4 



For two electrons comprising any bond, their total spin may be 1 or 0; 

 that is, the multiplicity may be 3 or 1. The state of multiplicity 1, 

 called the singlet state, has, in general, a lower energy than the states of 

 multiplicity 3, called the triplet states. (A notable exception to this is 

 the molecule 2 , whose lowest or ground state is a 3 S state; that is, 

 S = 1, A = 0.) According to the selection rule, a molecule in the 

 lowest triplet state is "forbidden" to radiate its energy to reach the still 

 lower singlet state. Thus, energy may be "trapped" in the molecule. 

 This type of trapping has been included in several schemes to explain 

 the action of chlorophyll during photosynthesis. 



In general, single, covalent bonds have relatively large spacings 

 between successive electronic-energy levels. It is usually not feasible to 

 measure absorption spectra due to single covalent bonds. On the 

 contrary, the compounds whose spectra are of greatest biological interest 

 usually have a system of alternate double and single bonds, called a 

 conjugated system. Examples include vitamin A and rhodopsin, discussed 

 in the chapter on vision, and heme groups and heme proteins, which are 

 so markedly colored. As more and more bonds are added to a con- 

 jugated system, the energy difference between the lowest and the first 

 excited states decreases; the absorption band is shifted progressively 

 toward the red. 



It is convenient to write a chain of alternate single and double bonds 

 as 



H H H 



/ C % / C % / C % / 



H 



H 



H 



Although simple, this form is probably misleading. A more significant 

 symbolism might be 



where the number 3 indicates that three pairs of electrons have orbitals 

 encompassing the dotted region. (Recall that orbital means the locus 

 of the most probable locations and not that the electrons travel around 

 fixed orbits.) 



The pairs of electrons within the dotted region cannot have spherically 

 symmetric orbits because the chain shown would be stretched out in the 



