ULTRAVIOLET ABSORPTION SPECTRA 



173 



Tsuboi, 1950; Loofbourow, 1940; Siiisheimer, 1954). The absorption 

 spectrum of ribonucleic acid also increases in intensity and shifts slightly 

 toward shorter wave lengths during depolymerization (Kunitz, 1946; 

 Tsuboi, 1950). 



If the radiant energy is plane polarized, absorption will be greatest 

 when the plane of the electric vector is parallel to the direction (s) of 

 greatest electron mobility and maximal induced dipole moment, i.e., 

 parallel to the direction of a chain of conjugated bonds, as in carotene, or 

 parallel to the plane of the ring in a planar aromatic or heterocyclic mole- 

 cule, as in benzene (Fig. 5-5). Spectrally distinct absorption bands may 

 appear, corresponding to transitions involving mutually perpendicular 

 changes in dipole moment (Lewis and Bigeleisen, 1943b; Scheibe and 

 Kandler, 1938; Scheibe et al., 1943; Coulson, 1948; Nakamoto, 1952). 

 In solution the random orientation of molecules will prevent detection 

 of any such preferred directions; however, in cellular structures or in 

 crystals, uniform molecular orientation may permit a preferential absorp- 

 tion for light polarized in these directions (Butenandt et al., 1942; 



X^ ^\ 



\/ \/ 



/\ /\ /\ 



\ 



KEKULE DEWAR 



Fig. 5-6. The canonical structures of benzene. {Sklar, 1937.) 



Schauenstein et al., 1949). This phenomenon of differential absorption 

 dependent on the plane of polarization is known as dichroism and can be 

 useful as an indication of molecular orientation. 



THEORETICAL DEVELOPMENTS 



As indicated, these empirical correlations have received support from 

 modern theories of the electronic structure of organic molecules and of 

 the change in electronic configuration attendant on the absorption of 

 radiation. These treatments have developed along two lines, the valence- 

 bond concept (MaccoU, 1947; Heitler, 1945; Pauling, 1945; Van Vleck 

 and Sherman, 1935; Sklar, 1937) and the molecular-orbital concept 

 (Coulson, 1947; Herzfeld, 1947; Mulliken and Rieke, 1941). 



In the valence-bond concept, the electrons involved in chemical bonds 

 are assumed to remain in atomic orbitals, which overlap with orbitals of 

 the neighboring atoms, and the energy of each bond may be calculated 

 from wave-mechanical principles. The energy level of the molecule is 

 dependent on the summation of the energy levels of each bond. It is 

 recognized that, with conjugated structures, the formulas usually written 

 represent but one of several possible canonical forms (Fig. 5-6), all of 

 which may be considered to contribute in varying degree to the actual 



