82 II. CHEMISTRY OF FATTY ACIDS AND GLYCEROL 



173 mju, while peaks for oleic and linoleic acids were found also at 183 and 

 190 mjLi, respectively."^ The corresponding trans isomers had higher ab- 

 sorption maxima, which were 4 and 2.5 m/x toward the visible spectrum; 

 the intensity of the absorption was increased 15% in the first case (elaidic 

 vs. oleic), while it was decreased 11% with the second trans isomer (linolen- 

 elaidic vs. linoleic). 



d. Raman Spectrum. The Raman spectrum has been of considerable 

 importance in helping to establish the structural relationships of isomeric 

 compounds, especially when used in connection with the results from 

 infrared spectroscopy. The Raman effect, discovered as recently as 1928 

 by Sir C. V. Raman, is produced by irradiating the substance to be in- 

 vestigated with rays as nearly monochromatic as possible; under such 

 conditions, the material upon which the light beam is focussed becomes a 

 site of emission of light of the same or different wave length from that of the 

 incident light. The emitted rays can best be observed at an angle of 90°C. 

 to the direction of the applied light. The nature of such emitted light 

 can be analyzed by the use of a spectrograph by which the intensity and 

 position of the light waves can be registered on a photographic plate. 



The emergent light usually owes its origin to several factors. The most 

 important of these is, of course, the true Raman scattering. However, 

 when homogeneous material is irradiated, considerable emitted light may 

 be the result of the so-called Rayleigh scattering from particles of colloidal 

 size, or even from the molecules themselves. Other causes for the produc- 

 tion of emergent rays may be direct reflection when the material is hetero- 

 geneous, or a fluorescent emission in cases in which this phenomenon is 

 possible. All of these factors may operate simultaneously. 



The Raman effect has been ascribed to changes which occur when a 

 quantum of light collides with a molecule of the irradiated substance. 

 The latter substance is usually decomposed. The consequent loss of 

 energy in the molecule is similar to that which takes place whenever an in- 

 elastic collision occurs in a reaction involving a quantum process. The 

 energy released is absorbed as increased vibrational or rotational energy. 

 The resulting emitted or scattered light is usually of greater wave length 

 than that of the impinging light, by a fixed amount M^hich depends upon the 

 quantum process involved. In some cases in which the molecule is in a 

 higher energy state than the normal, it may emit light of greater frequency, 

 i.e., of a lower wave length, than that of the incident light. Such spectral 

 bands are known as "anti-Stokes" lines. 



Characteristic Raman spectra are produced by ethylenic compounds at 

 approximately 1.G50 cm.~^ and 3.010 cm.~^ frequency units. The former 

 band is less intense for the cis than for the trans compound, Avhile the re- 



^^l. I. Rusoff, J. R. Piatt, H. B. Klevens, and G. O. Burr, J. Am. Chem. Soc, 67, 

 673-678 (1945). 



