27 : 3/ Quantum Mechanical Basis of Molecular Spectra 



515 



Qualitatively, the description of the simple harmonic vibrator 

 adequately describes the type of spectra observed and predicts that they 

 should be located somewhere in the infrared. However, such details as 

 the wavelength at which — C — H bonds absorb or the shape of the 

 absorption bands cannot be predicted a priori. Rather, the absorption 

 bands of known structures are measured, and these are used to interpret 



1,300 1,200 1,100 1,000 900 

 Wave Number (cm -1 ) 



Figure 3. Infrared absorption spectra of 3-desoxytigogen. 

 Computed and observed spectra are compared. The wave 

 number, used here for the abscissa, is the reciprocal of the 

 wavelength. After R. N. Jones et al., "Infrared Intensity 

 Measurements Applied to the Determination of Molecular 

 Structure," Annals of the New York Academy of Sciences 69: 38 

 (1957). 



the bonds present in unknown compounds. Even here, there are only 

 empirical rules to indicate how a particular compound will alter the 

 absorption band due to a bond as > C=0. A successful application of 

 these rules is illustrated in Figure 3. 



To recapitulate, then, vibrational absorption bands show the bonds 

 which are present. The spectral fingerprints of most biologically 

 important compounds are so complicated in the spectral region from 

 2-20/x that purified solutions are necessary. These absorption spectra 

 can be used to identify the bonds in a compound or determine the 



