514 



Quantum Mechanical Basis of Molecular Spectra /27 : 3 



gel (as nujol) or as part of a pressed KBr disc. The preparation by 

 either method does alter the spectrum. Some typical absorption bands 

 are given in Table II. 



Group 



>C =N 



>c =s, 



-N = N-, 



>so 4 



-OH 



TABLE II 

 Infrared Absorptions 



•N = 



S = 



°o} 



in 



r* 



5.95 or longer 



6.28-6.8 



8 -8.5 

 2.66-2.98 



Group 



in 



fi 



>NH 

 -SH 

 -OD 

 >ND 



CH 



2.88-3.28 

 3.72-3.9 

 3.6 -3.8 

 3.85-4.15 



3.05-3.7 



The various types of covalent bonds each have several absorption 

 maxima corresponding to different types of vibrations. Randal and 



5 6 



7 8 9 10 II 12 

 Wavelength (\l) 



Figure 2. Infrared absorption spectrum of 1-a-dimyristoyl 

 cephalin. Absorption bands in the infrared are due to molec- 

 ular vibrations and rotations. After H. P. Schwarz et al., 

 "Infrared Studies of Tissue Lipides," Annals of the New York 

 Academy of Sciences 69: 116 (1957). 



co-workers classify the vibrations responsible for the absorption spectra 

 as: (1) stretching (along the bond); (2) bending (across the bond); 

 (3) deformation — a bending which changes the bond angle; (4) wagging 

 — an entire group moving perpendicular to the plane of symmetry; 

 (5) rocking— similar to wagging but in the plane of symmetry; (6) 

 twisting — an entire group rotates around a bond to the rest of the 

 molecule; (7) breathing — completely symmetric stretching — usually in 

 rings; and (8) others not yet identified. 



