PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 



211 



the molecule has a permanent dipole moment) ; they involve much smaller 

 energy differences and hence lie in the infrared region. Transitions 

 between different rotational levels of the same vibrational level can also 

 occur; these lie still farther in the infrared. Information on the energy 

 levels of diatomic molecules is obtained chiefly from analysis of their 

 electronic and infrared spectra (emission and absorption), and from their 



8 - 



>- 



o 



cr 



UJ 



QL 



jtc.j 



etc. 



-V-m 



J = 0,1,2,- 

 etc. > 





|tc.f 



3 y,+ 



v = 18 

 17 

 16 

 15 

 14 

 13 

 12 

 II 

 10 

 9 

 8 

 7 

 6 

 5 

 4 

 3 

 2 

 1 

 



etc. 



'n„ 



-8 

 -7 

 -6 

 -5 

 ■4 

 -3 

 ■2 

 ■ \ 

 ■0 



^tc.| 



•4 

 •3 

 •2 

 ■ 1 

 •0 



'TTr, 



Fig. 3-2. Energy-level diagram of molecular nitrogen. (Only the lowest of the 

 electronic, vibrational, and rotational levels are shown. The separation of rotational 

 levels is not to scale.) 



Raman spectra (Herzberg, 1950) ; some further knowledge may be gained 

 from electron-impact experiments (Massey and Burhop, 1952). 



3-2b. Potential Curves. If a diatomic molecule were prevented from 

 vibrating and rotating, the two nuclei of its constituent atoms being con- 

 strained to a separation r, the molecule would have a total energy which 

 depends on r. This function E{r) is highly important for the under- 

 standing of many properties of molecules. Thus, if r is very great, the 



