PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 221 



With the exception of the case of certain unsaturated, organic mole- 

 cules with extended systems of conjugated double bonds, it is an exag- 

 geration to consider the valence electrons as belonging to the molecule as 

 a whole. Various geometrical regions ("radicals" or "groups of atoms") 

 may be more or less isolated and exhibit properties, such as vibrational 

 or even electronic absorption bands, to a greater or smaller extent inde- 

 pendent of the balance of the molecule. Of course, there is always some 

 interaction between different regions, and certain instances show an 

 intramolecular flow or transfer of electronic excitation energy, a process 

 which is conceptually impossible in a diatomic or even triatomic mole- 

 cule and which often has photochemical importance in very large mole- 

 cules. Another consequence of this "weak coupling" between groups 

 of atoms, usually mutually remote, is that in a given electronic state a 

 molecule may have in summation a great deal of vibrational energy, this 

 being distributed between various vibrational degrees of freedom. The 

 total vibrational energy can easily far exceed the differences in energies 

 of various electronic levels, or (as can never be the case with a diatomic 

 molecule) the dissociation energy of a particular bond, so that vibration- 

 rotation energy levels of a given electronic state exist far above the 

 lowest level of that state and usually overlap the vibration-rotation 

 levels of higher electronic states. 



Emission or absorption of light by polyatomic molecules gives rise to 

 emission and absorption spectra corresponding to transitions between 

 energy levels. As in the diatomic case, there are pure rotation and 

 vibration-rotation bands in the infrared, and electronic bands, with 

 vibrational and rotational structure, in the visible and ultraviolet. 

 However, the electronic bands of polyatomic molecules are usually 

 diffuse and often continuous. Bands with sharply defined vibration- 

 rotation structure are rare: some simple molecules (e.g., CO2, NO2, SO2, 

 H2CO) have, as a gas at low pressure, at least some regions of discrete 

 structure in their electronic spectra, but others (e.g., CH4, NH3) have 

 spectra which are entirely diffuse, as do almost all complicated molecules. 

 This important fact is explained in Sect. 3-3c. It is mentioned here 

 because it is responsible for the circumstance that little detailed informa- 

 tion has been obtained regarding excited electronic states of polyatomic 

 molecules (other than for certain small molecules, or molecules containing 

 ring structure and conjugation, like benzene and derivatives) . Obviously, 

 the complexity of the spectra and the extreme difficulty in their analysis, 

 even when they have discrete structure, also contribute to this unfortu- 

 nate situation. On the other hand, analysis of the infrared spectra can 

 often be achieved (as has been the case, to name a few important 

 examples, with CO2 and H2O), but this yields information only on the 

 ground electronic states — chiefly on interatomic distances and vibra- 

 tional frequencies — and hence is not what is needed for the understanding 



