ISOMERS AND MULTIPLETS 145 



copies of each character to make up the 25! Of course, not just any arrange- 

 ment of atoms gives a stable molecule; but on the other hand the number of 

 chemical groups of which a macromolecule is composed ( — CH 2 , — NH, 

 — CO, — C — S — , etc.) is certainly far more than five! .... One concludes 

 that the number of stable isomers of a macromolecule must be huge, but at 

 this stage of knowledge one really has no idea how many there are. Each 

 must have a unique set of physical and chemical properties. Just as in the 

 case of the simple alcohols, each must be a stable molecular entity. 



Excited States 



No molecule, even if anchored at some point, must be completely quiet 

 if T > 0°K. Indeed, in an environment at 98°F (37°C) such a molecule, 

 even if initially at rest, or quiet — i.e., in its vibrational and rotational 

 "ground state" as it is called — will soon be buffeted into motion by neigh- 

 boring molecules of gas, liquid, or solid, until its energy level or tempera- 

 ture is, on the average, that of the environment. Heat energy enters the 

 molecule as the energy of rotation or vibration if the molecule is anchored, 

 and enters also as the kinetic energy of translation (linear motion) if the 

 molecule is free. The vibrations and rotations may be thought of as standing 

 or traveling matter waves moving across the molecule. Parts of the mole- 

 cule can be fixed and immobile; other parts can be free. The distribution of 

 energy within the molecule will be continuously changing. 



Macromolecules accept and give up energy to the surroundings in discrete 

 bursts or bunches or quanta, if the quantum theory applies here as it is 

 known to apply to 2- and 3-atom molecules. However, the energy differ- 

 ences between mechanically excited states must be very small — so small that 

 almost a continuous exchange of energy must be possible. 



The important point is that all of the configurations which result from 

 heat exchange are configurations proper to one isomer; in principle the 

 isomer may assume many shapes. Consider the random coil configuration 

 of protein as an example. The one chemical entity may assume many shapes 

 simply as a result of thermal exchange. 



Electronically excited states also exist but these are different. It was seen 

 in Chapter 4 that electrons which make the bonds of molecules can absorb 

 and re-emit electromagnetic radiation, and that some excited states can be 

 reached by the absorption of such small amounts of energy that even local 

 heat energy sometimes will do the trick. It is a general rule-of-thumb that 

 whenever a bonding electron accepts energy of any kind and becomes itself 

 "excited," the bond is weakened. Once weakened, it is more susceptible to 

 thermal buffeting and to chemical attack. Its "defense" is to rid itself of the 

 extra energy and get back into the bond; this it does by reradiation, or by 

 transfer of energy into the mechanical motion of the molecule. 



