12 LIGHT AND LIFE 



interacting parts) . If there is a large number of parts having very 

 nearly the same energies, a general broadening and lowering of elec- 

 tronic transitions to such a group of states results, since the inter- 

 action tends to separate the multiple energy levels of the unperturbed 

 system (16a). The excited states, because they correspond to higher 

 electronic energy and thus are closer to the top of the barriers, will 

 feel such interactions more than the ground state of the molecule. In 

 addition to the effect on the energies, the interactions enable elec- 

 tronic energy transfer to take place within the molecule. 



The electronic energy states of small molecules, because of their 

 relative simplicity, form a good starting point for the understanding 

 of detailed effects in large molecules.- Many problems of current 

 interest in molecular biology appear to be associated with exactly 

 the same problems which are of current interest to the molecular 

 physicist. The reason for this, of course, is partly due to the in- 

 creased amount of interaction between these scientific groups and 

 partly, it is to be hoped, to the fact that these are important prob- 

 lems in both fields. Some of the problems are (1) intermolecular and 

 intramolecular energy transfer, (2) long-lived states, (3) electron 

 density in molecules and its change with electronic excitation, (4) 

 molecular geometry and its change with electronic excitation, and 

 (5) the effect of inter- and intra-molecular environment upon the 

 ground and excited electronic states of molecules. Some of these 

 effects as they pertain to small molecules will be discussed. 



2. Electronic Orbitals, States, and Transitions 



Molecular stationary states are characterized by solutions of the 

 time-independent Schrodinger equation for the "many electron-many 

 nucleus" system. Important contributions to the energy come from 

 electrostatic attractive interactions between electrons and nuclei, and 

 electrostatic repulsive interactions between electrons and between 

 nuclei. Smaller but no less interesting contributions arise because of 

 oscillation of the nuclear masses about their equilibrium positions, 

 rotation of the molecule as a whole, and magnetic interactions among 

 the magnetic moments of the spinning electrons and those associated 

 with rotating electronic and nuclear charges in the molecule. The 

 approximation is often made that electronic orliital, electronic spin, 

 and nuclear motions can be treated separately, and in this approxi- 

 mation molecular electronic states are characterized by solutions to 

 the approximately separable electronic part of the wave equation with 

 nuclei fixed in their equilibrium positions. It is just the cross terms 



