Summary of basic problems

Absorption of radiation by polyatomic molecules composed of light atoms follows, as regards change in multiplicity, the same rules as those for atoms and diatomic molecules. Since most polyatomic molecules have ground singlet states the vast majority of absorption processes form upper singlet states. In the presence 

Upper repulsive states exist for polyatomic molecules just as they do for diatomic molecules. The alkyl halides all show absorption which corresponds to dissociation into alkyl radicals and halogen atoms. Saturated hydrocarbons absorb only below 1800 A unless rings with considerable strain are present. The first absorption region for the simple alkanes is a continuum and dissociation must immediately follow the absorption act. The simple alcohols also show continua. 

It must be re-emphasized, however, that a true continuum in a spectrum is more difficult to identify than for diatomic molecules, and a primary yield of unity independent of total pressure and of scavengers is better proof of the presence of a continuum than is the appearance of the spectrum. 

Triplet states are, however, formed to larger extents and play more important roles than would have been thought to be true twenty or thirty years ago. The classical work of Lewis and Kasha129, whichshowed certain emissions to come from triplet states, opened the way for the rationalization of many phenomena which would otherwise prove to be quite incomprehensible. Perhaps, as Matsen etal.130 have pointed out, an undue emphasis has been placed on electron spin and on the multiplicity of states. The symmetry of the entire wave function is really the important point, and the contributions to it of all states of the molecule must be considered. Viewed in this light the triplet component , to use rather crude language, will depend on the vibrational quantum numbers in the excited state. If other isomers can exist, their contributions to the complete wave function must also be considered. 

One factor must, however, be kept in mind. Triplet states invariably lie at lower energies than do the corresponding singlet states131. While it may be true that the multiplicity per se will not affect reactivity for many processes, the difference in energy may be decisive and some reactions can occur with singlet states which would not be possible with triplet states. 

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Fig. 3.2. a oxidative, b reductive quenching of the sensitizer. Summary of basic problems to be solved in fuel production via photoredox processes... 

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