822 LIGHT AND LIFE 



electron to an antibonding orbital, and the effect is spread over all 

 six bonds in the ring. Almost always, the chemical bonds become 

 weaker and longer in an excited state of the molecule. A transition 

 of one electron thus effectively reduces the strength of a triple bond 

 to that of a double, the strength of a double bond to that of a 

 single, and disrupts a single bond. Bond angles are also changed. 

 The transition from a singlet state to a triplet state with the same 

 electronic configuration has been analyzed for formaldehyde. The 

 n ^ TT* singlet-singlet and singlet-triplet transitions alike involve 

 a promotion of a nonbonding electron to an antibonding orbital 

 affecting the C=0 bond. The bond length becomes intermediate be- 

 tween that of a single bond (C — O) and a double bond (C=0) , 

 and is 1.312 A in the triplet state and 1.326 A in the singlet state. 

 There is an indication from vibrational data respecting other mole- 

 cules that in general the singlet and triplet states with the same orbital 

 configuration have very nearly the same geometry, which may be 

 quite different from that of other states wtih different electronic con- 

 figurations. Nevertheless, it is not the geometry of the lowest triplet 

 state that makes it so important. According to Robinson, its signi- 

 ficance lies rather in that it is the excited state of very lowest energy, 

 and the only one likely to have a lifetime sufficient to convey electronic 

 excitation to chemical reactions. 



The interactions of solvent molecules with solute molecules lead 

 to changes in the reactivity of the excited species, as well as shifts in 

 the spectrum. In general, when the separation of the energy levels 

 increases (a blue shift) , the chemical reactivity decreases, and vice 

 versa. These related phenomena focus attention on the nature of the 

 solute-solvent interactions. Robinson discusses non-polar solvent in- 

 teractions, charge-transfer interactions, and dipole-dipole interactions. 



For non-polar solvents, attraction between solvent and solute re- 

 sults in shift of the electron density from both solvent and solute 

 molecules to the intermolecular spaces between them. This effect 

 weakens and lengthens the chemical bonds, and consequently pro- 

 duces a shift of the frequency spectrum toward the red. Repulsive 

 interactions conversely produce a blue shift. The latter is less com- 

 mon, but can be produced, for example, by trapping the molecules 

 in a low temperature solid. Transfer of charge is common in cer- 

 tain complexes, such as iodine-pyridine. which are bound by forces 

 weaker than valence forces but greater than van der Waals' forces. 

 As a consequence of a loss of vibrational energy by the donor mole- 

 cule, a red shift is found. In the example cited, the I-I bond loses 



