176 



Simon Freed 



transformation probably in an acid-base reaction. The quick readjustment 

 to equilibrium is shown by the interchange in relative intensities of the bands 

 in the red region. The band furthest towards the red grows in as the temperature 

 is reduced, at the expense of the band near it toward shorter wavelengths. 



That these reactions occur rapidly at such temperatures is not very sur- 

 prising since little heat of activation is required for this type of reaction. Figure 

 6 depicts a type of oxidation-reduction at low temperatures. When iodine 



3000 



4000 5000 6000 



WAVELENGTH ANGSTROMS 



7000 



Fig. 5. Chlorophyll b in 15% dipropylamine diluted with equal proportions of 

 propane and propene. A chemical readjustment toward equilibrium occurs 



between 170°K and 75°K. 



is finely divided it rapidly dissolves in isoprene at the temperature of dry-ice, 

 193°K. A brown solution forms at the solid-liquid interface but it decolorizes 

 very quickly, becoming colorless a little distance from the iodine surface. In 

 the light of other investigations it was surmised that the solution is brown 

 because of the presence of a 1:1 (molecular iodine-hydrocarbon molecule) 

 addition compound which possesses a characteristic absorption band in the 

 ultraviolet region. To build up any appreciable concentration of this compound 

 it would evidently be necessary to make solutions of iodine in isoprene below 

 193°K. When a solution of isoprene in propane (to which propene had been 

 added to increase the solubility of isoprene) at the temperature of liquid 

 nitrogen (77°K) is mixed with a solution of iodine in propane and propene. 



