Hot Radical Reactions Photolysis of Iodides

Hydrogen iodide has a continuous absorption spectrum in the ultraviolet starting at about 3850 A and extending through the far ultraviolet, the absorption coefficient becoming quite large near 2500 A. The absorption of light loads to the over-all reaction 

If wc assume stationary states for H and I, we find for the stationary rate of H2 production  

I -f- H2 — HI -f- H, I 4- HI — I2 + II would be in the ratio of 1/200, which would be much lower than any theory would predict. 

Note that the reactions of hot radicals will be picked up only if the steric factors for their reactions are fairly high because dea tivation by collision is always a very fast competing process. It is in fact somewhat surprising that hot CHg radical reactions are detected at all, since normal CHg radicals have stern factors of about 10 (Table XII.6) for II abstracition reactions. 

CTTs -f- III CH4 -h I C2HbT 4- M CjHbI + M CH2I -f- CH2I C2H4R C2II4 -I- I2 4-M C2H4I2 

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