BEHAVIOR OF ORGANIC COMPOUNDS 123 



10~^ sec or more). However, the initial ionization may be to an elec- 

 tronically excited state of the ion A"^. Ensuant internal conversion to 

 a lower state of A"*" and decomposition (that is, predissociation), as by 

 either of the reactions la and 16, may likewise take a long time. The 

 time involved will govern the phenomena observed. For example, in 

 mass spectrometry we detect all manner of peaks with masses less than 

 the parent peak. Some must have resulted from rearrangement in a time 

 short compared with that required to move from the slit system to the 

 magnetic field (<<C10"'^ sec). On the other hand, the ghost peaks in mass 

 spectrometry have been attributed by Hippie and his coworkers (21) to 

 decomposition of the metastable ions while within the accelerating field; 

 for example, 



C4H10+ -^ C3H7+ + CH3 (2 X 10-6 sec) 



C4H10+ -^ C3H6+ + CH4 (1.7 X 10-6 sec) 



In this discussion of radiation chemical processes in organic compounds 

 such entities as R, X, B, and C and their ions have not been identified 

 as other than possible decomposition products of A which may be 

 radicals or molecules. In a complicated or large molecule a great variety 

 of possibilities exists. In a homologous series many possible paths of 

 decomposition are equally probable. Thus, w^e have a principle (14) 

 which has been found to apply fairly well for straight-chain paraffins and 

 their normal carboxylic acids: 



Where the special chemistry of the substances involved does not 

 play a significant role, both nature and relative concentration of 

 products are determined by nature and relative frequency of occur- 

 rence of parent groups in the affected molecule. 



This principle is particularly noteworthy for its limitations. In very 

 few cases can chemistry be neglected. We might expect, on such a basis, 

 that all compounds are equally sensitive to the effect of ionizing radia- 

 tion. However, as a matter of fact, we know that aromatic compounds 

 in particular are quite resistant to effects of radiation as compared to 

 aliphatic compounds (13, 14). It is a matter of particular interest for 

 radiobiologists that the aromatic group may protect the side chain. 

 Hentz and Burton (22) have shown that such compounds as toluene, 

 mesitylene, and ethylbenzene are more easily decomposed than benzene 

 but are far more resistant to radiation (that is, by a factor of 10) than 

 alkanes. On the other hand, another feature interesting to radiobiologists 

 is that energy liberated in the aromatic group may be effective for de- 

 composition of the side chain (22). 



