84 CHEMICAL REACTIONS IN THE GAS PHASE 



the most completely antibonding states of the ion. The distribution 

 function which gives the number of molecular ions with a given energy 

 cannot be given as a mathematical expression at this time. A great deal 

 of work has been done (35) on the theory of collisions between electrons, 

 but thus far no satisfactory treatment is available for collisions with 

 electrons bound in a molecule. It is possible to derive a fairly simple 

 expression for the probability of a free electron losing a given amount of 

 energy in a single collision. In the present case, however, an electron 

 must necessarily undergo at least one ionizing and one or more non- 

 ionizing collisions in passing through a given molecule in order to give it 

 enough energy to cause the observed dissociation. This requirement fol- 

 lows from the fact that the experimental conditions are such as to make 

 the collision of two electrons with the same molecules highly improbable. 

 The spectra of the octane isomers have previously been referred to, 

 and it was pointed out that, where there existed one C — C bond appreci- 

 ably weaker than the other C — C bonds in the molecule, few fragments 

 larger than the largest formed by breaking this weakest bond were found. 

 The same rule holds for the other saturated hydrocarbons. There is 

 low probability of splitting off a CII3 from the straight-chain hydro- 

 carbon ion; this probability is greatly increased in molecules with the 

 (CH3)2CH — group, the factor ranging from 5 (hexanes) to 150 (octanes) 

 and 50 (nonanes) (23), while the factor assuming equal bond-breaking 

 probabilities will be 1.5. We lack the space necessary to point out the 

 many regularities of this sort in detail. The most important rules re- 

 garding the initial cracking and subsequent decomposition, consistent 

 with the idea that one initially has a group of high-temperature ions, we 

 believe to be the following: 



- (a) The first bond broken in the parent ion is a C — C bond, and the 

 relative probabilities of breaking the various C — C bonds lie in the same 

 order as the bond strengths in the un-ionized molecules. 



(5) At times a simultaneous transfer of a hydrogen atom from the ion 

 to the uncharged fragment occurs along with the bond breaking described 

 immediately above, producing an olefinic ion and a neutral, saturated 

 molecule. The probability of this occurrence is usually about }^ to ^i 

 that of the process (a), but on occasion it is more important than (a), 

 as with (23) 



C3H7 + C4H9+ (12%) 



CH3— CH— CH— CH2— CH3 



I I 

 CH3 CH3 



C3H8 + C4H8+ (17%) 



