80 CHEMICAL REACTIONS IN THE GAS PHASE 



a character seems significant nevertheless. For the butanes the appear- 

 ance potentials for certain metastable ion transitions have been deter- 

 mined (27) and require that the uncharged fragments be the molecules 

 H2 and CH4. 



A characteristic of all the experimental facts that have been presented 

 here is that they may all be interpreted by assuming that the dissociation 

 processes are essentially thermal in nature. By this we mean that the 

 molecules gain, in the course of ionization, various amounts of vibra- 

 tional energy and subsequently dissociate, as a sufficient amount of this 

 energy becomes concentrated in some bond or other. This assumption 

 is very strikingly borne out by the actual appearance of the spectra of 

 the octanes (Tables 1 and 2), where one finds a great deal of correlation 

 between the structure of the molecules, in terms of bond energies, and 

 the mass spectra. 



Let us now consider in some detail the basis for this interpretation. 

 It is surely reasonable to assume that in saturated hydrocarbons the 

 binding energies of the electrons in all C — C and all C — H bonds are 

 about the same, and since the normal electron beam energies are from 

 5 to 10 times the ionization potential of the molecule, which electron is 

 removed from the molecule on ionization will be largely a matter of 

 chance, with perhaps a slight weighting factor in favor of electron 

 removal from a C — C bond. That is, to a first approximation, the 

 numbers of ions formed by removal of an electron from different bonds 

 will be about the same. If there is any validity to the approximation 

 that the bonding electrons in the molecule are localized in pairs in the 

 various bonds, the removal of an electron must leave the ion with a one- 

 electron bond, a bond far weaker than all other bonds in the molecule. 

 If we then assume that ionization is followed immediately by rupture at 

 the bond from which the electron has been removed, we expect the mass 

 spectra of all members of an isomeric series to have patterns obviously 

 dependent on the carbon skeleton. For example, in the mass spectra 

 of the octanes, the amount of CyHis"*" should increase as the number of 

 branched methyl groups increases, since there would be more opportunity 

 for methyl groups to be lost. A glance at Table 2 shows that, far from 

 exhibiting such patterns, the octane spectra depend on structure in an 

 entirely different fashion. This dependence seems to be related directly 

 to the energies of the various types of bonds in the molecule. For 

 example, those octanes having a tertiary butyl group at the end of the 

 chain show only a very few per cent of ions with carbon number greater 

 than 4. (See Table 2.) This parallels the known low energy of this 

 bond in comparison to the other carbon-carbon bonds present (28). 

 In 4-methyl heptane, where splitting at the weakest bonds will allow 



