Molecule Reactions in the Gas Phase

So far we have mostly considered first-order reactions of radical ions and ions in the gas phase. Here we will consider bimolecular reactions between ions and neutral molecules. Such reactions can be studied by Cl mass spectrometry or ion cyclotron resonance (ICR) spectrometry (see Section 7.1). 

A simple and intriguing reaction of this type is observed on ionizing methane. The initally formed radical ions CH4 react as follows  

Since CHs contains only four pairs of electrons, two of the hydrogen atoms must be linked to carbon by a three-center bond. The reaction is exothermic because the bond energy of such a bond is greater than the sum of a two-center, two-electron bond and a two-center, one-electron bond. Under conditions where the reaction of equation (7.29) can take place, an intense peak is observed corresponding to CH. It might be added that Olah et al. have recently found evidence for the formation of by protonation of 

In the case of other paraffins RH, where the corresponding carbonium ion R is stabilized by — / or — E substituents, an alternative process is observed e.g.. 

Here the stabilization of the ion (CH3)3C by the three — / methyl groups is sufficient to make the decomposition of the(CH3)3CH2 ion into (CH3)3C + H2 exothermic. 

---