Why is the molecular peak not observed in the mass spectrum of 2,2-dimethylbutane?

Why isn't the molecular weight of 2,2-dimethylbutane on the mass spectrum? I can tell that a $\ce{-CH3}$ group is removed just by the math, but I do not know the reason why. The molecular weight is 86 and I know it has to do with fragmentation.


[![Mass spec](https://i.stack.imgur.com/4mBMy.png)](https://i.stack.imgur.com/4mBMy.png)




The answer probably has to do with how the mass spectrum was obtained.


Most mass spectra that are provided online, including the ones on the NIST WebBook, are [electron ionisation](https://en.wikipedia.org/wiki/Electron_ionization) mass spectra. That means that you put the sample in a chamber, accelerate electrons to a *really* high speed (giving them approximately $70~\mathrm{eV}$ of energy), and then bombard the sample with those high-energy electrons in order to ionise the sample.


The ionisation of these organic molecules doesn't take that much energy - $15~\mathrm{eV}$ is a good approximation. So you have an extra $55~\mathrm{eV}$ of energy in each molecule. How can the molecule get rid of this excess energy? The simplest way is to break chemical bonds; a typical bond strength lies between $3$ and $10~\mathrm{eV}$.


Branched compounds in general do tend to fragment more easily than straight-chain compounds. The reason is simple. If you think about *n*-hexane, no matter which bond you cleave, you're going to end up with a primary cation (or radical). However, if you cleave at one of the branching points of 2,2-dimethylbutane, you'll get a tertiary cation (or radical). A more stable product means that fragmentation occurs more readily.


However, the fact that the molecular ion is not observed is not really unique to 2,2-dimethylbutane. Lots of EI mass spectra don't have the $\ce{M+}$ peak. If you want to observe the $\ce{M+}$ peak, it's a better idea to use a gentler ionisation technique, such as [chemical ionisation](https://en.wikipedia.org/wiki/Chemical_ionization) or [electrospray ionisation](https://en.wikipedia.org/wiki/Electrospray_ionization).


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