Why doesn't TEMPO react with itself?

TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl, or (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl) has the following structure:


[![TEMPO structure](https://i.stack.imgur.com/XLVar.png)](https://i.stack.imgur.com/XLVar.png)


Given that it's a free radical, why doesn't it react with itself, despite being capable of reacting with *other* radicals in the system itself)?


In addition, according to Wikipedia, it's a red-orange, sublimable solid. How can it be a solid despite being a radical?




I believe the first part of your question is asking why the oxygen with the unpaired electron doesn't find a second one to form a peroxide.


The answer is actually quite simple. It is by design. Next to the nitrogen, you see the 4 methyl groups that give TEMPO the "TEM" part of it's name. These methyl groups give TEMPO a lot of steric bulk around the oxygen, and this bulk makes it very difficult for two TEMPO radicals to form an oxygen-oxygen bond with each other. There is just not enough space. You can build a model to convince yourself of this.


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Steric hindrance is the way to make them stable. 
I have never worked with TEMPO, but I have seen this one in our lab:


[![enter image description here](https://i.stack.imgur.com/q6mjc.gif)](https://i.stack.imgur.com/q6mjc.gif)


Here the steric hindrance is very evident. 
This is also a nice stable solid. 


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The stability of TEMPO and hence is formation is thought to result from resonance involving the nitrogen lone pair as well as hyperconjugation from the numerous methyl groups. See also [the Wikipedia article on 3 center 2 electron bond](https://en.wikipedia.org/wiki/Three-center_two-electron_bond).


The resonance involves oxoammonium contributors as those postulates in oxidation catalyse by TEMPO and carbanions, hence invoking sacrificial hyperconjugation.


[![Oxoammonium contributors in TEMPO](https://i.stack.imgur.com/AbxtF.jpg)](https://i.stack.imgur.com/AbxtF.jpg)


As stressed in other answers the steric hindrance of the methyl groups *also* plays a major role "caging" the radical center and leading to longer than usual bonds when TEMPO reacts.


Consider that coupling two TEMPO would lead to a peroxide. Peroxides are, with few exceptions, unstable and reactive.


The above effects makes a hypothetical $\ce{O-O}$ bond in TEMPO even less stable than that one in other peroxides, where it is already characterised by energy of about $\pu{50 Kcal/mol}$, i.e. less than the already unstable $\ce{O-H}$ bond in the same molecule and generally speaking less than a half of $\ce{C-C}$, $\ce{C-H}$ and $\ce{C-O}$.


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