Carbon tetraradical

I was just reading through *Reactions: The private life of atoms* by Peter Atkins and I noticed that in Chapter 3, the chapter on the combustion reaction, the author writes: 



> 
> As we watch we see $\ce {CH\_4}$ being whittled down to naked $\ce {C}$ as its $\ce{H}$ atoms are stripped away by radical attack.
> 
> 
> 


"Naked $\ce {C}$" here seems to refer to a carbon atom, with four unpaired electrons. This seems rather surprising to me as I was thinking that such a reactive and unstable species could not possibly form. Thus, I would like to ask if a "naked $\ce {C}$" can possibly exist, though it may have a really short lifetime. Additionally, I would like to ask if this "naked $\ce {C}$" is considered a tetraradical or a biradical because in the ground state of carbon, there are only two unpaired electrons. 




In its ground state, naked carbon is triplet $^3P$, with two metastable singlet states $^1D$ and $^1S$ ($^1D$ being the one that participates in most reactions) while the tetraradical is the least stable one:
[![enter image description here](https://i.stack.imgur.com/gT1B4.gif)](https://i.stack.imgur.com/gT1B4.gif)


It can be synthesized by several different methods. Graphite vaporization by an electric arc or laser can be employed. Photolysis of suitable precursors can also produce atomic carbon:


[![enter image description here](https://i.stack.imgur.com/2vTGK.jpg)](https://i.stack.imgur.com/2vTGK.jpg)
[![enter image description here](https://i.stack.imgur.com/zCWFw.jpg)](https://i.stack.imgur.com/zCWFw.jpg)
Finely designed diazo compounds could be good precursors for atomic carbon, e.g. the diazo compound in the image below produces on heating a carbene, which immediately collapses to form benzene and atomic carbon. The problem with this approach is that the products can react further.


[![enter image description here](https://i.stack.imgur.com/6Z0Po.jpg)](https://i.stack.imgur.com/6Z0Po.jpg)


Another approach is to use diazotetrazole, but the drawback here is that diazonoium chloride obtained after the first step is extremely explosive.
[![enter image description here](https://i.stack.imgur.com/Q3f42.jpg)](https://i.stack.imgur.com/Q3f42.jpg)
I would also like to comment about the reactivity of atomic carbon since it is rather interesting. This reactive species is known to insert in C-H bonds and it can also abstract carbonyl oxygens to form carbenes:


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


If it is reacted with water, carbohydrates form:
[![enter image description here](https://i.stack.imgur.com/dv3gk.jpg)](https://i.stack.imgur.com/dv3gk.jpg)
[![enter image description here](https://i.stack.imgur.com/796d6.jpg)](https://i.stack.imgur.com/796d6.jpg)


When reacted with alkenes, cumulenes are usually obtained:
[![enter image description here](https://i.stack.imgur.com/vj3JR.jpg)](https://i.stack.imgur.com/vj3JR.jpg)
They insert into C-Br and C-Cl bonds, but abstract fluorine from compounds containing C-F bonds.


More about this rather interesting chemistry can be read in "Reactive Intermediate Chemistry" by Robert A. Moss, Matthew S. Platz and Maitland Jones, Jr.


 13

The $^{5}S^{\circ}$ state, which lies just above the two singlet excited states referenced elsewhere here, could be considered a "tetraradical":


<https://physics.nist.gov/cgi-bin/ASD/energy1.pl>


The quintet state seems to be of little importance, however.


 0