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Friday, March 24, 2023

Gaseous carbon: The energetics of two types of tetracarbon, C4 and a problem!

The subject of dicarbon, C2, has been mentioned right here for a number of years now. It undoubtedly can be a fuel! This side of the species got here to the fore lately[1] when additional experiments on a possible chemical precursor of dicarbon, the zwitterion X(+)-C≡C(-), confirmed that totally different variants of X(+), comparable to not solely X=PhI(+), but in addition e.g. X=dibenzothiophenium(+) appeared to generate a gaseous species, which might be trapped as “C2” in a solvent-free related flask experiment.

A part of the thriller is that C2 itself is a particularly excessive power species, its dimerisation to C4 being round 107 kcal/mol exoenergic in free power. Now, earlier calculations[2] had revealed that the response of the precursor PhI(+)-C≡C(-) with itself can happen on a comparatively low power pathway which avoids the very excessive power of C2. The IRC for this response is proven beneath, displaying a modest barrier and a really exothermic response to the species PhI(+)-CCCC(-) and IPh. 

Right here I convey your consideration to an odd characteristic on the IRC, within the area of -5. On this area, successfully “free C4” is fashioned (at an power some 60 kcal/mol decrease than the reactants and 167 kcal/mol decrease than two molecules of free C2), however this species is instantly trapped by a PhI to type the ultimate merchandise with an extra lower in power of ~20 kcal/mol. Suppose nonetheless, in a molecular dynamics sense, some proportion of this “C4” may take a distinct trajectory and free itself at this level, therefore escaping being trapped by PhI? This response would then generate what once more is presumably a gaseous C4.

Right here I discover what would possibly occur subsequent, to reply the query of whether or not linear C4 is secure, or will it convert into one thing else? The scheme beneath exhibits a number of the potential pathways, resulting in the bicyclic type which I’ve beforehand mentioned extensively when it comes to its stabilising aromaticity. Calculations are on the CCSD(T)/Def2-TZVPPD fuel part stage, permitting biradicals to type (FAIR Knowledge DOI: 10.14469/hpc/11956).

You’ll be able to see that C4 is in a modest thermal effectively, with a free power barrier to cyclisation of ~22 kcal/mol. So generated at comparatively low energies, it’d retain its linear construction, whereas at room temperatures or larger, it’s going to in all probability find yourself because the bicyclic fragrant species.

The important thing calculation is perhaps that dimerisation response proven above. Would molecular dynamics present {that a} proportion of the response permits the escape of C4? Would that be temperature/strain dependent? I’m not about to strive these calculations, however gives of doing so gladly accepted! However that doesn’t essentially clear up the thriller of this response, alluded to above.[1] Is the trapped gaseous species C2 itself, C4 in some type, or certainly one thing else totally?

This put up has DOI: 10.14469/hpc/11959

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