Home Chemistry The energetics of two types of tetracarbon, C4 and a problem! « Henry Rzepa’s Weblog

The energetics of two types of tetracarbon, C4 and a problem! « Henry Rzepa’s Weblog

The energetics of two types of tetracarbon, C4 and a problem! « Henry Rzepa’s Weblog


The subject of dicarbon, C2, has been mentioned right here for a couple of years now. It undoubtedly can be a gasoline! 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 completely different variants of X(+), comparable to not solely X=PhI(+), but additionally e.g. X=dibenzothiophenium(+) appeared to generate a gaseous species, which may very well be trapped as “C2” in a solvent-free linked flask experiment.

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

Right here I carry your consideration to an odd characteristic on the IRC, within the area of -5. On this area, successfully “free C4” is fashioned (at an vitality 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 vitality of ~20 kcal/mol. Suppose nevertheless, in a molecular dynamics sense, some proportion of this “C4” might take a unique 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 may occur subsequent, to reply the query of whether or not linear C4 is steady, or will it convert into one thing else? The scheme under reveals a number of the doable 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 gasoline part stage, permitting biradicals to type (FAIR Information DOI: 10.14469/hpc/11956).

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

The important thing calculation may be that dimerisation response proven above. Would molecular dynamics present {that a} proportion of the response permits the escape of C4? Would that be temperature/stress dependent? I’m not about to attempt these calculations, however provides 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 completely?

This publish has DOI: 10.14469/hpc/11959



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