I’ve lengthy been fascinated by polymers of both carbon dioxide,† or carbon monoxide, or mixtures of each. One such molecule, known as dioxane tetraketone when it was featured on the ACS molecule-of-the-week web site and also referred to as the anhydride of oxalic acid, or extra formally 1,4-dioxane-2,3,5,6-tetraone, has been speculated upon for greater than a century.
The historical past of chemistry has many molecules whose existence has been speculated upon, however the place tried syntheses have failed and for which sound theoretical causes usually solely emerged a few years later.
The synthesis of dioxane tetraketone was lastly achieved in 1998 at low temperatures (243K), though it was famous that in CDCl3/Et2O options at 273K it rapidly decomposed to present equal portions of carbon monoxide and dioxide. The characterisation was by 13C NMR, for which a single sign at 144.9 ppm was noticed. The anticipated worth utilizing the ACD/CNMR Predictor 2.0 Program (a so-called additive rule-based technique) was 154 ppm (the worth obtained utilizing the same device out there in Chemdraw is 150.9 ppm). The monomer oxirane-2,3-dione was additionally eradicated due to its predicted 13C shift utilizing the identical technique of 167 ppm (155.3 utilizing Chemdraw). Right here I believed I’d verify these chemical shifts utilizing a DFT-based technique and in addition take a look at the barrier to the decomposition to see if it corresponds to a facile response at 273K (FAIR Information DOI: 10.14469/hpc/10619).
Firstly the NMR, utilizing eg ωB97XD/aug-cc-pvdz/SCRF=chloroform. The calculated worth of 148.2 ppm compares effectively with the noticed worth of 144.9 ppm. The worth calculated for oxirane-2,3-dione was 156.6 ppm, reasonably decrease than the ACD/Predictor technique however in settlement with the Chemdraw implementation. The anticipated IR spectrum (not reported) is proven beneath, ought to it ever be measured for this species.
Subsequent, the response power profile, this time calculated utilizing ωB97XD/Def2-TZVPP for the response mechanism proven beneath.
The IRC reveals that the mechanism (black arrows) is adopted, in a concerted course of that reveals completely no signal of any ionic intermediate (crimson) which may then result in oxirane-2,3-dione (blue). The barrier ΔG‡ is 36.9 kcal/mol (it’s decrease than the entire power inferred beneath as a result of the entropy could be very optimistic, one molecule being transformed to 4 in the course of the response) which is way to excessive to correspond to a response that simply happens at 273K. The worth in water as solvent could be very comparable, once more indicating that the ionic route isn’t enhanced by a polar solvent. The transition state has one other characteristic of curiosity. It has C2 chiral symmetry, typical of a pericyclic response with Möbius topology, as certainly could be applicable for an eight electron course of.
So what about that thriller then? Properly, experimentally dioxane tetraketone decomposes at 273K, which might correspond to a free power barrier of round 14-15 kcal/mol. The calculated worth is way greater, too excessive to be merely an error within the DFT technique. So here’s a suggestion. CDCl3, except very rigorously purified, comprises HCl, which may very simply catalyse the response. So if one other solvent have been to be tried, shall we say acetonitrile during which any hint of acid has been eliminated, would options of dioxane tetraketone then persist at room temperatures for much longer? An experiment maybe to be tried!