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Dimethylberyllium + CO2 → Fire! A DFT and ab Initio Study into the Photon Emission Observed in a Gas Phase Carbon Dioxide Activation Reaction

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journal contribution
posted on 19.07.2018, 13:49 by Ross Fairlie, Götz Bucher
Dimethylberyllium, Me2Be, is known to ignite when the neat compound is reacted with CO2. In this contribution, we present evidence from DFT and ab initio calculations demonstrating that, while the two-stage gas phase carboxylation of Me2Be to yield beryllium acetate is strongly exothermic, it is not sufficiently so to result in the formation of excited states, as required by a combustion process. The reaction, however, will liberate sufficient heat to drive endothermic unimolecular decomposition reactions. In the case of the reaction of diethylberyllium Et2Be, this results in the formation of beryllium hydride via a β-hydride elimination reaction, and potentially of Be atoms. Pyrolysis of Me2Be, which lacks β-hydrogen atoms, is predicted to give the extremely reactive methyleneberyllium CH2Be, a ground state triplet species. All reactive intermediates generated by pyrolysis of either Me2Be or Et2Be are calculated to react with CO2 in exothermic reactions. With one possible exception, however, none of the carboxylation reactions are predicted to be sufficiently exothermic to yield a product in an excited state. The photon emission observed experimentally is rationalized via the oligomerization of monomeric BeO, which was studied up to two different tetramers. Formation of (BeO)2, (BeO)3, and (BeO)4 ring structures was found to be so intensely exothermic that even relatively high-lying (5.6 eV) excited states will be populated with ease. Finally, the reaction of Be atoms with CO2, previously studied by matrix isolation spectroscopy (Andrews, L.; Tague, T. J., Jr. J. Am. Chem. Soc. 1994, 116, 6856−6859), was found to proceed via initial formation of a four-membered ring carbene-type structure, which had not been taken into account in the earlier experimental work.