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

Dimethylberyllium, Me₂Be, is known to ignite when the neat compound is reacted with CO₂. In this contribution, we present evidence from DFT and <i>ab initio</i> 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