Is the Elusive Trioxydehydroethene Neutral (O2C−CO) Detectable in the Gas Phase?

Abstract

The covalently bound radical anion [O2C−CO]-• is formed when 1,3-dioxolane-2,5-dione captures an electron followed by retro-cleavage of CH2O. Calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory indicate that there are a number of neutral isomers with formula C2O3, viz. van der Waals complex O2C- - -CO (rel. energy = 0 kcal mol-1), singlet oxiran dione (+60.9 kcal mol-1), triplet OCOCO (+99.4 kcal mol-1), covalently bound triplet O2C−CO (+ 107.6 kcal mol-1), singlet trioxapropellane (+171.9 kcal mol-1) and triplet trioxapropellane (+ 222.4 kcal mol-1). Of these, only triplet O2C−CO is accessible by vertical Franck−Condon one-electron oxidation of [O2C−CO]-•. Neutralization reionization experiments of [O2C−CO]-• (both -NR+ and -NR-) fail to produce recovery signals corresponding to ionized C2O3, which means that if neutral C2O3 is stable, the lifetime must be <10-6 sec. The -NR+ spectrum of [O2C−CO]-• shows peaks corresponding to CO+•, CO2+• and to [OCCO]+•. The last of these species can only be formed from a decomposing C2O3+• radical cation by a process endothermic by 47 kcal mol-1 [at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory]. Calculations at this same level of theory indicate that the vertical one-electron oxidation of [O2C−CO]-• to triplet O2C−CO ground state produces the neutral with essentially no excess energy. There are two dissociation pathways of this triplet neutral, (i) an endothermic process yielding 3CO + 1CO2 (+ 28.9 kcal mol-1), and (ii) an exothermic process (−6.8 kcal mol-1) with a barrier of 5.4 kcal mol-1 yielding 1CO + 3CO2. A combination of experimental and theoretical data, suggests that vertical oxidation of [O2C−CO]-• produces only one neutral C2O3 isomer; a transient triplet O2C−CO neutral whose lifetime is less than 10-6 sec.