Formation of two isomeric C3HO radicals from charged precursors in the gas phase. Potential interstellar molecules

Abstract

Theoretical calculations of the C3HO potential surface at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G* level indicate that the three radicals HCCCO, CCCHO, and (cyclo-C₃H)dO are stable, with HCCCO being the most stable of the three. A fourth isomer, CCHCO, is unstable with respect to cyclization to (cyclo-C₃H)dO. Two isomers have been prepared by neutralization of charged precursors, formed as follows: (i) HCCCO, by HCtCsC(O)-⁺O(H)(Me) → HC₃O+ + MeOH, and (ii) C₂CHO, by (a) Me₃SiC≡C-CHO + HO⁻ → -C≡C-CHO + Me₃SiOH and (b) -C≡C-CH(OH)-C≡CH → ⁻C≡C-CHO + C²H². A comparison of the CR and ⁻NR⁺ spectra of ⁻C₂CHO indicate that C₂CHO is (partially) rearranging to an isomer that shows significant formation of CO°⁺ in the ⁻NR⁺ spectrum of the anion. Ab initio calculations indicate that HCCCO is the product of the isomerism and that a proportion of these isomerized neutrals dissociate to CO and C₂H. The neutral HCCCO may be formed by (i) synchronous rearrangement of C₂CHO and/or (ii) stepwise rearrangement of C₂CHO through (cyclo-C₃H)=O. The second of these processes should have the higher rate, as it has the lower barrier in the rate-determining step and the higher Arrhenius pre-exponential A factor.