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

Theoretical calculations of the C3HO potential surface at the CCSD(T)/aug-c c-pVDu/B3LYP/6-31G* level indicate that the three radicals HCCCO, CCCHO, an d (cyclo-C3H)=O are stable, with HCCCO being the most stable of the three. A fourth isomer, CCHCO, is unstable with respect to cyclization to (cyclo-C 3H)=O. Two isomers have been prepared by neutralization of charged precurso rs, formed as follows: (i) HCCCO, by HC drop C-C(O)-O+(H)(Me) --> HC3O+ + M eOH, and (ii) C2CHO, by (a) Me3SiC drop C-CHO + HO- --> C- drop C-CHO + Me3 SiOH and (b) C- drop C-CH(OH)-C drop CH --> C- drop C-CHO + C2H2. A compari son 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 neut rals dissociate to CO and C2H. The neutral HCCCO may be formed by (i) synch ronous rearrangement of C2CHO and/or (ii) stepwise rearrangement of C2CHO t hrough (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 high er Arrhenius pre-exponential A factor.