1999MNRAS.302..482P

High-level ab initio quantum chemical calculations have been employed to investigate structural and thermochemical properties of the molecules c-C₃HM, MCCCH, MNCCC and MCCCN (M = Na, Mg, Al), which are feasible dissociative recombination products of the reactions of M⁺ with C₃H₂ and HC_2n+1N. A detailed study of the MCCCN and c-C₃HM optimized geometries, obtained using various basis sets, has furnished values for the rotational constants of these species to an expected accuracy of±2 per cent, which should assist in their laboratory or radioastronomical detection. In an examination of the mechanisms by which these species may arise, we conclude that MgCCCN is the best candidate for detection within the outer envelopes of carbon-rich AGB stars such as IRC+10216; that NaCCCN and AlCCCN may well arise via inner-envelope chemical processes within IRC+10216 (but are unlikely to be found within the outer envelope); and that the MC₃H structures, particularly c-C₃HMg, are worthwhile candidates for a radioastronomical search within cold dense clouds such as TMC-1.