AB INITIO CHARACTERIZATION OF MgCCH, MgCCH~+, AND MgC_2 AND PATHWAYS TO THEIR FORMATION IN THE INTERSTELLAR MEDIUM

摘要

A study of Mg-bearing compounds has been performed in order to determine molecular properties which are critical for planning new astronomical searches and laboratory studies. The primary focus of the work is on MgCCH, MgCCH~+, and the isomers of MgC_2. Only MgCCH has been identified in laboratory studies. Additional calculations have been carried out on MgH, MgNC, MgCN, and their cations in an effort to evaluate pathways to the formation of MgCCH and MgCCH~+ in the interstellar medium (ISM) or in circum-stellar envelopes. Correlated ab initio methods and correlation-consistent basis sets have been employed. Properties including structures, rotational constants, dipole moments, and harmonic frequencies are reported. A transition state between linear MgCC and cyclic MgC_2 has been characterized and was found to yield a minimal barrier (～0.5 kcal mole~(-1)), indicating easy interconversion to the cyclic form. Direct reactions in the ISM between Mg or Mg~+ and HCCH are precluded by energetic considerations, but a number of ion-molecule or neutral-neutral exchange reactions between CCH and various Mg-containing species offer plausible pathways to MgCCH or MgCCH~+. Weakly bound MgH may react with CCH to form MgCCH, but MgH has not been detected. Both MgNC and MgCN have been observed, but reactions with CCH are slightly endothermic by 1-3 kcal mole~(-1). Although MgH~+, MgNC~+, and MgCN~+ have not been detected, their reactions with CCH to form MgCCH~+ are all exothermic. With only a small barrier separating linear MgCC and cyclic MgC_2, the dissociative recombination of MgCCH~+ with an electron is expected to yield cyclic MgC_2 and regenerate Mg and CCH. New astronomical searches for MgCCH, MgCCH~+, cyclic MgC_2, MgNC~+, and MgCN~+ will provide further insight into organo-magnesium astrochemistry.